Safe Haskell  None 

Language  Haskell2010 
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.
This is identical to the Identity from Data.Functor.Identity
in "base" except for its Show
instance.
Identity  

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

(Functor f, Functor g) => Functor (Compose * * f g) Source  
(Applicative f, Applicative g) => Applicative (Compose * * f g) Source  
(Foldable f, Foldable g) => Foldable (Compose * * f g) Source  
(Traversable f, Traversable g) => Traversable (Compose * * f g) Source  
Storable (f (g x)) => Storable (Compose l k f g x) Source 
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 "base4.8". 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
"transformers0.5", they were not polykinded, making
them unusable for certain universes. However, in
"transformers0.5" 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
.