Language.Haskell.TH.Syntax.Compat

This module exists to make it possible to define code that works across a wide range of template-haskell versions with as little CPP as possible. To that end, this module currently backports the following template-haskell constructs:

• The Quote class
• The Code type

Refer to the Haddocks below for examples of how to use each of these in a backwards-compatible way.

The Quote class

The Quote class (first proposed in GHC Proposal 246) was introduced in template-haskell-2.17.0.0. This module defines a version of Quote that is backward-compatible with older template-haskell releases and is forward-compatible with the existing Quote class.

In addition to Quote, this module also backports versions of functions in Language.Haskell.TH.Syntax that work over any Quote instance instead of just Q. Since this module is designed to coexist with the existing definitions in template-haskell as much as possible, the backported functions are suffixed with -Quote to avoid name clashes. For instance, the backported version of lift is named liftQuote.

The one exception to the no-name-clashes policy is the backported newName method of Quote. We could have conceivably named it newNameQuote, but then it would not have been possible to define backwards-compatible Quote instances without the use of CPP. As a result, some care must be exercised when combining this module with Language.Haskell.TH or Language.Haskell.TH.Syntax on older versions of template-haskell, as they both export a version of newName with a different type. Here is an example of how to safely combine these modules:

{-# LANGUAGE GeneralizedNewtypeDeriving, TemplateHaskell #-}

import Control.Monad.State (MonadState(..), State, evalState)
import Language.Haskell.TH hiding (newName)
import Language.Haskell.TH.Syntax hiding (newName)
import Language.Haskell.TH.Syntax.Compat

newtype PureQ a = MkPureQ (State Uniq a)
  deriving (Functor, Applicative, Monad, MonadState Uniq)

runPureQ :: PureQ a -> a
runPureQ m = case m of MkPureQ m' -> evalState m' 0

instance Quote PureQ where
  newName s = state $ i -> (mkNameU s i, i + 1)

main :: IO ()
main = putStrLn $ runPureQ $ do
  a <- newName "a"
  return $ nameBase a

We do not make an effort to backport any combinators from the Language.Haskell.TH.Lib module, as the surface area is simply too large. If you wish to generalize code that uses these combinators to work over Quote in a backwards-compatible way, use the unsafeQToQuote function.

class Monad m => Quote m where

The Quote class implements the minimal interface which is necessary for desugaring quotations.

• The Monad m superclass is needed to stitch together the different AST fragments.
• newName is used when desugaring binding structures such as lambdas to generate fresh names.

Therefore the type of an untyped quotation in GHC is `Quote m => m Exp`

For many years the type of a quotation was fixed to be `Q Exp` but by more precisely specifying the minimal interface it enables the Exp to be extracted purely from the quotation without interacting with Q.

Methods

newName :: String -> m Name

Generate a fresh name, which cannot be captured.

For example, this:

f = $(do
    nm1 <- newName "x"
    let nm2 = mkName "x"
    return (LamE [VarP nm1] (LamE [VarP nm2] (VarE nm1)))
   )

will produce the splice

f = \x0 -> \x -> x0

In particular, the occurrence VarE nm1 refers to the binding VarP nm1, and is not captured by the binding VarP nm2.

Although names generated by newName cannot be captured, they can capture other names. For example, this:

g = $(do
  nm1 <- newName "x"
  let nm2 = mkName "x"
  return (LamE [VarP nm2] (LamE [VarP nm1] (VarE nm2)))
 )

will produce the splice

g = \x -> \x0 -> x0

since the occurrence VarE nm2 is captured by the innermost binding of x, namely VarP nm1.

Instances

Quote Q

Defined in Language.Haskell.TH.Syntax.Compat

Methods

newName :: String -> Q Name

Quote functionality

The unsafeQToQuote function

unsafeQToQuote :: Quote m => Q a -> m a

Use a Q computation in a Quote context. This function is only safe when the Q computation performs actions from the Quote instance for Q or any of Quote's subclasses (Functor, Applicative, and Monad). Attempting to perform actions from the MonadFail, MonadIO, or Quasi instances for Q will result in runtime errors.

