{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998


        Arity and eta expansion
-}

{-# LANGUAGE CPP #-}

-- | Arity and eta expansion
module CoreArity (
        manifestArity, joinRhsArity, exprArity, typeArity,
        exprEtaExpandArity, findRhsArity, etaExpand,
        etaExpandToJoinPoint, etaExpandToJoinPointRule,
        exprBotStrictness_maybe
    ) where

#include "HsVersions.h"

import GhcPrelude

import CoreSyn
import CoreFVs
import CoreUtils
import CoreSubst
import Demand
import Var
import VarEnv
import Id
import Type
import TyCon    ( initRecTc, checkRecTc )
import Coercion
import BasicTypes
import Unique
import DynFlags ( DynFlags, GeneralFlag(..), gopt )
import Outputable
import FastString
import Pair
import Util     ( debugIsOn )

{-
************************************************************************
*                                                                      *
              manifestArity and exprArity
*                                                                      *
************************************************************************

exprArity is a cheap-and-cheerful version of exprEtaExpandArity.
It tells how many things the expression can be applied to before doing
any work.  It doesn't look inside cases, lets, etc.  The idea is that
exprEtaExpandArity will do the hard work, leaving something that's easy
for exprArity to grapple with.  In particular, Simplify uses exprArity to
compute the ArityInfo for the Id.

Originally I thought that it was enough just to look for top-level lambdas, but
it isn't.  I've seen this

        foo = PrelBase.timesInt

We want foo to get arity 2 even though the eta-expander will leave it
unchanged, in the expectation that it'll be inlined.  But occasionally it
isn't, because foo is blacklisted (used in a rule).

Similarly, see the ok_note check in exprEtaExpandArity.  So
        f = __inline_me (\x -> e)
won't be eta-expanded.

And in any case it seems more robust to have exprArity be a bit more intelligent.
But note that   (\x y z -> f x y z)
should have arity 3, regardless of f's arity.
-}

manifestArity :: CoreExpr -> Arity
-- ^ manifestArity sees how many leading value lambdas there are,
--   after looking through casts
manifestArity :: CoreExpr -> Arity
manifestArity (Lam v :: CoreBndr
v e :: CoreExpr
e) | CoreBndr -> Bool
isId CoreBndr
v        = 1 Arity -> Arity -> Arity
forall a. Num a => a -> a -> a
+ CoreExpr -> Arity
manifestArity CoreExpr
e
                        | Bool
otherwise     = CoreExpr -> Arity
manifestArity CoreExpr
e
manifestArity (Tick t :: Tickish CoreBndr
t e :: CoreExpr
e) | Bool -> Bool
not (Tickish CoreBndr -> Bool
forall id. Tickish id -> Bool
tickishIsCode Tickish CoreBndr
t) =  CoreExpr -> Arity
manifestArity CoreExpr
e
manifestArity (Cast e :: CoreExpr
e _)                = CoreExpr -> Arity
manifestArity CoreExpr
e
manifestArity _                         = 0

joinRhsArity :: CoreExpr -> JoinArity
-- Join points are supposed to have manifestly-visible
-- lambdas at the top: no ticks, no casts, nothing
-- Moreover, type lambdas count in JoinArity
joinRhsArity :: CoreExpr -> Arity
joinRhsArity (Lam _ e :: CoreExpr
e) = 1 Arity -> Arity -> Arity
forall a. Num a => a -> a -> a
+ CoreExpr -> Arity
joinRhsArity CoreExpr
e
joinRhsArity _         = 0


---------------
exprArity :: CoreExpr -> Arity
-- ^ An approximate, fast, version of 'exprEtaExpandArity'
exprArity :: CoreExpr -> Arity
exprArity e :: CoreExpr
e = CoreExpr -> Arity
go CoreExpr
e
  where
    go :: CoreExpr -> Arity
go (Var v :: CoreBndr
v)                     = CoreBndr -> Arity
idArity CoreBndr
v
    go (Lam x :: CoreBndr
x e :: CoreExpr
e) | CoreBndr -> Bool
isId CoreBndr
x          = CoreExpr -> Arity
go CoreExpr
e Arity -> Arity -> Arity
forall a. Num a => a -> a -> a
+ 1
                 | Bool
otherwise       = CoreExpr -> Arity
go CoreExpr
e
    go (Tick t :: Tickish CoreBndr
t e :: CoreExpr
e) | Bool -> Bool
not (Tickish CoreBndr -> Bool
forall id. Tickish id -> Bool
tickishIsCode Tickish CoreBndr
t) = CoreExpr -> Arity
go CoreExpr
e
    go (Cast e :: CoreExpr
e co :: Coercion
co)                 = Arity -> Type -> Arity
trim_arity (CoreExpr -> Arity
go CoreExpr
e) (Pair Type -> Type
forall a. Pair a -> a
pSnd (Coercion -> Pair Type
coercionKind Coercion
co))
                                        -- Note [exprArity invariant]
    go (App e :: CoreExpr
e (Type _))            = CoreExpr -> Arity
go CoreExpr
e
    go (App f :: CoreExpr
f a :: CoreExpr
a) | CoreExpr -> Bool
exprIsTrivial CoreExpr
a = (CoreExpr -> Arity
go CoreExpr
f Arity -> Arity -> Arity
forall a. Num a => a -> a -> a
- 1) Arity -> Arity -> Arity
forall a. Ord a => a -> a -> a
`max` 0
        -- See Note [exprArity for applications]
        -- NB: coercions count as a value argument

    go _                           = 0

    trim_arity :: Arity -> Type -> Arity
    trim_arity :: Arity -> Type -> Arity
trim_arity arity :: Arity
arity ty :: Type
ty = Arity
arity Arity -> Arity -> Arity
forall a. Ord a => a -> a -> a
`min` [OneShotInfo] -> Arity
forall (t :: * -> *) a. Foldable t => t a -> Arity
length (Type -> [OneShotInfo]
typeArity Type
ty)

---------------
typeArity :: Type -> [OneShotInfo]
-- How many value arrows are visible in the type?
-- We look through foralls, and newtypes
-- See Note [exprArity invariant]
typeArity :: Type -> [OneShotInfo]
typeArity ty :: Type
ty
  = RecTcChecker -> Type -> [OneShotInfo]
go RecTcChecker
initRecTc Type
ty
  where
    go :: RecTcChecker -> Type -> [OneShotInfo]
go rec_nts :: RecTcChecker
rec_nts ty :: Type
ty
      | Just (_, ty' :: Type
ty')  <- Type -> Maybe (CoreBndr, Type)
splitForAllTy_maybe Type
ty
      = RecTcChecker -> Type -> [OneShotInfo]
go RecTcChecker
rec_nts Type
ty'

      | Just (arg :: Type
arg,res :: Type
res) <- Type -> Maybe (Type, Type)
splitFunTy_maybe Type
ty
      = Type -> OneShotInfo
typeOneShot Type
arg OneShotInfo -> [OneShotInfo] -> [OneShotInfo]
forall a. a -> [a] -> [a]
: RecTcChecker -> Type -> [OneShotInfo]
go RecTcChecker
rec_nts Type
res

      | Just (tc :: TyCon
tc,tys :: [Type]
tys) <- HasDebugCallStack => Type -> Maybe (TyCon, [Type])
Type -> Maybe (TyCon, [Type])
splitTyConApp_maybe Type
ty
      , Just (ty' :: Type
ty', _) <- TyCon -> [Type] -> Maybe (Type, Coercion)
instNewTyCon_maybe TyCon
tc [Type]
tys
      , Just rec_nts' :: RecTcChecker
rec_nts' <- RecTcChecker -> TyCon -> Maybe RecTcChecker
checkRecTc RecTcChecker
rec_nts TyCon
tc  -- See Note [Expanding newtypes]
                                                -- in TyCon
--   , not (isClassTyCon tc)    -- Do not eta-expand through newtype classes
--                              -- See Note [Newtype classes and eta expansion]
--                              (no longer required)
      = RecTcChecker -> Type -> [OneShotInfo]
go RecTcChecker
rec_nts' Type
ty'
        -- Important to look through non-recursive newtypes, so that, eg
        --      (f x)   where f has arity 2, f :: Int -> IO ()
        -- Here we want to get arity 1 for the result!
        --
        -- AND through a layer of recursive newtypes
        -- e.g. newtype Stream m a b = Stream (m (Either b (a, Stream m a b)))

      | Bool
otherwise
      = []

---------------
exprBotStrictness_maybe :: CoreExpr -> Maybe (Arity, StrictSig)
-- A cheap and cheerful function that identifies bottoming functions
-- and gives them a suitable strictness signatures.  It's used during
-- float-out
exprBotStrictness_maybe :: CoreExpr -> Maybe (Arity, StrictSig)
exprBotStrictness_maybe e :: CoreExpr
e
  = case ArityType -> Maybe Arity
getBotArity (ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
e) of
        Nothing -> Maybe (Arity, StrictSig)
forall a. Maybe a
Nothing
        Just ar :: Arity
ar -> (Arity, StrictSig) -> Maybe (Arity, StrictSig)
forall a. a -> Maybe a
Just (Arity
ar, Arity -> StrictSig
sig Arity
ar)
  where
    env :: ArityEnv
env    = AE :: CheapFun -> Bool -> ArityEnv
AE { ae_ped_bot :: Bool
ae_ped_bot = Bool
True, ae_cheap_fn :: CheapFun
ae_cheap_fn = \ _ _ -> Bool
False }
    sig :: Arity -> StrictSig
sig ar :: Arity
ar = [Demand] -> DmdResult -> StrictSig
mkClosedStrictSig (Arity -> Demand -> [Demand]
forall a. Arity -> a -> [a]
replicate Arity
ar Demand
topDmd) DmdResult
exnRes
                  -- For this purpose we can be very simple
                  -- exnRes is a bit less aggressive than botRes

