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Remove identity type family

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This commit is contained in:
Taylor Fausak 2021-05-10 19:12:06 -04:00
parent 8f7fb55e5a
commit 99d601b222
9 changed files with 31 additions and 100 deletions
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13
src/ghc-8.10/Witch/Lift.hs
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@ -1,12 +1,9 @@
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE TypeFamilies #-}
module Witch.Lift where
import qualified Data.Typeable as Typeable
import qualified Language.Haskell.TH.Syntax as TH
import qualified Witch.Identity as Identity
import qualified Witch.TryCast as TryCast
import qualified Witch.Utility as Utility
@ -39,9 +36,8 @@ liftedCast = TH.liftTyped . Utility.unsafeCast
-- > -- Prefer this:
-- > $$(liftedFrom @s "some literal")
liftedFrom
:: forall s target source
. ( Identity.Identity s ~ source
, TryCast.TryCast source target
:: forall source target
. ( TryCast.TryCast source target
, TH.Lift target
, Show source
, Typeable.Typeable source
@ -60,9 +56,8 @@ liftedFrom = liftedCast
-- > -- Prefer this:
-- > $$(liftedInto @t "some literal")
liftedInto
:: forall t source target
. ( Identity.Identity t ~ target
, TryCast.TryCast source target
:: forall target source
. ( TryCast.TryCast source target
, TH.Lift target
, Show source
, Typeable.Typeable source

13
src/ghc-8.8/Witch/Lift.hs
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@ -1,12 +1,9 @@
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module Witch.Lift where
import qualified Data.Typeable as Typeable
import qualified Language.Haskell.TH.Syntax as TH
import qualified Witch.Identity as Identity
import qualified Witch.TryCast as TryCast
import qualified Witch.Utility as Utility
@ -39,9 +36,8 @@ liftedCast s = TH.unsafeTExpCoerce $ TH.lift (Utility.unsafeCast s :: target)
-- > -- Prefer this:
-- > $$(liftedFrom @s "some literal")
liftedFrom
:: forall s target source
. ( Identity.Identity s ~ source
, TryCast.TryCast source target
:: forall source target
. ( TryCast.TryCast source target
, TH.Lift target
, Show source
, Typeable.Typeable source
@ -60,9 +56,8 @@ liftedFrom = liftedCast
-- > -- Prefer this:
-- > $$(liftedInto @t "some literal")
liftedInto
:: forall t source target
. ( Identity.Identity t ~ target
, TryCast.TryCast source target
:: forall target source
. ( TryCast.TryCast source target
, TH.Lift target
, Show source
, Typeable.Typeable source

13
src/ghc-9.0/Witch/Lift.hs
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@ -1,12 +1,9 @@
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE TypeFamilies #-}
module Witch.Lift where
import qualified Data.Typeable as Typeable
import qualified Language.Haskell.TH.Syntax as TH
import qualified Witch.Identity as Identity
import qualified Witch.TryCast as TryCast
import qualified Witch.Utility as Utility
@ -40,9 +37,8 @@ liftedCast = TH.liftTyped . Utility.unsafeCast
-- > -- Prefer this:
-- > $$(liftedFrom @s "some literal")
liftedFrom
:: forall s target m source
. ( Identity.Identity s ~ source
, TryCast.TryCast source target
:: forall source target m
. ( TryCast.TryCast source target
, TH.Lift target
, Show source
, Typeable.Typeable source
@ -62,9 +58,8 @@ liftedFrom = liftedCast
-- > -- Prefer this:
-- > $$(liftedInto @t "some literal")
liftedInto
:: forall t source m target
. ( Identity.Identity t ~ target
, TryCast.TryCast source target
:: forall target source m
. ( TryCast.TryCast source target
, TH.Lift target
, Show source
, Typeable.Typeable source

28
src/lib/Witch.hs
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@ -159,34 +159,6 @@ module Witch
-- 'Witch.TryCast.TryCast' instance for it. And if it is technically
-- possible to convert from @s@ to @t@ but there are a lot of caveats, you
-- probably should not write any instances at all.
-- ** Type applications
-- | This library is designed to be used with the [@TypeApplications@](https://downloads.haskell.org/~ghc/9.0.1/docs/html/users_guide/exts/type_applications.html)
-- language extension. Although it is not required for basic functionality,
-- it is strongly encouraged. You can use 'Witch.Cast.cast',
-- 'Witch.TryCast.tryCast', 'Witch.Utility.unsafeCast', and
-- 'Witch.Lift.liftedCast' without type applications. Everything else
-- requires a type application.
-- ** Ambiguous types
-- | You may see @Identity@ show up in some type signatures. Anywhere you see
-- @Identity a@, you can mentally replace it with @a@. It is a type family
-- used to trick GHC into requiring type applications for certain functions.
-- If you forget to give a type application, you will see an error like
-- this:
--
-- >>> from (1 :: Int8) :: Int16
-- <interactive>:1:1: error:
-- * Couldn't match type `Identity s0' with `Int8'
-- arising from a use of `from'
-- The type variable `s0' is ambiguous
-- * In the expression: from (1 :: Int8) :: Int16
-- In an equation for `it': it = from (1 :: Int8) :: Int16
--
-- You can fix the problem by giving a type application:
--
-- >>> from @Int8 1 :: Int16
-- 1
) where
import qualified Witch.Cast

