Data.Vinyl.Functor

Introduction

This module provides functors and functor compositions that can be used as the interpretation function for a Rec. For a more full discussion of this, scroll down to the bottom.

newtype Identity a

This is identical to the Identity from Data.Functor.Identity in "base" except for its Show instance.

Constructors

Identity

Fields

• getIdentity :: a

Instances

Monad Identity

Methods

(>>=) :: Identity a -> (a -> Identity b) -> Identity b

(>>) :: Identity a -> Identity b -> Identity b

return :: a -> Identity a

fail :: String -> Identity a

Functor Identity

Methods

fmap :: (a -> b) -> Identity a -> Identity b

(<$) :: a -> Identity b -> Identity a

Applicative Identity

Methods

pure :: a -> Identity a

(<*>) :: Identity (a -> b) -> Identity a -> Identity b

(*>) :: Identity a -> Identity b -> Identity b

(<*) :: Identity a -> Identity b -> Identity a

Foldable Identity

Methods

fold :: Monoid m => Identity m -> m

foldMap :: Monoid m => (a -> m) -> Identity a -> m

foldr :: (a -> b -> b) -> b -> Identity a -> b

foldr' :: (a -> b -> b) -> b -> Identity a -> b

foldl :: (b -> a -> b) -> b -> Identity a -> b

foldl' :: (b -> a -> b) -> b -> Identity a -> b

foldr1 :: (a -> a -> a) -> Identity a -> a

foldl1 :: (a -> a -> a) -> Identity a -> a

toList :: Identity a -> [a]

null :: Identity a -> Bool

length :: Identity a -> Int

elem :: Eq a => a -> Identity a -> Bool

maximum :: Ord a => Identity a -> a

minimum :: Ord a => Identity a -> a

sum :: Num a => Identity a -> a

product :: Num a => Identity a -> a

Traversable Identity

Methods

traverse :: Applicative f => (a -> f b) -> Identity a -> f (Identity b)

sequenceA :: Applicative f => Identity (f a) -> f (Identity a)

mapM :: Monad m => (a -> m b) -> Identity a -> m (Identity b)

sequence :: Monad m => Identity (m a) -> m (Identity a)

Eq a => Eq (Identity a)

Methods

(==) :: Identity a -> Identity a -> Bool

(/=) :: Identity a -> Identity a -> Bool

Ord a => Ord (Identity a)

Methods

compare :: Identity a -> Identity a -> Ordering

(<) :: Identity a -> Identity a -> Bool

(<=) :: Identity a -> Identity a -> Bool

(>) :: Identity a -> Identity a -> Bool

(>=) :: Identity a -> Identity a -> Bool

max :: Identity a -> Identity a -> Identity a

min :: Identity a -> Identity a -> Identity a

Show a => Show (Identity a)

Methods

showsPrec :: Int -> Identity a -> ShowS

show :: Identity a -> String

showList :: [Identity a] -> ShowS

Storable a => Storable (Identity a)

Methods

sizeOf :: Identity a -> Int

alignment :: Identity a -> Int

peekElemOff :: Ptr (Identity a) -> Int -> IO (Identity a)

pokeElemOff :: Ptr (Identity a) -> Int -> Identity a -> IO ()

peekByteOff :: Ptr b -> Int -> IO (Identity a)

pokeByteOff :: Ptr b -> Int -> Identity a -> IO ()

peek :: Ptr (Identity a) -> IO (Identity a)

poke :: Ptr (Identity a) -> Identity a -> IO ()

data Thunk a

Used this instead of Identity to make a record lazy in its fields.

Constructors

Thunk

Fields

• getThunk :: a

Instances

Monad Thunk

Methods

(>>=) :: Thunk a -> (a -> Thunk b) -> Thunk b

(>>) :: Thunk a -> Thunk b -> Thunk b

return :: a -> Thunk a

fail :: String -> Thunk a

Functor Thunk

Methods

fmap :: (a -> b) -> Thunk a -> Thunk b

(<$) :: a -> Thunk b -> Thunk a

Applicative Thunk

Methods

pure :: a -> Thunk a

(<*>) :: Thunk (a -> b) -> Thunk a -> Thunk b

(*>) :: Thunk a -> Thunk b -> Thunk b

(<*) :: Thunk a -> Thunk b -> Thunk a

Foldable Thunk

Methods

fold :: Monoid m => Thunk m -> m

foldMap :: Monoid m => (a -> m) -> Thunk a -> m

foldr :: (a -> b -> b) -> b -> Thunk a -> b

foldr' :: (a -> b -> b) -> b -> Thunk a -> b

foldl :: (b -> a -> b) -> b -> Thunk a -> b

foldl' :: (b -> a -> b) -> b -> Thunk a -> b

foldr1 :: (a -> a -> a) -> Thunk a -> a

foldl1 :: (a -> a -> a) -> Thunk a -> a

toList :: Thunk a -> [a]

null :: Thunk a -> Bool

length :: Thunk a -> Int

elem :: Eq a => a -> Thunk a -> Bool

maximum :: Ord a => Thunk a -> a

minimum :: Ord a => Thunk a -> a

sum :: Num a => Thunk a -> a

product :: Num a => Thunk a -> a

Traversable Thunk

Methods

traverse :: Applicative f => (a -> f b) -> Thunk a -> f (Thunk b)

sequenceA :: Applicative f => Thunk (f a) -> f (Thunk a)

mapM :: Monad m => (a -> m b) -> Thunk a -> m (Thunk b)

sequence :: Monad m => Thunk (m a) -> m (Thunk a)