{-
Note [exprArity invariant]
~~~~~~~~~~~~~~~~~~~~~~~~~~
exprArity has the following invariant:

  (1) If typeArity (exprType e) = n,
      then manifestArity (etaExpand e n) = n

      That is, etaExpand can always expand as much as typeArity says
      So the case analysis in etaExpand and in typeArity must match

  (2) exprArity e <= typeArity (exprType e)

  (3) Hence if (exprArity e) = n, then manifestArity (etaExpand e n) = n

      That is, if exprArity says "the arity is n" then etaExpand really
      can get "n" manifest lambdas to the top.

Why is this important?  Because
  - In TidyPgm we use exprArity to fix the *final arity* of
    each top-level Id, and in
  - In CorePrep we use etaExpand on each rhs, so that the visible lambdas
    actually match that arity, which in turn means
    that the StgRhs has the right number of lambdas

An alternative would be to do the eta-expansion in TidyPgm, at least
for top-level bindings, in which case we would not need the trim_arity
in exprArity.  That is a less local change, so I'm going to leave it for today!

Note [Newtype classes and eta expansion]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    NB: this nasty special case is no longer required, because
    for newtype classes we don't use the class-op rule mechanism
    at all.  See Note [Single-method classes] in TcInstDcls. SLPJ May 2013

-------- Old out of date comments, just for interest -----------
We have to be careful when eta-expanding through newtypes.  In general
it's a good idea, but annoyingly it interacts badly with the class-op
rule mechanism.  Consider

   class C a where { op :: a -> a }
   instance C b => C [b] where
     op x = ...

These translate to

   co :: forall a. (a->a) ~ C a

   $copList :: C b -> [b] -> [b]
   $copList d x = ...

   $dfList :: C b -> C [b]
   {-# DFunUnfolding = [$copList] #-}
   $dfList d = $copList d |> co@[b]

Now suppose we have:

   dCInt :: C Int

   blah :: [Int] -> [Int]
   blah = op ($dfList dCInt)

Now we want the built-in op/$dfList rule will fire to give
   blah = $copList dCInt

But with eta-expansion 'blah' might (and in Trac #3772, which is
slightly more complicated, does) turn into

   blah = op (\eta. ($dfList dCInt |> sym co) eta)

and now it is *much* harder for the op/$dfList rule to fire, because
exprIsConApp_maybe won't hold of the argument to op.  I considered
trying to *make* it hold, but it's tricky and I gave up.

The test simplCore/should_compile/T3722 is an excellent example.
-------- End of old out of date comments, just for interest -----------


Note [exprArity for applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we come to an application we check that the arg is trivial.
   eg  f (fac x) does not have arity 2,
                 even if f has arity 3!

* We require that is trivial rather merely cheap.  Suppose f has arity 2.
  Then    f (Just y)
  has arity 0, because if we gave it arity 1 and then inlined f we'd get
          let v = Just y in \w. <f-body>
  which has arity 0.  And we try to maintain the invariant that we don't
  have arity decreases.

*  The `max 0` is important!  (\x y -> f x) has arity 2, even if f is
   unknown, hence arity 0


************************************************************************
*                                                                      *
           Computing the "arity" of an expression
*                                                                      *
************************************************************************

Note [Definition of arity]
~~~~~~~~~~~~~~~~~~~~~~~~~~
The "arity" of an expression 'e' is n if
   applying 'e' to *fewer* than n *value* arguments
   converges rapidly

Or, to put it another way

   there is no work lost in duplicating the partial
   application (e x1 .. x(n-1))

In the divegent case, no work is lost by duplicating because if the thing
is evaluated once, that's the end of the program.

Or, to put it another way, in any context C

   C[ (\x1 .. xn. e x1 .. xn) ]
         is as efficient as
   C[ e ]

It's all a bit more subtle than it looks:

Note [One-shot lambdas]
~~~~~~~~~~~~~~~~~~~~~~~
Consider one-shot lambdas
                let x = expensive in \y z -> E
We want this to have arity 1 if the \y-abstraction is a 1-shot lambda.

Note [Dealing with bottom]
~~~~~~~~~~~~~~~~~~~~~~~~~~
A Big Deal with computing arities is expressions like

   f = \x -> case x of
               True  -> \s -> e1
               False -> \s -> e2

This happens all the time when f :: Bool -> IO ()
In this case we do eta-expand, in order to get that \s to the
top, and give f arity 2.

This isn't really right in the presence of seq.  Consider
        (f bot) `seq` 1

This should diverge!  But if we eta-expand, it won't.  We ignore this
"problem" (unless -fpedantic-bottoms is on), because being scrupulous
would lose an important transformation for many programs. (See
Trac #5587 for an example.)

Consider also
        f = \x -> error "foo"
Here, arity 1 is fine.  But if it is
        f = \x -> case x of
                        True  -> error "foo"
                        False -> \y -> x+y
then we want to get arity 2.  Technically, this isn't quite right, because
        (f True) `seq` 1
should diverge, but it'll converge if we eta-expand f.  Nevertheless, we
do so; it improves some programs significantly, and increasing convergence
isn't a bad thing.  Hence the ABot/ATop in ArityType.

So these two transformations aren't always the Right Thing, and we
have several tickets reporting unexpected behaviour resulting from
this transformation.  So we try to limit it as much as possible:

 (1) Do NOT move a lambda outside a known-bottom case expression
       case undefined of { (a,b) -> \y -> e }
     This showed up in Trac #5557

 (2) Do NOT move a lambda outside a case if all the branches of
     the case are known to return bottom.
        case x of { (a,b) -> \y -> error "urk" }
     This case is less important, but the idea is that if the fn is
     going to diverge eventually anyway then getting the best arity
     isn't an issue, so we might as well play safe

 (3) Do NOT move a lambda outside a case unless
     (a) The scrutinee is ok-for-speculation, or
     (b) more liberally: the scrutinee is cheap (e.g. a variable), and
         -fpedantic-bottoms is not enforced (see Trac #2915 for an example)

Of course both (1) and (2) are readily defeated by disguising the bottoms.

4. Note [Newtype arity]
~~~~~~~~~~~~~~~~~~~~~~~~
Non-recursive newtypes are transparent, and should not get in the way.
We do (currently) eta-expand recursive newtypes too.  So if we have, say

        newtype T = MkT ([T] -> Int)

Suppose we have
        e = coerce T f
where f has arity 1.  Then: etaExpandArity e = 1;
that is, etaExpandArity looks through the coerce.

When we eta-expand e to arity 1: eta_expand 1 e T
we want to get:                  coerce T (\x::[T] -> (coerce ([T]->Int) e) x)

  HOWEVER, note that if you use coerce bogusly you can ge
        coerce Int negate
  And since negate has arity 2, you might try to eta expand.  But you can't
  decopose Int to a function type.   Hence the final case in eta_expand.

Note [The state-transformer hack]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
        f = e
where e has arity n.  Then, if we know from the context that f has
a usage type like
        t1 -> ... -> tn -1-> t(n+1) -1-> ... -1-> tm -> ...
then we can expand the arity to m.  This usage type says that
any application (x e1 .. en) will be applied to uniquely to (m-n) more args
Consider f = \x. let y = <expensive>
                 in case x of
                      True  -> foo
                      False -> \(s:RealWorld) -> e
where foo has arity 1.  Then we want the state hack to
apply to foo too, so we can eta expand the case.

Then we expect that if f is applied to one arg, it'll be applied to two
(that's the hack -- we don't really know, and sometimes it's false)
See also Id.isOneShotBndr.

Note [State hack and bottoming functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's a terrible idea to use the state hack on a bottoming function.
Here's what happens (Trac #2861):

  f :: String -> IO T
  f = \p. error "..."

Eta-expand, using the state hack:

  f = \p. (\s. ((error "...") |> g1) s) |> g2
  g1 :: IO T ~ (S -> (S,T))
  g2 :: (S -> (S,T)) ~ IO T

Extrude the g2

  f' = \p. \s. ((error "...") |> g1) s
  f = f' |> (String -> g2)

Discard args for bottomming function

  f' = \p. \s. ((error "...") |> g1 |> g3
  g3 :: (S -> (S,T)) ~ (S,T)

Extrude g1.g3

  f'' = \p. \s. (error "...")
  f' = f'' |> (String -> S -> g1.g3)

And now we can repeat the whole loop.  Aargh!  The bug is in applying the
state hack to a function which then swallows the argument.

This arose in another guise in Trac #3959.  Here we had

     catch# (throw exn >> return ())

Note that (throw :: forall a e. Exn e => e -> a) is called with [a = IO ()].
After inlining (>>) we get

     catch# (\_. throw {IO ()} exn)

We must *not* eta-expand to

     catch# (\_ _. throw {...} exn)

because 'catch#' expects to get a (# _,_ #) after applying its argument to
a State#, not another function!

In short, we use the state hack to allow us to push let inside a lambda,
but not to introduce a new lambda.


Note [ArityType]
~~~~~~~~~~~~~~~~
ArityType is the result of a compositional analysis on expressions,
from which we can decide the real arity of the expression (extracted
with function exprEtaExpandArity).

Here is what the fields mean. If an arbitrary expression 'f' has
ArityType 'at', then

 * If at = ABot n, then (f x1..xn) definitely diverges. Partial
   applications to fewer than n args may *or may not* diverge.

   We allow ourselves to eta-expand bottoming functions, even
   if doing so may lose some `seq` sharing,
       let x = <expensive> in \y. error (g x y)
       ==> \y. let x = <expensive> in error (g x y)

 * If at = ATop as, and n=length as,
   then expanding 'f' to (\x1..xn. f x1 .. xn) loses no sharing,
   assuming the calls of f respect the one-shot-ness of
   its definition.

   NB 'f' is an arbitrary expression, eg (f = g e1 e2).  This 'f'
   can have ArityType as ATop, with length as > 0, only if e1 e2 are
   themselves.

 * In both cases, f, (f x1), ... (f x1 ... f(n-1)) are definitely
   really functions, or bottom, but *not* casts from a data type, in
   at least one case branch.  (If it's a function in one case branch but
   an unsafe cast from a data type in another, the program is bogus.)
   So eta expansion is dynamically ok; see Note [State hack and
   bottoming functions], the part about catch#

Example:
      f = \x\y. let v = <expensive> in
          \s(one-shot) \t(one-shot). blah
      'f' has ArityType [ManyShot,ManyShot,OneShot,OneShot]
      The one-shot-ness means we can, in effect, push that
      'let' inside the \st.


