[Git][ghc/ghc][wip/wither-eq1-and-friends] Relax instances for Functor combinators; put superclass on Class1 and Class2 to make non-breaking
sheaf (@sheaf)
gitlab at gitlab.haskell.org
Mon Sep 19 13:13:11 UTC 2022
sheaf pushed to branch wip/wither-eq1-and-friends at Glasgow Haskell Compiler / GHC
Commits:
b3b50765 by John Ericson at 2022-09-19T15:13:01+02:00
Relax instances for Functor combinators; put superclass on Class1 and Class2 to make non-breaking
This change is approved by the Core Libraries commitee in
https://github.com/haskell/core-libraries-committee/issues/10
The first change makes the `Eq`, `Ord`, `Show`, and `Read` instances for
`Sum`, `Product`, and `Compose` match those for `:+:`, `:*:`, and `:.:`.
These have the proper flexible contexts that are exactly what the
instance needs:
For example, instead of
```haskell
instance (Eq1 f, Eq1 g, Eq a) => Eq (Compose f g a) where
(==) = eq1
```
we do
```haskell
deriving instance Eq (f (g a)) => Eq (Compose f g a)
```
But, that change alone is rather breaking, because until now `Eq (f a)`
and `Eq1 f` (and respectively the other classes and their `*1`
equivalents too) are *incomparable* constraints. This has always been an
annoyance of working with the `*1` classes, and now it would rear it's
head one last time as an pesky migration.
Instead, we give the `*1` classes superclasses, like so:
```haskell
(forall a. Eq a => Eq (f a)) => Eq1 f
```
along with some laws that canonicity is preserved, like:
```haskell
liftEq (==) = (==)
```
and likewise for `*2` classes:
```haskell
(forall a. Eq a => Eq1 (f a)) => Eq2 f
```
and laws:
```haskell
liftEq2 (==) = liftEq1
```
The `*1` classes also have default methods using the `*2` classes where
possible.
What this means, as explained in the docs, is that `*1` classes really
are generations of the regular classes, indicating that the methods can
be split into a canonical lifting combined with a canonical inner, with
the super class "witnessing" the laws[1] in a fashion.
Circling back to the pragmatics of migrating, note that the superclass
means evidence for the old `Sum`, `Product`, and `Compose` instances is
(more than) sufficient, so breakage is less likely --- as long no
instances are "missing", existing polymorphic code will continue to
work.
Breakage can occur when a datatype implements the `*1` class but not the
corresponding regular class, but this is almost certainly an oversight.
For example, containers made that mistake for `Tree` and `Ord`, which I
fixed in https://github.com/haskell/containers/pull/761, but fixing the
issue by adding `Ord1` was extremely *un*controversial.
`Generically1` was also missing `Eq`, `Ord`, `Read,` and `Show`
instances. It is unlikely this would have been caught without
implementing this change.
-----
[1]: In fact, someday, when the laws are part of the language and not
only documentation, we might be able to drop the superclass field of the
dictionary by using the laws to recover the superclass in an
instance-agnostic manner, e.g. with a *non*-overloaded function with
type:
```haskell
DictEq1 f -> DictEq a -> DictEq (f a)
```
But I don't wish to get into optomizations now, just demonstrate the
close relationship between the law and the superclass.
Bump haddock submodule because of test output changing.
- - - - -
6 changed files:
- libraries/base/Data/Functor/Classes.hs
- libraries/base/Data/Functor/Compose.hs
- libraries/base/Data/Functor/Product.hs
- libraries/base/Data/Functor/Sum.hs
- libraries/base/changelog.md
- utils/haddock
Changes:
=====================================
libraries/base/Data/Functor/Classes.hs
=====================================
@@ -1,7 +1,10 @@
{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE Safe #-}
+{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE QuantifiedConstraints #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Functor.Classes
@@ -91,8 +94,18 @@ import Text.Show (showListWith)
-- | Lifting of the 'Eq' class to unary type constructors.
--
+-- Any instance should be subject to the following law that canonicity
+-- is preserved:
+--
+-- @liftEq (==)@ = @(==)@
+--
+-- This class therefore represents the generalization of 'Eq' by
+-- decomposing its main method into a canonical lifting on a canonical
+-- inner method, so that the lifting can be reused for other arguments
+-- than the canonical one.
+--
-- @since 4.9.0.0
-class Eq1 f where
+class (forall a. Eq a => Eq (f a)) => Eq1 f where
-- | Lift an equality test through the type constructor.
