src/Data/Patch/Class.hs · patch

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1{-# LANGUAGE CPP #-}

2{-# LANGUAGE ScopedTypeVariables #-}

3{-# LANGUAGE TypeFamilies #-}

4-- | The interface for types which represent changes made to other types

5module Data.Patch.Class where

7import Data.Functor.Identity

8import Data.Kind (Type)

9import Data.Maybe

10#if !MIN_VERSION_base(4,11,0)

11import Data.Semigroup (Semigroup(..))

12#endif

13import Data.Proxy

15-- | A 'Patch' type represents a kind of change made to a datastructure.

17-- If an instance of 'Patch' is also an instance of 'Semigroup', it should obey

18-- the law that @applyAlways (f <> g) == applyAlways f . applyAlways g@.

19class Patch p where

20 type PatchTarget p :: Type

21 -- | Apply the patch @p a@ to the value @a@. If no change is needed, return

22 -- 'Nothing'.

23 apply :: p -> PatchTarget p -> Maybe (PatchTarget p)

25-- | Apply a 'Patch'; if it does nothing, return the original value

26applyAlways :: Patch p => p -> PatchTarget p -> PatchTarget p

27applyAlways p t = fromMaybe t $ apply p t

29-- | 'Identity' can be used as a 'Patch' that always fully replaces the value

30instance Patch (Identity a) where

31 type PatchTarget (Identity a) = a

32 apply (Identity a) _ = Just a

34-- | 'Proxy' can be used as a 'Patch' that does nothing.

35instance forall (a :: Type). Patch (Proxy a) where

36 type PatchTarget (Proxy a) = a

37 apply ~Proxy _ = Nothing

39-- | Like '(.)', but composes functions that return patches rather than

40-- functions that return new values. The Semigroup instance for patches must

41-- apply patches right-to-left, like '(.)'.

42composePatchFunctions :: (Patch p, Semigroup p) => (PatchTarget p -> p) -> (PatchTarget p -> p) -> PatchTarget p -> p

43composePatchFunctions g f a =

44 let fp = f a

45 in g (applyAlways fp a) <> fp