Suppose f = \xy. x+y
Then  f             :: AT [False,False] ATop
      f v           :: AT [False]       ATop
      f <expensive> :: AT []            ATop

-------------------- Main arity code ----------------------------
-}

-- See Note [ArityType]
data ArityType = ATop [OneShotInfo] | ABot Arity
     -- There is always an explicit lambda
     -- to justify the [OneShot], or the Arity

instance Outputable ArityType where
  ppr :: ArityType -> SDoc
ppr (ATop os :: [OneShotInfo]
os) = String -> SDoc
text "ATop" SDoc -> SDoc -> SDoc
<> SDoc -> SDoc
parens (Arity -> SDoc
forall a. Outputable a => a -> SDoc
ppr ([OneShotInfo] -> Arity
forall (t :: * -> *) a. Foldable t => t a -> Arity
length [OneShotInfo]
os))
  ppr (ABot n :: Arity
n)  = String -> SDoc
text "ABot" SDoc -> SDoc -> SDoc
<> SDoc -> SDoc
parens (Arity -> SDoc
forall a. Outputable a => a -> SDoc
ppr Arity
n)

vanillaArityType :: ArityType
vanillaArityType :: ArityType
vanillaArityType = [OneShotInfo] -> ArityType
ATop []      -- Totally uninformative

-- ^ The Arity returned is the number of value args the
-- expression can be applied to without doing much work
exprEtaExpandArity :: DynFlags -> CoreExpr -> Arity
-- exprEtaExpandArity is used when eta expanding
--      e  ==>  \xy -> e x y
exprEtaExpandArity :: DynFlags -> CoreExpr -> Arity
exprEtaExpandArity dflags :: DynFlags
dflags e :: CoreExpr
e
  = case (ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
e) of
      ATop oss :: [OneShotInfo]
oss -> [OneShotInfo] -> Arity
forall (t :: * -> *) a. Foldable t => t a -> Arity
length [OneShotInfo]
oss
      ABot n :: Arity
n   -> Arity
n
  where
    env :: ArityEnv
env = AE :: CheapFun -> Bool -> ArityEnv
AE { ae_cheap_fn :: CheapFun
ae_cheap_fn = DynFlags -> CheapAppFun -> CheapFun
mk_cheap_fn DynFlags
dflags CheapAppFun
isCheapApp
             , ae_ped_bot :: Bool
ae_ped_bot  = GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_PedanticBottoms DynFlags
dflags }

getBotArity :: ArityType -> Maybe Arity
-- Arity of a divergent function
getBotArity :: ArityType -> Maybe Arity
getBotArity (ABot n :: Arity
n) = Arity -> Maybe Arity
forall a. a -> Maybe a
Just Arity
n
getBotArity _        = Maybe Arity
forall a. Maybe a
Nothing

mk_cheap_fn :: DynFlags -> CheapAppFun -> CheapFun
mk_cheap_fn :: DynFlags -> CheapAppFun -> CheapFun
mk_cheap_fn dflags :: DynFlags
dflags cheap_app :: CheapAppFun
cheap_app
  | Bool -> Bool
not (GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_DictsCheap DynFlags
dflags)
  = \e :: CoreExpr
e _     -> CheapAppFun -> CoreExpr -> Bool
exprIsCheapX CheapAppFun
cheap_app CoreExpr
e
  | Bool
otherwise
  = \e :: CoreExpr
e mb_ty :: Maybe Type
mb_ty -> CheapAppFun -> CoreExpr -> Bool
exprIsCheapX CheapAppFun
cheap_app CoreExpr
e
             Bool -> Bool -> Bool
|| case Maybe Type
mb_ty of
                  Nothing -> Bool
False
                  Just ty :: Type
ty -> Type -> Bool
isDictLikeTy Type
ty


----------------------
findRhsArity :: DynFlags -> Id -> CoreExpr -> Arity -> (Arity, Bool)
-- This implements the fixpoint loop for arity analysis
-- See Note [Arity analysis]
-- If findRhsArity e = (n, is_bot) then
--  (a) any application of e to <n arguments will not do much work,
--      so it is safe to expand e  ==>  (\x1..xn. e x1 .. xn)
--  (b) if is_bot=True, then e applied to n args is guaranteed bottom
findRhsArity :: DynFlags -> CoreBndr -> CoreExpr -> Arity -> (Arity, Bool)
findRhsArity dflags :: DynFlags
dflags bndr :: CoreBndr
bndr rhs :: CoreExpr
rhs old_arity :: Arity
old_arity
  = (Arity, Bool) -> (Arity, Bool)
go (CheapAppFun -> (Arity, Bool)
get_arity CheapAppFun
init_cheap_app)
       -- We always call exprEtaExpandArity once, but usually
       -- that produces a result equal to old_arity, and then
       -- we stop right away (since arities should not decrease)
       -- Result: the common case is that there is just one iteration
  where
    is_lam :: Bool
is_lam = CoreExpr -> Bool
has_lam CoreExpr
rhs

    has_lam :: CoreExpr -> Bool
has_lam (Tick _ e :: CoreExpr
e) = CoreExpr -> Bool
has_lam CoreExpr
e
    has_lam (Lam b :: CoreBndr
b e :: CoreExpr
e)  = CoreBndr -> Bool
isId CoreBndr
b Bool -> Bool -> Bool
|| CoreExpr -> Bool
has_lam CoreExpr
e
    has_lam _          = Bool
False

    init_cheap_app :: CheapAppFun
    init_cheap_app :: CheapAppFun
init_cheap_app fn :: CoreBndr
fn n_val_args :: Arity
n_val_args
      | CoreBndr
fn CoreBndr -> CoreBndr -> Bool
forall a. Eq a => a -> a -> Bool
== CoreBndr
bndr = Bool
True   -- On the first pass, this binder gets infinite arity
      | Bool
otherwise  = CheapAppFun
isCheapApp CoreBndr
fn Arity
n_val_args

    go :: (Arity, Bool) -> (Arity, Bool)
    go :: (Arity, Bool) -> (Arity, Bool)
go cur_info :: (Arity, Bool)
cur_info@(cur_arity :: Arity
cur_arity, _)
      | Arity
cur_arity Arity -> Arity -> Bool
forall a. Ord a => a -> a -> Bool
<= Arity
old_arity = (Arity, Bool)
cur_info
      | Arity
new_arity Arity -> Arity -> Bool
forall a. Eq a => a -> a -> Bool
== Arity
cur_arity = (Arity, Bool)
cur_info
      | Bool
otherwise = ASSERT( new_arity < cur_arity )
#if defined(DEBUG)
                    pprTrace "Exciting arity"
                       (vcat [ ppr bndr <+> ppr cur_arity <+> ppr new_arity
                             , ppr rhs])
#endif
                    (Arity, Bool) -> (Arity, Bool)
go (Arity, Bool)
new_info
      where
        new_info :: (Arity, Bool)
new_info@(new_arity :: Arity
new_arity, _) = CheapAppFun -> (Arity, Bool)
get_arity CheapAppFun
cheap_app

        cheap_app :: CheapAppFun
        cheap_app :: CheapAppFun
cheap_app fn :: CoreBndr
fn n_val_args :: Arity
n_val_args
          | CoreBndr
fn CoreBndr -> CoreBndr -> Bool
forall a. Eq a => a -> a -> Bool
== CoreBndr
bndr = Arity
n_val_args Arity -> Arity -> Bool
forall a. Ord a => a -> a -> Bool
< Arity
cur_arity
          | Bool
otherwise  = CheapAppFun
isCheapApp CoreBndr
fn Arity
n_val_args

    get_arity :: CheapAppFun -> (Arity, Bool)
    get_arity :: CheapAppFun -> (Arity, Bool)
get_arity cheap_app :: CheapAppFun
cheap_app
      = case (ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
rhs) of
          ABot n :: Arity
n -> (Arity
n, Bool
True)
          ATop (os :: OneShotInfo
os:oss :: [OneShotInfo]
oss) | OneShotInfo -> Bool
isOneShotInfo OneShotInfo
os Bool -> Bool -> Bool
|| Bool
is_lam
                  -> (1 Arity -> Arity -> Arity
forall a. Num a => a -> a -> a
+ [OneShotInfo] -> Arity
forall (t :: * -> *) a. Foldable t => t a -> Arity
length [OneShotInfo]
oss, Bool
False)    -- Don't expand PAPs/thunks
          ATop _  -> (0,              Bool
False)    -- Note [Eta expanding thunks]
       where
         env :: ArityEnv
env = AE :: CheapFun -> Bool -> ArityEnv
AE { ae_cheap_fn :: CheapFun
ae_cheap_fn = DynFlags -> CheapAppFun -> CheapFun
mk_cheap_fn DynFlags
dflags CheapAppFun
cheap_app
                  , ae_ped_bot :: Bool
ae_ped_bot  = GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_PedanticBottoms DynFlags
dflags }