This is useful when you have some Q-valued functions that only performs actions from Quote and wish to generalise it from Q to Quote without having to rewrite the internals of the function. This is especially handy for code defined in terms of combinators from Language.Haskell.TH.Lib, which were all hard-coded to Q prior to template-haskell-2.17.0.0. For instance, consider this function:

apply :: Exp -> Exp -> Q Exp
apply f x = appE (return x) (return y)

There are two ways to generalize this function to use Quote in a backwards-compatible way. One way to do so is to rewrite apply to avoid the use of appE, like so:

applyQuote :: Quote m => Exp -> Exp -> m Exp
applyQuote f x = return (AppE x y)

For a small example like applyQuote, there isn't much work involved. But this can become tiresome for larger examples. In such cases, unsafeQToQuote can do the heavy lifting for you. For example, applyQuote can also be defined as:

applyQuote :: Quote m => Exp -> Exp -> m Exp
applyQuote f x = unsafeQToQuote (apply f x)

Functions from Language.Haskell.TH.Syntax

unTypeQQuote :: forall (r :: RuntimeRep) (a :: TYPE r) m. Quote m => m (TExp a) -> m Exp

Discard the type annotation and produce a plain Template Haskell expression

Levity-polymorphic since template-haskell-2.16.0.0.

This is a variant of the unTypeQ function that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

As this function interacts with typed Template Haskell, this function is only defined on template-haskell-2.9.0.0 (GHC 7.8) or later.

unsafeTExpCoerceQuote :: forall (r :: RuntimeRep) (a :: TYPE r) m. Quote m => m Exp -> m (TExp a)

Annotate the Template Haskell expression with a type

This is unsafe because GHC cannot check for you that the expression really does have the type you claim it has.

Levity-polymorphic since template-haskell-2.16.0.0.

This is a variant of the unsafeTExpCoerce function that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

As this function interacts with typed Template Haskell, this function is only defined on template-haskell-2.9.0.0 (GHC 7.8) or later.

liftQuote :: forall t m. (Lift t, Quote m) => t -> m Exp

Turn a value into a Template Haskell expression, suitable for use in a splice.

This is a variant of the lift method of Lift that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

Levity-polymorphic since template-haskell-2.17.0.0.

liftTypedQuote :: forall t m. (Lift t, Quote m) => t -> Code m t

Turn a value into a Template Haskell typed expression, suitable for use in a typed splice.

This is a variant of the liftTyped method of Lift that is always guaranteed to use a Quote constraint and return a Code, even on old versions of template-haskell.

As this function interacts with typed Template Haskell, this function is only defined on template-haskell-2.9.0.0 (GHC 7.8) or later. While the liftTyped method of Lift was first introduced in template-haskell-2.16.0.0, we are able to backport it back to template-haskell-2.9.0.0 by making use of the lift method on older versions of template-haskell. This crucially relies on the Lift law that lift x ≡ unTypeQ (liftTyped x) to work, so beware if you use liftTypedQuote with an unlawful Lift instance.

Levity-polymorphic since template-haskell-2.17.0.0.

liftStringQuote :: Quote m => String -> m Exp

This is a variant of the liftString function that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

The Code and CodeQ types

The Code type (first proposed in GHC Proposal 195) was introduced in template-haskell-2.17.0.0. This module defines a version of Code that is backward-compatible with older template-haskell releases and is forward-compatible with the existing Code class. In addition to Code, this module also backports the functions in Language.Haskell.TH.Syntax that manipulate Code values.

One troublesome aspect of writing backwards-compatible code involving Code is that GHC 9.0 changed the types of typed Template Haskell splices. Before, they were of type Q (TExp a), but they are now of type Code Q a. The IsCode class can be used to paper over the difference between these two types. For more details, consult the Haddocks for IsCode.

Because Code interacts with typed Template Haskell, the Code type and any function that mentions Code in its type are only defined on template-haskell-2.9.0.0 (GHC 7.8) or later.

newtype Code m (a :: TYPE (r :: RuntimeRep))

Levity-polymorphic since template-haskell-2.16.0.0.

Constructors

Code

Fields

• examineCode :: m (TExp a)

  Underlying monadic value

Instances

Quote q => IsCode q (a :: TYPE r) (Code q a)

Levity-polymorphic since template-haskell-2.16.0.0.