Show a => Show (Thunk a)

Methods

showsPrec :: Int -> Thunk a -> ShowS

show :: Thunk a -> String

showList :: [Thunk a] -> ShowS

newtype Lift op f g x

Constructors

Lift

Fields

• getLift :: op (f x) (g x)

Instances

(Functor f, Functor g) => Functor (Lift * * * Either f g)

Methods

fmap :: (a -> b) -> Lift * * * Either f g a -> Lift * * * Either f g b

(<$) :: a -> Lift * * * Either f g b -> Lift * * * Either f g a

(Functor f, Functor g) => Functor (Lift * * * (,) f g)

Methods

fmap :: (a -> b) -> Lift * * * (,) f g a -> Lift * * * (,) f g b

(<$) :: a -> Lift * * * (,) f g b -> Lift * * * (,) f g a

(Applicative f, Applicative g) => Applicative (Lift * * * (,) f g)

Methods

pure :: a -> Lift * * * (,) f g a

(<*>) :: Lift * * * (,) f g (a -> b) -> Lift * * * (,) f g a -> Lift * * * (,) f g b

(*>) :: Lift * * * (,) f g a -> Lift * * * (,) f g b -> Lift * * * (,) f g b

(<*) :: Lift * * * (,) f g a -> Lift * * * (,) f g b -> Lift * * * (,) f g a

newtype Compose f g x

Constructors

Compose

Fields

• getCompose :: f (g x)

Instances

(Functor f, Functor g) => Functor (Compose * * f g)

Methods

fmap :: (a -> b) -> Compose * * f g a -> Compose * * f g b

(<$) :: a -> Compose * * f g b -> Compose * * f g a

(Applicative f, Applicative g) => Applicative (Compose * * f g)

Methods

pure :: a -> Compose * * f g a

(<*>) :: Compose * * f g (a -> b) -> Compose * * f g a -> Compose * * f g b

(*>) :: Compose * * f g a -> Compose * * f g b -> Compose * * f g b

(<*) :: Compose * * f g a -> Compose * * f g b -> Compose * * f g a

(Foldable f, Foldable g) => Foldable (Compose * * f g)

Methods

fold :: Monoid m => Compose * * f g m -> m

foldMap :: Monoid m => (a -> m) -> Compose * * f g a -> m

foldr :: (a -> b -> b) -> b -> Compose * * f g a -> b

foldr' :: (a -> b -> b) -> b -> Compose * * f g a -> b

foldl :: (b -> a -> b) -> b -> Compose * * f g a -> b

foldl' :: (b -> a -> b) -> b -> Compose * * f g a -> b

foldr1 :: (a -> a -> a) -> Compose * * f g a -> a

foldl1 :: (a -> a -> a) -> Compose * * f g a -> a

toList :: Compose * * f g a -> [a]

null :: Compose * * f g a -> Bool

length :: Compose * * f g a -> Int

elem :: Eq a => a -> Compose * * f g a -> Bool

maximum :: Ord a => Compose * * f g a -> a

minimum :: Ord a => Compose * * f g a -> a

sum :: Num a => Compose * * f g a -> a

product :: Num a => Compose * * f g a -> a

(Traversable f, Traversable g) => Traversable (Compose * * f g)

Methods

traverse :: Applicative f => (a -> f b) -> Compose * * f g a -> f (Compose * * f g b)

sequenceA :: Applicative f => Compose * * f g (f a) -> f (Compose * * f g a)

mapM :: Monad m => (a -> m b) -> Compose * * f g a -> m (Compose * * f g b)

sequence :: Monad m => Compose * * f g (m a) -> m (Compose * * f g a)

Storable (f (g x)) => Storable (Compose k l f g x)

Methods

sizeOf :: Compose k l f g x -> Int

alignment :: Compose k l f g x -> Int

peekElemOff :: Ptr (Compose k l f g x) -> Int -> IO (Compose k l f g x)

pokeElemOff :: Ptr (Compose k l f g x) -> Int -> Compose k l f g x -> IO ()

peekByteOff :: Ptr b -> Int -> IO (Compose k l f g x)

pokeByteOff :: Ptr b -> Int -> Compose k l f g x -> IO ()

peek :: Ptr (Compose k l f g x) -> IO (Compose k l f g x)

poke :: Ptr (Compose k l f g x) -> Compose k l f g x -> IO ()

type (:.) f g = Compose f g infixr 9

newtype Const a b

Constructors

Const

Fields

• getConst :: a

Instances

Functor (Const * a)