{-
Note [Arity analysis]
~~~~~~~~~~~~~~~~~~~~~
The motivating example for arity analysis is this:

  f = \x. let g = f (x+1)
          in \y. ...g...

What arity does f have?  Really it should have arity 2, but a naive
look at the RHS won't see that.  You need a fixpoint analysis which
says it has arity "infinity" the first time round.

This example happens a lot; it first showed up in Andy Gill's thesis,
fifteen years ago!  It also shows up in the code for 'rnf' on lists
in Trac #4138.

The analysis is easy to achieve because exprEtaExpandArity takes an
argument
     type CheapFun = CoreExpr -> Maybe Type -> Bool
used to decide if an expression is cheap enough to push inside a
lambda.  And exprIsCheapX in turn takes an argument
     type CheapAppFun = Id -> Int -> Bool
which tells when an application is cheap. This makes it easy to
write the analysis loop.

The analysis is cheap-and-cheerful because it doesn't deal with
mutual recursion.  But the self-recursive case is the important one.


Note [Eta expanding through dictionaries]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If the experimental -fdicts-cheap flag is on, we eta-expand through
dictionary bindings.  This improves arities. Thereby, it also
means that full laziness is less prone to floating out the
application of a function to its dictionary arguments, which
can thereby lose opportunities for fusion.  Example:
        foo :: Ord a => a -> ...
     foo = /\a \(d:Ord a). let d' = ...d... in \(x:a). ....
        -- So foo has arity 1

     f = \x. foo dInt $ bar x

The (foo DInt) is floated out, and makes ineffective a RULE
     foo (bar x) = ...

One could go further and make exprIsCheap reply True to any
dictionary-typed expression, but that's more work.

See Note [Dictionary-like types] in TcType.hs for why we use
isDictLikeTy here rather than isDictTy

Note [Eta expanding thunks]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We don't eta-expand
   * Trivial RHSs     x = y
   * PAPs             x = map g
   * Thunks           f = case y of p -> \x -> blah

When we see
     f = case y of p -> \x -> blah
should we eta-expand it? Well, if 'x' is a one-shot state token
then 'yes' because 'f' will only be applied once.  But otherwise
we (conservatively) say no.  My main reason is to avoid expanding
PAPSs
        f = g d  ==>  f = \x. g d x
because that might in turn make g inline (if it has an inline pragma),
which we might not want.  After all, INLINE pragmas say "inline only
when saturated" so we don't want to be too gung-ho about saturating!
-}

arityLam :: Id -> ArityType -> ArityType
arityLam :: CoreBndr -> ArityType -> ArityType
arityLam id :: CoreBndr
id (ATop as :: [OneShotInfo]
as) = [OneShotInfo] -> ArityType
ATop (CoreBndr -> OneShotInfo
idStateHackOneShotInfo CoreBndr
id OneShotInfo -> [OneShotInfo] -> [OneShotInfo]
forall a. a -> [a] -> [a]
: [OneShotInfo]
as)
arityLam _  (ABot n :: Arity
n)  = Arity -> ArityType
ABot (Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
+1)

floatIn :: Bool -> ArityType -> ArityType
-- We have something like (let x = E in b),
-- where b has the given arity type.
floatIn :: Bool -> ArityType -> ArityType
floatIn _     (ABot n :: Arity
n)  = Arity -> ArityType
ABot Arity
n
floatIn True  (ATop as :: [OneShotInfo]
as) = [OneShotInfo] -> ArityType
ATop [OneShotInfo]
as
floatIn False (ATop as :: [OneShotInfo]
as) = [OneShotInfo] -> ArityType
ATop ((OneShotInfo -> Bool) -> [OneShotInfo] -> [OneShotInfo]
forall a. (a -> Bool) -> [a] -> [a]
takeWhile OneShotInfo -> Bool
isOneShotInfo [OneShotInfo]
as)
   -- If E is not cheap, keep arity only for one-shots

arityApp :: ArityType -> Bool -> ArityType
-- Processing (fun arg) where at is the ArityType of fun,
-- Knock off an argument and behave like 'let'
arityApp :: ArityType -> Bool -> ArityType
arityApp (ABot 0)      _     = Arity -> ArityType
ABot 0
arityApp (ABot n :: Arity
n)      _     = Arity -> ArityType
ABot (Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
-1)
arityApp (ATop [])     _     = [OneShotInfo] -> ArityType
ATop []
arityApp (ATop (_:as :: [OneShotInfo]
as)) cheap :: Bool
cheap = Bool -> ArityType -> ArityType
floatIn Bool
cheap ([OneShotInfo] -> ArityType
ATop [OneShotInfo]
as)

andArityType :: ArityType -> ArityType -> ArityType   -- Used for branches of a 'case'
andArityType :: ArityType -> ArityType -> ArityType
andArityType (ABot n1 :: Arity
n1) (ABot n2 :: Arity
n2)  = Arity -> ArityType
ABot (Arity
n1 Arity -> Arity -> Arity
forall a. Ord a => a -> a -> a
`max` Arity
n2) -- Note [ABot branches: use max]
andArityType (ATop as :: [OneShotInfo]
as)  (ABot _)  = [OneShotInfo] -> ArityType
ATop [OneShotInfo]
as
andArityType (ABot _)   (ATop bs :: [OneShotInfo]
bs) = [OneShotInfo] -> ArityType
ATop [OneShotInfo]
bs
andArityType (ATop as :: [OneShotInfo]
as)  (ATop bs :: [OneShotInfo]
bs) = [OneShotInfo] -> ArityType
ATop ([OneShotInfo]
as [OneShotInfo] -> [OneShotInfo] -> [OneShotInfo]
`combine` [OneShotInfo]
bs)
  where      -- See Note [Combining case branches]
    combine :: [OneShotInfo] -> [OneShotInfo] -> [OneShotInfo]
combine (a :: OneShotInfo
a:as :: [OneShotInfo]
as) (b :: OneShotInfo
b:bs :: [OneShotInfo]
bs) = (OneShotInfo
a OneShotInfo -> OneShotInfo -> OneShotInfo
`bestOneShot` OneShotInfo
b) OneShotInfo -> [OneShotInfo] -> [OneShotInfo]
forall a. a -> [a] -> [a]
: [OneShotInfo] -> [OneShotInfo] -> [OneShotInfo]
combine [OneShotInfo]
as [OneShotInfo]
bs
    combine []     bs :: [OneShotInfo]
bs     = (OneShotInfo -> Bool) -> [OneShotInfo] -> [OneShotInfo]
forall a. (a -> Bool) -> [a] -> [a]
takeWhile OneShotInfo -> Bool
isOneShotInfo [OneShotInfo]
bs
    combine as :: [OneShotInfo]
as     []     = (OneShotInfo -> Bool) -> [OneShotInfo] -> [OneShotInfo]
forall a. (a -> Bool) -> [a] -> [a]
takeWhile OneShotInfo -> Bool
isOneShotInfo [OneShotInfo]
as

{- Note [ABot branches: use max]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider   case x of
             True  -> \x.  error "urk"
             False -> \xy. error "urk2"

Remember: ABot n means "if you apply to n args, it'll definitely diverge".
So we need (ABot 2) for the whole thing, the /max/ of the ABot arities.

Note [Combining case branches]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
  go = \x. let z = go e0
               go2 = \x. case x of
                           True  -> z
                           False -> \s(one-shot). e1
           in go2 x
We *really* want to eta-expand go and go2.
When combining the barnches of the case we have
     ATop [] `andAT` ATop [OneShotLam]
and we want to get ATop [OneShotLam].  But if the inner
lambda wasn't one-shot we don't want to do this.
(We need a proper arity analysis to justify that.)

So we combine the best of the two branches, on the (slightly dodgy)
basis that if we know one branch is one-shot, then they all must be.

Note [Arity trimming]
~~~~~~~~~~~~~~~~~~~~~
Consider ((\x y. blah) |> co), where co :: (Int->Int->Int) ~ (Int -> F a) , and
F is some type family.

Because of Note [exprArity invariant], item (2), we must return with arity at
most 1, because typeArity (Int -> F a) = 1.  So we have to trim the result of
calling arityType on (\x y. blah).  Failing to do so, and hence breaking the
exprArity invariant, led to #5441.

How to trim?  For ATop, it's easy.  But we must take great care with ABot.
Suppose the expression was (\x y. error "urk"), we'll get (ABot 2).  We
absolutely must not trim that to (ABot 1), because that claims that
((\x y. error "urk") |> co) diverges when given one argument, which it
absolutely does not. And Bad Things happen if we think something returns bottom
when it doesn't (#16066).

So, do not reduce the 'n' in (ABot n); rather, switch (conservatively) to ATop.

Historical note: long ago, we unconditionally switched to ATop when we
encountered a cast, but that is far too conservative: see #5475
-}

---------------------------
type CheapFun = CoreExpr -> Maybe Type -> Bool
        -- How to decide if an expression is cheap
        -- If the Maybe is Just, the type is the type
        -- of the expression; Nothing means "don't know"

data ArityEnv
  = AE { ArityEnv -> CheapFun
ae_cheap_fn :: CheapFun
       , ArityEnv -> Bool
ae_ped_bot  :: Bool       -- True <=> be pedantic about bottoms
  }

arityType :: ArityEnv -> CoreExpr -> ArityType

arityType :: ArityEnv -> CoreExpr -> ArityType
arityType env :: ArityEnv
env (Cast e :: CoreExpr
e co :: Coercion
co)
  = case ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
e of
      ATop os :: [OneShotInfo]
os -> [OneShotInfo] -> ArityType
ATop (Arity -> [OneShotInfo] -> [OneShotInfo]
forall a. Arity -> [a] -> [a]
take Arity
co_arity [OneShotInfo]
os)
      -- See Note [Arity trimming]
      ABot n :: Arity
n | Arity
co_arity Arity -> Arity -> Bool
forall a. Ord a => a -> a -> Bool
< Arity
n -> [OneShotInfo] -> ArityType
ATop (Arity -> OneShotInfo -> [OneShotInfo]
forall a. Arity -> a -> [a]
replicate Arity
co_arity OneShotInfo
noOneShotInfo)
             | Bool
otherwise    -> Arity -> ArityType
ABot Arity
n
  where
    co_arity :: Arity
co_arity = [OneShotInfo] -> Arity
forall (t :: * -> *) a. Foldable t => t a -> Arity
length (Type -> [OneShotInfo]
typeArity (Pair Type -> Type
forall a. Pair a -> a
pSnd (Coercion -> Pair Type
coercionKind Coercion
co)))
    -- See Note [exprArity invariant] (2); must be true of
    -- arityType too, since that is how we compute the arity
    -- of variables, and they in turn affect result of exprArity
    -- Trac #5441 is a nice demo
    -- However, do make sure that ATop -> ATop and ABot -> ABot!
    --   Casts don't affect that part. Getting this wrong provoked #5475

arityType _ (Var v :: CoreBndr
v)
  | StrictSig
strict_sig <- CoreBndr -> StrictSig
idStrictness CoreBndr
v
  , Bool -> Bool
not (Bool -> Bool) -> Bool -> Bool
forall a b. (a -> b) -> a -> b
$ StrictSig -> Bool
isTopSig StrictSig
strict_sig
  , (ds :: [Demand]
ds, res :: DmdResult
res) <- StrictSig -> ([Demand], DmdResult)
splitStrictSig StrictSig
strict_sig
  , let arity :: Arity
arity = [Demand] -> Arity
forall (t :: * -> *) a. Foldable t => t a -> Arity
length [Demand]
ds
  = if DmdResult -> Bool
isBotRes DmdResult
res then Arity -> ArityType
ABot Arity
arity
                    else [OneShotInfo] -> ArityType
ATop (Arity -> [OneShotInfo] -> [OneShotInfo]
forall a. Arity -> [a] -> [a]
take Arity
arity [OneShotInfo]
one_shots)
  | Bool
otherwise
  = [OneShotInfo] -> ArityType
ATop (Arity -> [OneShotInfo] -> [OneShotInfo]
forall a. Arity -> [a] -> [a]
take (CoreBndr -> Arity
idArity CoreBndr
v) [OneShotInfo]
one_shots)
  where
    one_shots :: [OneShotInfo]  -- One-shot-ness derived from the type
    one_shots :: [OneShotInfo]
one_shots = Type -> [OneShotInfo]
typeArity (CoreBndr -> Type
idType CoreBndr
v)