Defined in Language.Haskell.TH.Syntax.Compat

Methods

toCode :: Code q a -> Code q a

fromCode :: Code q a -> Code q a

type CodeQ = Code Q :: TYPE r -> *

Code functionality

The IsCode class

class IsCode q (a :: TYPE r) c | c -> a q where

A class that allows one to smooth over the differences between Code m a (the type of typed Template Haskell quotations on template-haskell-2.17.0.0 or later) and m (TExp a) (the type of typed Template Haskell quotations on older versions of template-haskell). Here are two examples that demonstrate how to use each method of IsCode:

{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Language.Haskell.TH.Syntax.Compat

-- toCode will ensure that the end result is a Code, regardless of
-- whether the quote itself returns a Code or a TExp.
myCode :: Code Q Int
myCode = toCode [|| 42 ||]

-- fromCode will ensure that the input Code is suitable for splicing
-- (i.e., it will return a Code or a TExp depending on the
-- template-haskell version in use).
fortyTwo :: Int
fortyTwo = $$(fromCode myCode)

Levity-polymorphic since template-haskell-2.16.0.0.

Methods

toCode :: c -> Code q a

Convert something to a Code.

fromCode :: Code q a -> c

Convert to something from a Code.

Instances

texp ~ TExp a => IsCode Q (a :: TYPE r) (Q texp)

Levity-polymorphic since template-haskell-2.16.0.0.

Defined in Language.Haskell.TH.Syntax.Compat

Methods

toCode :: Q texp -> Code Q a

fromCode :: Code Q a -> Q texp

Quote q => IsCode q (a :: TYPE r) (Code q a)

Levity-polymorphic since template-haskell-2.16.0.0.

Defined in Language.Haskell.TH.Syntax.Compat

Methods

toCode :: Code q a -> Code q a

fromCode :: Code q a -> Code q a

Limitations of IsCode

IsCode makes it possible to backport code involving typed Template Haskell quotations and splices where the types are monomorphized to Q. GHC 9.0 and later, however, make it possible to use typed TH quotations and splices that are polymorphic over any Quote instance. Unfortunately, the th-compat library does not yet have a good story for backporting Quote-polymorphic quotations or splices. For example, consider this code:

instance (Lift a, Quote q, Num a) => Num (Code q a) where
  -- ...
  x + y = [|| $$x + $$y ||]
  -- ...

How might we backport this code? If we were in a setting where q were monomorphized to Q, we could simply write this:

  x + y = toCode [|| $$(fromCode x) + $$(fromCode y) ||]

In a Quote-polymorphic setting, however, we run into issues. While this will compile on GHC 9.0 or later, it will not compile on earlier GHC versions because all typed TH quotations and splices must use Q. At present, the th-compat library does not offer any solution to this problem.

unsafeCodeCoerce :: forall (r :: RuntimeRep) (a :: TYPE r) m. Quote m => m Exp -> Code m a

Unsafely convert an untyped code representation into a typed code representation.

Levity-polymorphic since template-haskell-2.16.0.0.

liftCode :: forall (r :: RuntimeRep) (a :: TYPE r) m. m (TExp a) -> Code m a

Lift a monadic action producing code into the typed Code representation

Levity-polymorphic since template-haskell-2.16.0.0.

unTypeCode :: forall (r :: RuntimeRep) (a :: TYPE r) m. Quote m => Code m a -> m Exp

Extract the untyped representation from the typed representation

Levity-polymorphic since template-haskell-2.16.0.0.

hoistCode :: forall m n (r :: RuntimeRep) (a :: TYPE r). Monad m => (forall x. m x -> n x) -> Code m a -> Code n a

Modify the ambient monad used during code generation. For example, you can use hoistCode to handle a state effect:

 handleState :: Code (StateT Int Q) a -> Code Q a
 handleState = hoistCode (flip runState 0)

Levity-polymorphic since template-haskell-2.16.0.0.

bindCode :: forall m a (r :: RuntimeRep) (b :: TYPE r). Monad m => m a -> (a -> Code m b) -> Code m b

Variant of (>>=) which allows effectful computations to be injected into code generation.

Levity-polymorphic since template-haskell-2.16.0.0.

bindCode_ :: forall m a (r :: RuntimeRep) (b :: TYPE r). Monad m => m a -> Code m b -> Code m b

Variant of (>>) which allows effectful computations to be injected into code generation.