14
src/lib/Witch/Identity.hs
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@ -1,14 +0,0 @@
{-# LANGUAGE TypeFamilies #-}
module Witch.Identity where
-- | This is an ugly hack used to make GHC require type applications for
-- certain functions. See this Twitter thread for a discussion:
-- <https://twitter.com/taylorfausak/status/1329084033003782148>.
type family Identity a where
Identity Never = ()
Identity a = a
-- | Never use this type for anything! It only exists to make the 'Identity'
-- type family non-trivial.
data Never

2
src/lib/Witch/Instances.hs
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@ -1,8 +1,6 @@
{-# OPTIONS_GHC -Wno-orphans #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
module Witch.Instances where

45
src/lib/Witch/Utility.hs
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@ -1,6 +1,5 @@
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module Witch.Utility where
@ -8,7 +7,6 @@ import qualified Control.Exception as Exception
import qualified Data.Typeable as Typeable
import qualified GHC.Stack as Stack
import qualified Witch.Cast as Cast
import qualified Witch.Identity as Identity
import qualified Witch.TryCast as TryCast
import qualified Witch.TryCastException as TryCastException
@ -21,7 +19,7 @@ import qualified Witch.TryCastException as TryCastException
-- >
-- > -- Prefer this:
-- > f . as @Int . g
as :: forall s source . Identity.Identity s ~ source => source -> source
as :: forall source . source -> source
as = id
-- | This is the same as 'Cast.cast' except that it requires a type
@ -33,8 +31,8 @@ as = id
-- > -- Prefer this:
-- > from @s x
from
:: forall s target source
. (Identity.Identity s ~ source, Cast.Cast source target)
:: forall source target
. Cast.Cast source target
=> source
-> target
from = Cast.cast
@ -48,8 +46,8 @@ from = Cast.cast
-- > -- Prefer this:
-- > into @t x
into
:: forall t source target
. (Identity.Identity t ~ target, Cast.Cast source target)
:: forall target source
. Cast.Cast source target
=> source
-> target
into = Cast.cast
@ -65,9 +63,8 @@ into = Cast.cast
-- > -- Prefer this:
-- > over @t f
over
:: forall t source target
. ( Identity.Identity t ~ target
, Cast.Cast source target
:: forall target source
. ( Cast.Cast source target
, Cast.Cast target source
)
=> (target -> target)
@ -86,9 +83,8 @@ over f = Cast.cast . f . Cast.cast
-- > -- Prefer this:
-- > via @u
via
:: forall u source target through
. ( Identity.Identity u ~ through
, Cast.Cast source through
:: forall through source target
. ( Cast.Cast source through
, Cast.Cast through target
)
=> source
@ -104,8 +100,8 @@ via = Cast.cast . (\x -> x :: through) . Cast.cast
-- > -- Prefer this:
-- > tryFrom @s x
tryFrom
:: forall s target source
. (Identity.Identity s ~ source, TryCast.TryCast source target)
:: forall source target
. TryCast.TryCast source target
=> source
-> Either (TryCastException.TryCastException source target) target
tryFrom = TryCast.tryCast
@ -119,8 +115,8 @@ tryFrom = TryCast.tryCast
-- > -- Prefer this:
-- > tryInto @t x
tryInto
:: forall t source target
. (Identity.Identity t ~ target, TryCast.TryCast source target)
:: forall target source
. TryCast.TryCast source target
=> source
-> Either (TryCastException.TryCastException source target) target
tryInto = TryCast.tryCast
@ -139,9 +135,8 @@ tryInto = TryCast.tryCast
-- > -- Prefer this:
-- > tryVia @u
tryVia
:: forall u source target through
. ( Identity.Identity u ~ through
, TryCast.TryCast source through
:: forall through source target
. ( TryCast.TryCast source through
, TryCast.TryCast through target
)
=> source
@ -221,9 +216,8 @@ unsafeCast = either Exception.throw id . TryCast.tryCast
-- > -- Prefer this:
-- > unsafeFrom @s
unsafeFrom
:: forall s target source
. ( Identity.Identity s ~ source
, Stack.HasCallStack
:: forall source target
. ( Stack.HasCallStack
, TryCast.TryCast source target
, Show source
, Typeable.Typeable source
@ -242,9 +236,8 @@ unsafeFrom = unsafeCast
-- > -- Prefer this:
-- > unsafeInto @t
unsafeInto
:: forall t source target
. ( Identity.Identity t ~ target
, Stack.HasCallStack
:: forall target source
. ( Stack.HasCallStack
, TryCast.TryCast source target
, Show source
, Typeable.Typeable source

2
src/test/Main.hs
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@ -1,5 +1,3 @@
{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}

1
witch.cabal
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@ -47,7 +47,6 @@ library
exposed-modules:
Witch
Witch.Cast
Witch.Identity
Witch.Instances
Witch.Lift
Witch.TryCast