        -- Lambdas; increase arity
arityType env :: ArityEnv
env (Lam x :: CoreBndr
x e :: CoreExpr
e)
  | CoreBndr -> Bool
isId CoreBndr
x    = CoreBndr -> ArityType -> ArityType
arityLam CoreBndr
x (ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
e)
  | Bool
otherwise = ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
e

        -- Applications; decrease arity, except for types
arityType env :: ArityEnv
env (App fun :: CoreExpr
fun (Type _))
   = ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
fun
arityType env :: ArityEnv
env (App fun :: CoreExpr
fun arg :: CoreExpr
arg )
   = ArityType -> Bool -> ArityType
arityApp (ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
fun) (ArityEnv -> CheapFun
ae_cheap_fn ArityEnv
env CoreExpr
arg Maybe Type
forall a. Maybe a
Nothing)

        -- Case/Let; keep arity if either the expression is cheap
        -- or it's a 1-shot lambda
        -- The former is not really right for Haskell
        --      f x = case x of { (a,b) -> \y. e }
        --  ===>
        --      f x y = case x of { (a,b) -> e }
        -- The difference is observable using 'seq'
        --
arityType env :: ArityEnv
env (Case scrut :: CoreExpr
scrut _ _ alts :: [Alt CoreBndr]
alts)
  | CoreExpr -> Bool
exprIsBottom CoreExpr
scrut Bool -> Bool -> Bool
|| [Alt CoreBndr] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Alt CoreBndr]
alts
  = Arity -> ArityType
ABot 0     -- Do not eta expand
               -- See Note [Dealing with bottom (1)]
  | Bool
otherwise
  = case ArityType
alts_type of
     ABot n :: Arity
n  | Arity
nArity -> Arity -> Bool
forall a. Ord a => a -> a -> Bool
>0       -> [OneShotInfo] -> ArityType
ATop []    -- Don't eta expand
             | Bool
otherwise -> Arity -> ArityType
ABot 0     -- if RHS is bottomming
                                       -- See Note [Dealing with bottom (2)]

     ATop as :: [OneShotInfo]
as | Bool -> Bool
not (ArityEnv -> Bool
ae_ped_bot ArityEnv
env)    -- See Note [Dealing with bottom (3)]
             , ArityEnv -> CheapFun
ae_cheap_fn ArityEnv
env CoreExpr
scrut Maybe Type
forall a. Maybe a
Nothing -> [OneShotInfo] -> ArityType
ATop [OneShotInfo]
as
             | CoreExpr -> Bool
exprOkForSpeculation CoreExpr
scrut    -> [OneShotInfo] -> ArityType
ATop [OneShotInfo]
as
             | Bool
otherwise                     -> [OneShotInfo] -> ArityType
ATop ((OneShotInfo -> Bool) -> [OneShotInfo] -> [OneShotInfo]
forall a. (a -> Bool) -> [a] -> [a]
takeWhile OneShotInfo -> Bool
isOneShotInfo [OneShotInfo]
as)
  where
    alts_type :: ArityType
alts_type = (ArityType -> ArityType -> ArityType) -> [ArityType] -> ArityType
forall (t :: * -> *) a. Foldable t => (a -> a -> a) -> t a -> a
foldr1 ArityType -> ArityType -> ArityType
andArityType [ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
rhs | (_,_,rhs :: CoreExpr
rhs) <- [Alt CoreBndr]
alts]

arityType env :: ArityEnv
env (Let b :: Bind CoreBndr
b e :: CoreExpr
e)
  = Bool -> ArityType -> ArityType
floatIn (Bind CoreBndr -> Bool
cheap_bind Bind CoreBndr
b) (ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
e)
  where
    cheap_bind :: Bind CoreBndr -> Bool
cheap_bind (NonRec b :: CoreBndr
b e :: CoreExpr
e) = (CoreBndr, CoreExpr) -> Bool
is_cheap (CoreBndr
b,CoreExpr
e)
    cheap_bind (Rec prs :: [(CoreBndr, CoreExpr)]
prs)    = ((CoreBndr, CoreExpr) -> Bool) -> [(CoreBndr, CoreExpr)] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all (CoreBndr, CoreExpr) -> Bool
is_cheap [(CoreBndr, CoreExpr)]
prs
    is_cheap :: (CoreBndr, CoreExpr) -> Bool
is_cheap (b :: CoreBndr
b,e :: CoreExpr
e) = ArityEnv -> CheapFun
ae_cheap_fn ArityEnv
env CoreExpr
e (Type -> Maybe Type
forall a. a -> Maybe a
Just (CoreBndr -> Type
idType CoreBndr
b))

arityType env :: ArityEnv
env (Tick t :: Tickish CoreBndr
t e :: CoreExpr
e)
  | Bool -> Bool
not (Tickish CoreBndr -> Bool
forall id. Tickish id -> Bool
tickishIsCode Tickish CoreBndr
t)     = ArityEnv -> CoreExpr -> ArityType
arityType ArityEnv
env CoreExpr
e

arityType _ _ = ArityType
vanillaArityType

{-
%************************************************************************
%*                                                                      *
              The main eta-expander
%*                                                                      *
%************************************************************************

We go for:
   f = \x1..xn -> N  ==>   f = \x1..xn y1..ym -> N y1..ym
                                 (n >= 0)

where (in both cases)

        * The xi can include type variables

        * The yi are all value variables

        * N is a NORMAL FORM (i.e. no redexes anywhere)
          wanting a suitable number of extra args.

The biggest reason for doing this is for cases like

        f = \x -> case x of
                    True  -> \y -> e1
                    False -> \y -> e2

Here we want to get the lambdas together.  A good example is the nofib
program fibheaps, which gets 25% more allocation if you don't do this
eta-expansion.

We may have to sandwich some coerces between the lambdas
to make the types work.   exprEtaExpandArity looks through coerces
when computing arity; and etaExpand adds the coerces as necessary when
actually computing the expansion.

Note [No crap in eta-expanded code]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The eta expander is careful not to introduce "crap".  In particular,
given a CoreExpr satisfying the 'CpeRhs' invariant (in CorePrep), it
returns a CoreExpr satisfying the same invariant. See Note [Eta
expansion and the CorePrep invariants] in CorePrep.

This means the eta-expander has to do a bit of on-the-fly
simplification but it's not too hard.  The alernative, of relying on
a subsequent clean-up phase of the Simplifier to de-crapify the result,
means you can't really use it in CorePrep, which is painful.

Note [Eta expansion for join points]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The no-crap rule is very tiresome to guarantee when
we have join points. Consider eta-expanding
   let j :: Int -> Int -> Bool
       j x = e
   in b

The simple way is
  \(y::Int). (let j x = e in b) y

The no-crap way is
  \(y::Int). let j' :: Int -> Bool
                 j' x = e y
             in b[j'/j] y
where I have written to stress that j's type has
changed.  Note that (of course!) we have to push the application
inside the RHS of the join as well as into the body.  AND if j
has an unfolding we have to push it into there too.  AND j might
be recursive...

So for now I'm abandonig the no-crap rule in this case. I think
that for the use in CorePrep it really doesn't matter; and if
it does, then CoreToStg.myCollectArgs will fall over.

(Moreover, I think that casts can make the no-crap rule fail too.)

Note [Eta expansion and SCCs]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note that SCCs are not treated specially by etaExpand.  If we have
        etaExpand 2 (\x -> scc "foo" e)
        = (\xy -> (scc "foo" e) y)
So the costs of evaluating 'e' (not 'e y') are attributed to "foo"

Note [Eta expansion and source notes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
CorePrep puts floatable ticks outside of value applications, but not
type applications. As a result we might be trying to eta-expand an
expression like

  (src<...> v) @a

which we want to lead to code like

  \x -> src<...> v @a x

This means that we need to look through type applications and be ready
to re-add floats on the top.

-}

-- | @etaExpand n e@ returns an expression with
-- the same meaning as @e@, but with arity @n@.
--
-- Given:
--
-- > e' = etaExpand n e
--
-- We should have that:
--
-- > ty = exprType e = exprType e'
etaExpand :: Arity              -- ^ Result should have this number of value args
          -> CoreExpr           -- ^ Expression to expand
          -> CoreExpr
-- etaExpand arity e = res
-- Then 'res' has at least 'arity' lambdas at the top
--
-- etaExpand deals with for-alls. For example:
--              etaExpand 1 E
-- where  E :: forall a. a -> a
-- would return
--      (/\b. \y::a -> E b y)
--
-- It deals with coerces too, though they are now rare
-- so perhaps the extra code isn't worth it

etaExpand :: Arity -> CoreExpr -> CoreExpr
etaExpand n :: Arity
n orig_expr :: CoreExpr
orig_expr
  = Arity -> CoreExpr -> CoreExpr
go Arity
n CoreExpr
orig_expr
  where
      -- Strip off existing lambdas and casts
      -- Note [Eta expansion and SCCs]
    go :: Arity -> CoreExpr -> CoreExpr
go 0 expr :: CoreExpr
expr = CoreExpr
expr
    go n :: Arity
n (Lam v :: CoreBndr
v body :: CoreExpr
body) | CoreBndr -> Bool
isTyVar CoreBndr
v = CoreBndr -> CoreExpr -> CoreExpr
forall b. b -> Expr b -> Expr b
Lam CoreBndr
v (Arity -> CoreExpr -> CoreExpr
go Arity
n     CoreExpr
body)
                      | Bool
otherwise = CoreBndr -> CoreExpr -> CoreExpr
forall b. b -> Expr b -> Expr b
Lam CoreBndr
v (Arity -> CoreExpr -> CoreExpr
go (Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
-1) CoreExpr
body)
    go n :: Arity
n (Cast expr :: CoreExpr
expr co :: Coercion
co)           = CoreExpr -> Coercion -> CoreExpr
forall b. Expr b -> Coercion -> Expr b
Cast (Arity -> CoreExpr -> CoreExpr
go Arity
n CoreExpr
expr) Coercion
co
    go n :: Arity
n expr :: CoreExpr
expr
      = -- pprTrace "ee" (vcat [ppr orig_expr, ppr expr, ppr etas]) $
        CoreExpr -> CoreExpr
retick (CoreExpr -> CoreExpr) -> CoreExpr -> CoreExpr
forall a b. (a -> b) -> a -> b
$ [EtaInfo] -> CoreExpr -> CoreExpr
etaInfoAbs [EtaInfo]
etas (Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
etaInfoApp Subst
subst' CoreExpr
sexpr [EtaInfo]
etas)
      where
          in_scope :: InScopeSet
in_scope = VarSet -> InScopeSet
mkInScopeSet (CoreExpr -> VarSet
exprFreeVars CoreExpr
expr)
          (in_scope' :: InScopeSet
in_scope', etas :: [EtaInfo]
etas) = Arity -> CoreExpr -> InScopeSet -> Type -> (InScopeSet, [EtaInfo])
mkEtaWW Arity
n CoreExpr
orig_expr InScopeSet
in_scope (CoreExpr -> Type
exprType CoreExpr
expr)
          subst' :: Subst
subst' = InScopeSet -> Subst
mkEmptySubst InScopeSet
in_scope'