Levity-polymorphic since template-haskell-2.16.0.0.

joinCode :: forall m (r :: RuntimeRep) (a :: TYPE r). Monad m => m (Code m a) -> Code m a

A useful combinator for embedding monadic actions into Code myCode :: ... => Code m a myCode = joinCode $ do x <- someSideEffect return (makeCodeWith x)

Levity-polymorphic since template-haskell-2.16.0.0.

Splice

type Splice m (a :: TYPE r) = m (TExp a)

Splice m a is a type alias for:

• Code m a, if using template-haskell-2.17.0.0 or later, or
• m (TExp a), if using an older version of template-haskell.

This should be used with caution, as its definition differs depending on which version of template-haskell you are using. It is mostly useful for contexts in which one is writing a definition that is intended to be used directly in a typed Template Haskell splice, as the types of TH splices differ between template-haskell versions as well. One example of a type that uses Splice is the type signature for lifTypedFromUntypedSplice.

Levity-polymorphic since template-haskell-2.16.0.0.

type SpliceQ (a :: TYPE r) = Splice Q a

SpliceQ a is a type alias for:

• Code Q a, if using template-haskell-2.17.0.0 or later, or
• Q (TExp a), if using an older version of template-haskell.

This should be used with caution, as its definition differs depending on which version of template-haskell you are using. It is mostly useful for contexts in which one is writing a definition that is intended to be used directly in a typed Template Haskell splice, as the types of TH splices differ between template-haskell versions as well.

Levity-polymorphic since template-haskell-2.16.0.0.

liftTypedFromUntypedSplice :: (Lift t, Quote m) => t -> Splice m t

A variant of liftTypedQuote that is:

1. Always implemented in terms of lift behind the scenes, and
2. Returns a Splice. This means that the return type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

This is primarily useful for minimizing CPP in one particular scenario: implementing liftTyped in hand-written Lift instances where the corresponding lift implementation cannot be derived. For instance, consider this example from the text library:

instance Lift Text where
  lift = appE (varE 'pack) . stringE . unpack
#if MIN_VERSION_template_haskell(2,17,0)
  liftTyped = unsafeCodeCoerce . lift
#elif MIN_VERSION_template_haskell(2,16,0)
  liftTyped = unsafeTExpCoerce . lift
#endif

The precise details of how this lift implementation works are not important, only that it is something that DeriveLift could not generate. The main point of this example is to illustrate how tiresome it is to write the CPP necessary to define liftTyped in a way that works across multiple versions of template-haskell. With liftTypedFromUntypedSplice, however, this becomes slightly easier to manage:

instance Lift Text where
  lift = appE (varE 'pack) . stringE . unpack
#if MIN_VERSION_template_haskell(2,16,0)
  liftTyped = liftTypedFromUntypedSplice
#endif

Note that due to the way this function is defined, this will only work for Lift instances t such that (t :: Type). If you wish to manually define liftTyped for a type with a different kind, you will have to use unsafeSpliceCoerce to overcome levity polymorphism restrictions.

unsafeSpliceCoerce :: forall (r :: RuntimeRep) (a :: TYPE r) m. Quote m => m Exp -> Splice m a

Unsafely convert an untyped code representation into a typed code representation, where:

• The splice representation is Code m a, if using template-haskell-2.17.0.0 or later, or
• The splice representation is m (TExp a), if using an older version of template-haskell.

This is primarily useful for minimizing CPP when the following two conditions are met:

1. You need to implement liftTyped in a hand-written Lift instance where the corresponding lift implementation cannot be derived, and
2. The data type receiving a Lift instance has a kind besides Type.

Condition (2) is important because while it is possible to simply define 'Syntax.liftTyped = liftTypedFromUntypedSplice for Lift instances t such that (t :: Type), this will not work for types with different types, such as unboxed types or unlifted newtypes. This is because GHC restrictions prevent defining liftTypedFromUntypedSplice in a levity polymorphic fashion, so one must use unsafeSpliceCoerce to work around these restrictions. Here is an example of how to use unsafeSpliceCoerce:

instance Lift Int# where
  lift x = litE (intPrimL (fromIntegral (I# x)))
#if MIN_VERSION_template_haskell(2,16,0)
  liftTyped x = unsafeSpliceCoerce (lift x)
#endif

Levity-polymorphic since template-haskell-2.16.0.0.