Methods

fmap :: (a -> b) -> Const * a a -> Const * a b

(<$) :: a -> Const * a b -> Const * a a

Foldable (Const * a)

Methods

fold :: Monoid m => Const * a m -> m

foldMap :: Monoid m => (a -> m) -> Const * a a -> m

foldr :: (a -> b -> b) -> b -> Const * a a -> b

foldr' :: (a -> b -> b) -> b -> Const * a a -> b

foldl :: (b -> a -> b) -> b -> Const * a a -> b

foldl' :: (b -> a -> b) -> b -> Const * a a -> b

foldr1 :: (a -> a -> a) -> Const * a a -> a

foldl1 :: (a -> a -> a) -> Const * a a -> a

toList :: Const * a a -> [a]

null :: Const * a a -> Bool

length :: Const * a a -> Int

elem :: Eq a => a -> Const * a a -> Bool

maximum :: Ord a => Const * a a -> a

minimum :: Ord a => Const * a a -> a

sum :: Num a => Const * a a -> a

product :: Num a => Const * a a -> a

Traversable (Const * a)

Methods

traverse :: Applicative f => (a -> f b) -> Const * a a -> f (Const * a b)

sequenceA :: Applicative f => Const * a (f a) -> f (Const * a a)

mapM :: Monad m => (a -> m b) -> Const * a a -> m (Const * a b)

sequence :: Monad m => Const * a (m a) -> m (Const * a a)

Show a => Show (Const k a b)

Methods

showsPrec :: Int -> Const k a b -> ShowS

show :: Const k a b -> String

showList :: [Const k a b] -> ShowS

Storable a => Storable (Const k a b)

Methods

sizeOf :: Const k a b -> Int

alignment :: Const k a b -> Int

peekElemOff :: Ptr (Const k a b) -> Int -> IO (Const k a b)

pokeElemOff :: Ptr (Const k a b) -> Int -> Const k a b -> IO ()

peekByteOff :: Ptr b -> Int -> IO (Const k a b)

pokeByteOff :: Ptr b -> Int -> Const k a b -> IO ()

peek :: Ptr (Const k a b) -> IO (Const k a b)

poke :: Ptr (Const k a b) -> Const k a b -> IO ()

Discussion

Example

The data types in this module are used to build interpretation fuctions for a Rec. To build a Rec that is simply a heterogeneous list, use Identity:

>>> :{
let myRec1 :: Rec Identity '[Int,Bool,Char]
    myRec1 = Identity 4 :& Identity True :& Identity 'c' :& RNil
:}

For a record in which the fields are optional, you could alternatively write:

>>> :{
let myRec2 :: Rec Maybe '[Int,Bool,Char]
    myRec2 = Just 4 :& Nothing :& Nothing :& RNil
:}

And we can gather all of the effects with rtraverse:

>>> let r2 = rtraverse (fmap Identity) myRec2
>>> :t r2
r2 :: Maybe (Rec Identity '[Int, Bool, Char])
>>> r2
Nothing

If the fields only exist once an environment is provided, you can build the record as follows:

>>> :{
let myRec3 :: Rec ((->) Int) '[Int,Bool,Char]
    myRec3 = (+5) :& (const True) :& (head . show) :& RNil
:}

And again, we can collect these effects with "rtraverse":

>>> (rtraverse (fmap Identity) myRec3) 8
{13, True, '8'}

If you want the composition of these two effects, you can use Compose:

>>> import Data.Char (chr)
>>> :{
let safeDiv a b = if b == 0 then Nothing else Just (div a b)
    safeChr i = if i >= 32 && i <= 126 then Just (chr i) else Nothing
    myRec4 :: Rec (Compose ((->) Int) Maybe) '[Int,Char]
    myRec4 = (Compose $ safeDiv 42) :& (Compose safeChr) :& RNil
:}

Ecosystem

Of the five data types provided by this modules, three can be found in others places: Identity, Compose, and Const. They are included with "vinyl" to help keep the dependency list small. The differences will be discussed here.

The Data.Functor.Identity module was originally provided by "transformers". When GHC 7.10 was released, it was moved into "base-4.8". The Identity data type provided by that module is well recognized across the haskell ecosystem and has typeclass instances for lots of common typeclasses. The significant difference between it and the copy of it provided here is that this one has a different Show instance. This is illustrated below:

>>> Identity "hello"
"hello"

But, when using Identity from "base":

>>> import qualified Data.Functor.Identity as Base
>>> Base.Identity "hello"
Identity "hello"

This Show instance makes records look nicer in GHCi. Feel free to use Data.Functor.Identity if you do not need the prettier output or if you need the many additional typeclass instances that are provided for the standard Identity.

The story with Compose and Const is much more simple. These also exist in "transformers", although Const is named Constant there. Prior to the release of "transformers-0.5", they were not polykinded, making them unusable for certain universes. However, in "transformers-0.5" and forward, they have been made polykinded. This means that they are just as usable with Rec as the vinyl equivalents but with many more typeclass instances such as Ord and Show.