          -- Find ticks behind type apps.
          -- See Note [Eta expansion and source notes]
          (expr' :: CoreExpr
expr', args :: [CoreExpr]
args) = CoreExpr -> (CoreExpr, [CoreExpr])
forall b. Expr b -> (Expr b, [Expr b])
collectArgs CoreExpr
expr
          (ticks :: [Tickish CoreBndr]
ticks, expr'' :: CoreExpr
expr'') = (Tickish CoreBndr -> Bool)
-> CoreExpr -> ([Tickish CoreBndr], CoreExpr)
forall b.
(Tickish CoreBndr -> Bool)
-> Expr b -> ([Tickish CoreBndr], Expr b)
stripTicksTop Tickish CoreBndr -> Bool
forall id. Tickish id -> Bool
tickishFloatable CoreExpr
expr'
          sexpr :: CoreExpr
sexpr = (CoreExpr -> CoreExpr -> CoreExpr)
-> CoreExpr -> [CoreExpr] -> CoreExpr
forall (t :: * -> *) b a.
Foldable t =>
(b -> a -> b) -> b -> t a -> b
foldl' CoreExpr -> CoreExpr -> CoreExpr
forall b. Expr b -> Expr b -> Expr b
App CoreExpr
expr'' [CoreExpr]
args
          retick :: CoreExpr -> CoreExpr
retick expr :: CoreExpr
expr = (Tickish CoreBndr -> CoreExpr -> CoreExpr)
-> CoreExpr -> [Tickish CoreBndr] -> CoreExpr
forall (t :: * -> *) a b.
Foldable t =>
(a -> b -> b) -> b -> t a -> b
foldr Tickish CoreBndr -> CoreExpr -> CoreExpr
mkTick CoreExpr
expr [Tickish CoreBndr]
ticks

                                -- Abstraction    Application
--------------
data EtaInfo = EtaVar Var       -- /\a. []        [] a
                                -- \x.  []        [] x
             | EtaCo Coercion   -- [] |> sym co   [] |> co

instance Outputable EtaInfo where
   ppr :: EtaInfo -> SDoc
ppr (EtaVar v :: CoreBndr
v) = String -> SDoc
text "EtaVar" SDoc -> SDoc -> SDoc
<+> CoreBndr -> SDoc
forall a. Outputable a => a -> SDoc
ppr CoreBndr
v
   ppr (EtaCo co :: Coercion
co) = String -> SDoc
text "EtaCo"  SDoc -> SDoc -> SDoc
<+> Coercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr Coercion
co

pushCoercion :: Coercion -> [EtaInfo] -> [EtaInfo]
pushCoercion :: Coercion -> [EtaInfo] -> [EtaInfo]
pushCoercion co1 :: Coercion
co1 (EtaCo co2 :: Coercion
co2 : eis :: [EtaInfo]
eis)
  | Coercion -> Bool
isReflCo Coercion
co = [EtaInfo]
eis
  | Bool
otherwise   = Coercion -> EtaInfo
EtaCo Coercion
co EtaInfo -> [EtaInfo] -> [EtaInfo]
forall a. a -> [a] -> [a]
: [EtaInfo]
eis
  where
    co :: Coercion
co = Coercion
co1 Coercion -> Coercion -> Coercion
`mkTransCo` Coercion
co2

pushCoercion co :: Coercion
co eis :: [EtaInfo]
eis = Coercion -> EtaInfo
EtaCo Coercion
co EtaInfo -> [EtaInfo] -> [EtaInfo]
forall a. a -> [a] -> [a]
: [EtaInfo]
eis

--------------
etaInfoAbs :: [EtaInfo] -> CoreExpr -> CoreExpr
etaInfoAbs :: [EtaInfo] -> CoreExpr -> CoreExpr
etaInfoAbs []               expr :: CoreExpr
expr = CoreExpr
expr
etaInfoAbs (EtaVar v :: CoreBndr
v : eis :: [EtaInfo]
eis) expr :: CoreExpr
expr = CoreBndr -> CoreExpr -> CoreExpr
forall b. b -> Expr b -> Expr b
Lam CoreBndr
v ([EtaInfo] -> CoreExpr -> CoreExpr
etaInfoAbs [EtaInfo]
eis CoreExpr
expr)
etaInfoAbs (EtaCo co :: Coercion
co : eis :: [EtaInfo]
eis) expr :: CoreExpr
expr = CoreExpr -> Coercion -> CoreExpr
forall b. Expr b -> Coercion -> Expr b
Cast ([EtaInfo] -> CoreExpr -> CoreExpr
etaInfoAbs [EtaInfo]
eis CoreExpr
expr) (Coercion -> Coercion
mkSymCo Coercion
co)

--------------
etaInfoApp :: Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
-- (etaInfoApp s e eis) returns something equivalent to
--             ((substExpr s e) `appliedto` eis)

etaInfoApp :: Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
etaInfoApp subst :: Subst
subst (Lam v1 :: CoreBndr
v1 e :: CoreExpr
e) (EtaVar v2 :: CoreBndr
v2 : eis :: [EtaInfo]
eis)
  = Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
etaInfoApp (Subst -> CoreBndr -> CoreBndr -> Subst
CoreSubst.extendSubstWithVar Subst
subst CoreBndr
v1 CoreBndr
v2) CoreExpr
e [EtaInfo]
eis

etaInfoApp subst :: Subst
subst (Cast e :: CoreExpr
e co1 :: Coercion
co1) eis :: [EtaInfo]
eis
  = Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
etaInfoApp Subst
subst CoreExpr
e (Coercion -> [EtaInfo] -> [EtaInfo]
pushCoercion Coercion
co' [EtaInfo]
eis)
  where
    co' :: Coercion
co' = Subst -> Coercion -> Coercion
CoreSubst.substCo Subst
subst Coercion
co1

etaInfoApp subst :: Subst
subst (Case e :: CoreExpr
e b :: CoreBndr
b ty :: Type
ty alts :: [Alt CoreBndr]
alts) eis :: [EtaInfo]
eis
  = CoreExpr -> CoreBndr -> Type -> [Alt CoreBndr] -> CoreExpr
forall b. Expr b -> b -> Type -> [Alt b] -> Expr b
Case (Subst -> CoreExpr -> CoreExpr
subst_expr Subst
subst CoreExpr
e) CoreBndr
b1 Type
ty' [Alt CoreBndr]
alts'
  where
    (subst1 :: Subst
subst1, b1 :: CoreBndr
b1) = Subst -> CoreBndr -> (Subst, CoreBndr)
substBndr Subst
subst CoreBndr
b
    alts' :: [Alt CoreBndr]
alts' = (Alt CoreBndr -> Alt CoreBndr) -> [Alt CoreBndr] -> [Alt CoreBndr]
forall a b. (a -> b) -> [a] -> [b]
map Alt CoreBndr -> Alt CoreBndr
forall a. (a, [CoreBndr], CoreExpr) -> (a, [CoreBndr], CoreExpr)
subst_alt [Alt CoreBndr]
alts
    ty' :: Type
ty'   = Type -> [EtaInfo] -> Type
etaInfoAppTy (Subst -> Type -> Type
CoreSubst.substTy Subst
subst Type
ty) [EtaInfo]
eis
    subst_alt :: (a, [CoreBndr], CoreExpr) -> (a, [CoreBndr], CoreExpr)
subst_alt (con :: a
con, bs :: [CoreBndr]
bs, rhs :: CoreExpr
rhs) = (a
con, [CoreBndr]
bs', Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
etaInfoApp Subst
subst2 CoreExpr
rhs [EtaInfo]
eis)
              where
                 (subst2 :: Subst
subst2,bs' :: [CoreBndr]
bs') = Subst -> [CoreBndr] -> (Subst, [CoreBndr])
substBndrs Subst
subst1 [CoreBndr]
bs

etaInfoApp subst :: Subst
subst (Let b :: Bind CoreBndr
b e :: CoreExpr
e) eis :: [EtaInfo]
eis
  | Bool -> Bool
not (Bind CoreBndr -> Bool
isJoinBind Bind CoreBndr
b)
    -- See Note [Eta expansion for join points]
  = Bind CoreBndr -> CoreExpr -> CoreExpr
forall b. Bind b -> Expr b -> Expr b
Let Bind CoreBndr
b' (Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
etaInfoApp Subst
subst' CoreExpr
e [EtaInfo]
eis)
  where
    (subst' :: Subst
subst', b' :: Bind CoreBndr
b') = Subst -> Bind CoreBndr -> (Subst, Bind CoreBndr)
substBindSC Subst
subst Bind CoreBndr
b

etaInfoApp subst :: Subst
subst (Tick t :: Tickish CoreBndr
t e :: CoreExpr
e) eis :: [EtaInfo]
eis
  = Tickish CoreBndr -> CoreExpr -> CoreExpr
forall b. Tickish CoreBndr -> Expr b -> Expr b
Tick (Subst -> Tickish CoreBndr -> Tickish CoreBndr
substTickish Subst
subst Tickish CoreBndr
t) (Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
etaInfoApp Subst
subst CoreExpr
e [EtaInfo]
eis)

etaInfoApp subst :: Subst
subst expr :: CoreExpr
expr _
  | (Var fun :: CoreBndr
fun, _) <- CoreExpr -> (CoreExpr, [CoreExpr])
forall b. Expr b -> (Expr b, [Expr b])
collectArgs CoreExpr
expr
  , Var fun' :: CoreBndr
fun' <- SDoc -> Subst -> CoreBndr -> CoreExpr
lookupIdSubst (String -> SDoc
text "etaInfoApp" SDoc -> SDoc -> SDoc
<+> CoreBndr -> SDoc
forall a. Outputable a => a -> SDoc
ppr CoreBndr
fun) Subst
subst CoreBndr
fun
  , CoreBndr -> Bool
isJoinId CoreBndr
fun'
  = Subst -> CoreExpr -> CoreExpr
subst_expr Subst
subst CoreExpr
expr

etaInfoApp subst :: Subst
subst e :: CoreExpr
e eis :: [EtaInfo]
eis
  = CoreExpr -> [EtaInfo] -> CoreExpr
forall b. Expr b -> [EtaInfo] -> Expr b
go (Subst -> CoreExpr -> CoreExpr
subst_expr Subst
subst CoreExpr
e) [EtaInfo]
eis
  where
    go :: Expr b -> [EtaInfo] -> Expr b
go e :: Expr b
e []                  = Expr b
e
    go e :: Expr b
e (EtaVar v :: CoreBndr
v    : eis :: [EtaInfo]
eis) = Expr b -> [EtaInfo] -> Expr b
go (Expr b -> Expr b -> Expr b
forall b. Expr b -> Expr b -> Expr b
App Expr b
e (CoreBndr -> Expr b
forall b. CoreBndr -> Expr b
varToCoreExpr CoreBndr
v)) [EtaInfo]
eis
    go e :: Expr b
e (EtaCo co :: Coercion
co    : eis :: [EtaInfo]
eis) = Expr b -> [EtaInfo] -> Expr b
go (Expr b -> Coercion -> Expr b
forall b. Expr b -> Coercion -> Expr b
Cast Expr b
e Coercion
co) [EtaInfo]
eis


--------------
etaInfoAppTy :: Type -> [EtaInfo] -> Type
-- If                    e :: ty
-- then   etaInfoApp e eis :: etaInfoApp ty eis
etaInfoAppTy :: Type -> [EtaInfo] -> Type
etaInfoAppTy ty :: Type
ty []               = Type
ty
etaInfoAppTy ty :: Type
ty (EtaVar v :: CoreBndr
v : eis :: [EtaInfo]
eis) = Type -> [EtaInfo] -> Type
etaInfoAppTy (Type -> CoreExpr -> Type
applyTypeToArg Type
ty (CoreBndr -> CoreExpr
forall b. CoreBndr -> Expr b
varToCoreExpr CoreBndr
v)) [EtaInfo]
eis
etaInfoAppTy _  (EtaCo co :: Coercion
co : eis :: [EtaInfo]
eis) = Type -> [EtaInfo] -> Type
etaInfoAppTy (Pair Type -> Type
forall a. Pair a -> a
pSnd (Coercion -> Pair Type
coercionKind Coercion
co)) [EtaInfo]
eis

--------------
mkEtaWW :: Arity -> CoreExpr -> InScopeSet -> Type
        -> (InScopeSet, [EtaInfo])
        -- EtaInfo contains fresh variables,
        --   not free in the incoming CoreExpr
        -- Outgoing InScopeSet includes the EtaInfo vars
        --   and the original free vars

mkEtaWW :: Arity -> CoreExpr -> InScopeSet -> Type -> (InScopeSet, [EtaInfo])
mkEtaWW orig_n :: Arity
orig_n orig_expr :: CoreExpr
orig_expr in_scope :: InScopeSet
in_scope orig_ty :: Type
orig_ty
  = Arity -> TCvSubst -> Type -> [EtaInfo] -> (InScopeSet, [EtaInfo])
go Arity
orig_n TCvSubst
empty_subst Type
orig_ty []
  where
    empty_subst :: TCvSubst
empty_subst = InScopeSet -> TCvSubst
mkEmptyTCvSubst InScopeSet
in_scope

    go :: Arity              -- Number of value args to expand to
       -> TCvSubst -> Type   -- We are really looking at subst(ty)
       -> [EtaInfo]          -- Accumulating parameter
       -> (InScopeSet, [EtaInfo])
    go :: Arity -> TCvSubst -> Type -> [EtaInfo] -> (InScopeSet, [EtaInfo])
go n :: Arity
n subst :: TCvSubst
subst ty :: Type
ty eis :: [EtaInfo]
eis       -- See Note [exprArity invariant]

       ----------- Done!  No more expansion needed
       | Arity
n Arity -> Arity -> Bool
forall a. Eq a => a -> a -> Bool
== 0
       = (TCvSubst -> InScopeSet
getTCvInScope TCvSubst
subst, [EtaInfo] -> [EtaInfo]
forall a. [a] -> [a]
reverse [EtaInfo]
eis)

       ----------- Forall types  (forall a. ty)
       | Just (tcv :: CoreBndr
tcv,ty' :: Type
ty') <- Type -> Maybe (CoreBndr, Type)
splitForAllTy_maybe Type
ty
       , let (subst' :: TCvSubst
subst', tcv' :: CoreBndr
tcv') = HasCallStack => TCvSubst -> CoreBndr -> (TCvSubst, CoreBndr)
TCvSubst -> CoreBndr -> (TCvSubst, CoreBndr)
Type.substVarBndr TCvSubst
subst CoreBndr
tcv
       = let ((n_subst :: TCvSubst
n_subst, n_tcv :: CoreBndr
n_tcv), n_n :: Arity
n_n)
               -- We want to have at least 'n' lambdas at the top.
               -- If tcv is a tyvar, it corresponds to one Lambda (/\).
               --   And we won't reduce n.
               -- If tcv is a covar, we could eta-expand the expr with one
               --   lambda \co:ty. e co. In this case we generate a new variable
               --   of the coercion type, update the scope, and reduce n by 1.
               | CoreBndr -> Bool
isTyVar CoreBndr
tcv = ((TCvSubst
subst', CoreBndr
tcv'), Arity
n)
               | Bool
otherwise   = (Arity -> TCvSubst -> Type -> (TCvSubst, CoreBndr)
freshEtaId Arity
n TCvSubst
subst' (CoreBndr -> Type
varType CoreBndr
tcv'), Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
-1)
           -- Avoid free vars of the original expression
         in Arity -> TCvSubst -> Type -> [EtaInfo] -> (InScopeSet, [EtaInfo])
go Arity
n_n TCvSubst
n_subst Type
ty' (CoreBndr -> EtaInfo
EtaVar CoreBndr
n_tcv EtaInfo -> [EtaInfo] -> [EtaInfo]
forall a. a -> [a] -> [a]
: [EtaInfo]
eis)

       ----------- Function types  (t1 -> t2)
       | Just (arg_ty :: Type
arg_ty, res_ty :: Type
res_ty) <- Type -> Maybe (Type, Type)
splitFunTy_maybe Type
ty
       , Bool -> Bool
not (Type -> Bool
isTypeLevPoly Type
arg_ty)
          -- See Note [Levity polymorphism invariants] in CoreSyn
          -- See also test case typecheck/should_run/EtaExpandLevPoly

       , let (subst' :: TCvSubst
subst', eta_id' :: CoreBndr
eta_id') = Arity -> TCvSubst -> Type -> (TCvSubst, CoreBndr)
freshEtaId Arity
n TCvSubst
subst Type
arg_ty
           -- Avoid free vars of the original expression
       = Arity -> TCvSubst -> Type -> [EtaInfo] -> (InScopeSet, [EtaInfo])
go (Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
-1) TCvSubst
subst' Type
res_ty (CoreBndr -> EtaInfo
EtaVar CoreBndr
eta_id' EtaInfo -> [EtaInfo] -> [EtaInfo]
forall a. a -> [a] -> [a]
: [EtaInfo]
eis)

       ----------- Newtypes
       -- Given this:
       --      newtype T = MkT ([T] -> Int)
       -- Consider eta-expanding this
       --      eta_expand 1 e T
       -- We want to get
       --      coerce T (\x::[T] -> (coerce ([T]->Int) e) x)
       | Just (co :: Coercion
co, ty' :: Type
ty') <- Type -> Maybe (Coercion, Type)
topNormaliseNewType_maybe Type
ty
       , let co' :: Coercion
co' = HasCallStack => TCvSubst -> Coercion -> Coercion
TCvSubst -> Coercion -> Coercion
Coercion.substCo TCvSubst
subst Coercion
co
             -- Remember to apply the substitution to co (#16979)
             -- (or we could have applied to ty, but then
             --  we'd have had to zap it for the recursive call)
       = Arity -> TCvSubst -> Type -> [EtaInfo] -> (InScopeSet, [EtaInfo])
go Arity
n TCvSubst
subst Type
ty' (Coercion -> [EtaInfo] -> [EtaInfo]
pushCoercion Coercion
co' [EtaInfo]
eis)

       | Bool
otherwise       -- We have an expression of arity > 0,
                         -- but its type isn't a function, or a binder
                         -- is levity-polymorphic
       = WARN( True, (ppr orig_n <+> ppr orig_ty) $$ ppr orig_expr )
         (TCvSubst -> InScopeSet
getTCvInScope TCvSubst
subst, [EtaInfo] -> [EtaInfo]
forall a. [a] -> [a]
reverse [EtaInfo]
eis)
        -- This *can* legitmately happen:
        -- e.g.  coerce Int (\x. x) Essentially the programmer is
        -- playing fast and loose with types (Happy does this a lot).
        -- So we simply decline to eta-expand.  Otherwise we'd end up
        -- with an explicit lambda having a non-function type



--------------
-- Don't use short-cutting substitution - we may be changing the types of join
-- points, so applying the in-scope set is necessary
-- TODO Check if we actually *are* changing any join points' types

subst_expr :: Subst -> CoreExpr -> CoreExpr
subst_expr :: Subst -> CoreExpr -> CoreExpr
subst_expr = SDoc -> Subst -> CoreExpr -> CoreExpr
substExpr (String -> SDoc
text "CoreArity:substExpr")


--------------

-- | Split an expression into the given number of binders and a body,
-- eta-expanding if necessary. Counts value *and* type binders.
etaExpandToJoinPoint :: JoinArity -> CoreExpr -> ([CoreBndr], CoreExpr)
etaExpandToJoinPoint :: Arity -> CoreExpr -> ([CoreBndr], CoreExpr)
etaExpandToJoinPoint join_arity :: Arity
join_arity expr :: CoreExpr
expr
  = Arity -> [CoreBndr] -> CoreExpr -> ([CoreBndr], CoreExpr)
go Arity
join_arity [] CoreExpr
expr
  where
    go :: Arity -> [CoreBndr] -> CoreExpr -> ([CoreBndr], CoreExpr)
go 0 rev_bs :: [CoreBndr]
rev_bs e :: CoreExpr
e         = ([CoreBndr] -> [CoreBndr]
forall a. [a] -> [a]
reverse [CoreBndr]
rev_bs, CoreExpr
e)
    go n :: Arity
n rev_bs :: [CoreBndr]
rev_bs (Lam b :: CoreBndr
b e :: CoreExpr
e) = Arity -> [CoreBndr] -> CoreExpr -> ([CoreBndr], CoreExpr)
go (Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
-1) (CoreBndr
b CoreBndr -> [CoreBndr] -> [CoreBndr]
forall a. a -> [a] -> [a]
: [CoreBndr]
rev_bs) CoreExpr
e
    go n :: Arity
n rev_bs :: [CoreBndr]
rev_bs e :: CoreExpr
e         = case Arity -> CoreExpr -> ([CoreBndr], CoreExpr)
etaBodyForJoinPoint Arity
n CoreExpr
e of
                              (bs :: [CoreBndr]
bs, e' :: CoreExpr
e') -> ([CoreBndr] -> [CoreBndr]
forall a. [a] -> [a]
reverse [CoreBndr]
rev_bs [CoreBndr] -> [CoreBndr] -> [CoreBndr]
forall a. [a] -> [a] -> [a]
++ [CoreBndr]
bs, CoreExpr
e')

etaExpandToJoinPointRule :: JoinArity -> CoreRule -> CoreRule
etaExpandToJoinPointRule :: Arity -> CoreRule -> CoreRule
etaExpandToJoinPointRule _ rule :: CoreRule
rule@(BuiltinRule {})
  = WARN(True, (sep [text "Can't eta-expand built-in rule:", ppr rule]))
      -- How did a local binding get a built-in rule anyway? Probably a plugin.
    CoreRule
rule
etaExpandToJoinPointRule join_arity :: Arity
join_arity rule :: CoreRule
rule@(Rule { ru_bndrs :: CoreRule -> [CoreBndr]
ru_bndrs = [CoreBndr]
bndrs, ru_rhs :: CoreRule -> CoreExpr
ru_rhs = CoreExpr
rhs
                                               , ru_args :: CoreRule -> [CoreExpr]
ru_args  = [CoreExpr]
args })
  | Arity
need_args Arity -> Arity -> Bool
forall a. Eq a => a -> a -> Bool
== 0
  = CoreRule
rule
  | Arity
need_args Arity -> Arity -> Bool
forall a. Ord a => a -> a -> Bool
< 0
  = String -> SDoc -> CoreRule
forall a. HasCallStack => String -> SDoc -> a
pprPanic "etaExpandToJoinPointRule" (Arity -> SDoc
forall a. Outputable a => a -> SDoc
ppr Arity
join_arity SDoc -> SDoc -> SDoc
$$ CoreRule -> SDoc
forall a. Outputable a => a -> SDoc
ppr CoreRule
rule)
  | Bool
otherwise
  = CoreRule
rule { ru_bndrs :: [CoreBndr]
ru_bndrs = [CoreBndr]
bndrs [CoreBndr] -> [CoreBndr] -> [CoreBndr]
forall a. [a] -> [a] -> [a]
++ [CoreBndr]
new_bndrs, ru_args :: [CoreExpr]
ru_args = [CoreExpr]
args [CoreExpr] -> [CoreExpr] -> [CoreExpr]
forall a. [a] -> [a] -> [a]
++ [CoreExpr]
forall b. [Expr b]
new_args
         , ru_rhs :: CoreExpr
ru_rhs = CoreExpr
new_rhs }
  where
    need_args :: Arity
need_args = Arity
join_arity Arity -> Arity -> Arity
forall a. Num a => a -> a -> a
- [CoreExpr] -> Arity
forall (t :: * -> *) a. Foldable t => t a -> Arity
length [CoreExpr]
args
    (new_bndrs :: [CoreBndr]
new_bndrs, new_rhs :: CoreExpr
new_rhs) = Arity -> CoreExpr -> ([CoreBndr], CoreExpr)
etaBodyForJoinPoint Arity
need_args CoreExpr
rhs
    new_args :: [Expr b]
new_args = [CoreBndr] -> [Expr b]
forall b. [CoreBndr] -> [Expr b]
varsToCoreExprs [CoreBndr]
new_bndrs

-- Adds as many binders as asked for; assumes expr is not a lambda
etaBodyForJoinPoint :: Int -> CoreExpr -> ([CoreBndr], CoreExpr)
etaBodyForJoinPoint :: Arity -> CoreExpr -> ([CoreBndr], CoreExpr)
etaBodyForJoinPoint need_args :: Arity
need_args body :: CoreExpr
body
  = Arity
-> Type
-> TCvSubst
-> [CoreBndr]
-> CoreExpr
-> ([CoreBndr], CoreExpr)
forall b.
Arity
-> Type -> TCvSubst -> [CoreBndr] -> Expr b -> ([CoreBndr], Expr b)
go Arity
need_args (CoreExpr -> Type
exprType CoreExpr
body) (CoreExpr -> TCvSubst
init_subst CoreExpr
body) [] CoreExpr
body
  where
    go :: Arity
-> Type -> TCvSubst -> [CoreBndr] -> Expr b -> ([CoreBndr], Expr b)
go 0 _  _     rev_bs :: [CoreBndr]
rev_bs e :: Expr b
e
      = ([CoreBndr] -> [CoreBndr]
forall a. [a] -> [a]
reverse [CoreBndr]
rev_bs, Expr b
e)
    go n :: Arity
n ty :: Type
ty subst :: TCvSubst
subst rev_bs :: [CoreBndr]
rev_bs e :: Expr b
e
      | Just (tv :: CoreBndr
tv, res_ty :: Type
res_ty) <- Type -> Maybe (CoreBndr, Type)
splitForAllTy_maybe Type
ty
      , let (subst' :: TCvSubst
subst', tv' :: CoreBndr
tv') = HasCallStack => TCvSubst -> CoreBndr -> (TCvSubst, CoreBndr)
TCvSubst -> CoreBndr -> (TCvSubst, CoreBndr)
Type.substVarBndr TCvSubst
subst CoreBndr
tv
      = Arity
-> Type -> TCvSubst -> [CoreBndr] -> Expr b -> ([CoreBndr], Expr b)
go (Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
-1) Type
res_ty TCvSubst
subst' (CoreBndr
tv' CoreBndr -> [CoreBndr] -> [CoreBndr]
forall a. a -> [a] -> [a]
: [CoreBndr]
rev_bs) (Expr b
e Expr b -> Expr b -> Expr b
forall b. Expr b -> Expr b -> Expr b
`App` CoreBndr -> Expr b
forall b. CoreBndr -> Expr b
varToCoreExpr CoreBndr
tv')
      | Just (arg_ty :: Type
arg_ty, res_ty :: Type
res_ty) <- Type -> Maybe (Type, Type)
splitFunTy_maybe Type
ty
      , let (subst' :: TCvSubst
subst', b :: CoreBndr
b) = Arity -> TCvSubst -> Type -> (TCvSubst, CoreBndr)
freshEtaId Arity
n TCvSubst
subst Type
arg_ty
      = Arity
-> Type -> TCvSubst -> [CoreBndr] -> Expr b -> ([CoreBndr], Expr b)
go (Arity
nArity -> Arity -> Arity
forall a. Num a => a -> a -> a
-1) Type
res_ty TCvSubst
subst' (CoreBndr
b CoreBndr -> [CoreBndr] -> [CoreBndr]
forall a. a -> [a] -> [a]
: [CoreBndr]
rev_bs) (Expr b
e Expr b -> Expr b -> Expr b
forall b. Expr b -> Expr b -> Expr b
`App` CoreBndr -> Expr b
forall b. CoreBndr -> Expr b
Var CoreBndr
b)
      | Bool
otherwise
      = String -> SDoc -> ([CoreBndr], Expr b)
forall a. HasCallStack => String -> SDoc -> a
pprPanic "etaBodyForJoinPoint" (SDoc -> ([CoreBndr], Expr b)) -> SDoc -> ([CoreBndr], Expr b)
forall a b. (a -> b) -> a -> b
$ Arity -> SDoc
int Arity
need_args SDoc -> SDoc -> SDoc
$$
                                         CoreExpr -> SDoc
forall a. Outputable a => a -> SDoc
ppr CoreExpr
body SDoc -> SDoc -> SDoc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CoreExpr -> Type
exprType CoreExpr
body)

    init_subst :: CoreExpr -> TCvSubst
init_subst e :: CoreExpr
e = InScopeSet -> TCvSubst
mkEmptyTCvSubst (VarSet -> InScopeSet
mkInScopeSet (CoreExpr -> VarSet
exprFreeVars CoreExpr
e))

--------------
freshEtaId :: Int -> TCvSubst -> Type -> (TCvSubst, Id)
-- Make a fresh Id, with specified type (after applying substitution)
-- It should be "fresh" in the sense that it's not in the in-scope set
-- of the TvSubstEnv; and it should itself then be added to the in-scope
-- set of the TvSubstEnv
--
-- The Int is just a reasonable starting point for generating a unique;
-- it does not necessarily have to be unique itself.
freshEtaId :: Arity -> TCvSubst -> Type -> (TCvSubst, CoreBndr)
freshEtaId n :: Arity
n subst :: TCvSubst
subst ty :: Type
ty
      = (TCvSubst
subst', CoreBndr
eta_id')
      where
        ty' :: Type
ty'     = HasCallStack => TCvSubst -> Type -> Type
TCvSubst -> Type -> Type
Type.substTy TCvSubst
subst Type
ty
        eta_id' :: CoreBndr
eta_id' = InScopeSet -> CoreBndr -> CoreBndr
uniqAway (TCvSubst -> InScopeSet
getTCvInScope TCvSubst
subst) (CoreBndr -> CoreBndr) -> CoreBndr -> CoreBndr
forall a b. (a -> b) -> a -> b
$
                  FastString -> Unique -> Type -> CoreBndr
mkSysLocalOrCoVar (String -> FastString
fsLit "eta") (Arity -> Unique
mkBuiltinUnique Arity
n) Type
ty'
        subst' :: TCvSubst
subst'  = TCvSubst -> CoreBndr -> TCvSubst
extendTCvInScope TCvSubst
subst CoreBndr
eta_id'