--
-- The function will usually be applied to an equality function,
@@ -102,6 +115,10 @@ class Eq1 f where
--
-- @since 4.9.0.0
liftEq :: (a -> b -> Bool) -> f a -> f b -> Bool
+ default liftEq
+ :: (f ~ f' c, Eq2 f', Eq c)
+ => (a -> b -> Bool) -> f a -> f b -> Bool
+ liftEq = liftEq2 (==)
-- | Lift the standard @('==')@ function through the type constructor.
--
@@ -111,8 +128,18 @@ eq1 = liftEq (==)
-- | Lifting of the 'Ord' class to unary type constructors.
--
+-- Any instance should be subject to the following law that canonicity
+-- is preserved:
+--
+-- @liftCompare compare@ = 'compare'
+--
+-- This class therefore represents the generalization of 'Ord' by
+-- decomposing its main method into a canonical lifting on a canonical
+-- inner method, so that the lifting can be reused for other arguments
+-- than the canonical one.
+--
-- @since 4.9.0.0
-class (Eq1 f) => Ord1 f where
+class (Eq1 f, forall a. Ord a => Ord (f a)) => Ord1 f where
-- | Lift a 'compare' function through the type constructor.
--
-- The function will usually be applied to a comparison function,
@@ -122,6 +149,10 @@ class (Eq1 f) => Ord1 f where
--
-- @since 4.9.0.0
liftCompare :: (a -> b -> Ordering) -> f a -> f b -> Ordering
+ default liftCompare
+ :: (f ~ f' c, Ord2 f', Ord c)
+ => (a -> b -> Ordering) -> f a -> f b -> Ordering
+ liftCompare = liftCompare2 compare
-- | Lift the standard 'compare' function through the type constructor.
--
@@ -131,6 +162,22 @@ compare1 = liftCompare compare
-- | Lifting of the 'Read' class to unary type constructors.
--
+-- Any instance should be subject to the following laws that canonicity
+-- is preserved:
+--
+-- @liftReadsPrec readsPrec readList@ = 'readsPrec'
+--
+-- @liftReadList readsPrec readList@ = 'readList'
+--
+-- @liftReadPrec readPrec readListPrec@ = 'readPrec'
+--
+-- @liftReadListPrec readPrec readListPrec@ = 'readListPrec'
+--
+-- This class therefore represents the generalization of 'Read' by
+-- decomposing it's methods into a canonical lifting on a canonical
+-- inner method, so that the lifting can be reused for other arguments
+-- than the canonical one.
+--
-- Both 'liftReadsPrec' and 'liftReadPrec' exist to match the interface
-- provided in the 'Read' type class, but it is recommended to implement
-- 'Read1' instances using 'liftReadPrec' as opposed to 'liftReadsPrec', since
@@ -145,7 +192,7 @@ compare1 = liftCompare compare
-- For more information, refer to the documentation for the 'Read' class.
--
-- @since 4.9.0.0
-class Read1 f where
+class (forall a. Read a => Read (f a)) => Read1 f where
{-# MINIMAL liftReadsPrec | liftReadPrec #-}
-- | 'readsPrec' function for an application of the type constructor
@@ -219,14 +266,30 @@ liftReadListPrecDefault rp rl = list (liftReadPrec rp rl)
-- | Lifting of the 'Show' class to unary type constructors.
--
+-- Any instance should be subject to the following laws that canonicity
+-- is preserved:
+--
+-- @liftShowsPrec showsPrec showList@ = 'showsPrec'
+--
+-- @liftShowList showsPrec showList@ = 'showList'
+--
+-- This class therefore represents the generalization of 'Show' by
+-- decomposing it's methods into a canonical lifting on a canonical
+-- inner method, so that the lifting can be reused for other arguments
+-- than the canonical one.
+--
-- @since 4.9.0.0
-class Show1 f where
+class (forall a. Show a => Show (f a)) => Show1 f where
-- | 'showsPrec' function for an application of the type constructor
-- based on 'showsPrec' and 'showList' functions for the argument type.
--
-- @since 4.9.0.0
liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) ->
Int -> f a -> ShowS
+ default liftShowsPrec
+ :: (f ~ f' b, Show2 f', Show b)
+ => (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> f a -> ShowS
+ liftShowsPrec = liftShowsPrec2 showsPrec showList
-- | 'showList' function for an application of the type constructor
-- based on 'showsPrec' and 'showList' functions for the argument type.
@@ -248,7 +311,7 @@ showsPrec1 = liftShowsPrec showsPrec showList
-- | Lifting of the 'Eq' class to binary type constructors.
--
-- @since 4.9.0.0
-class Eq2 f where
+class (forall a. Eq a => Eq1 (f a)) => Eq2 f where
-- | Lift equality tests through the type constructor.
--
-- The function will usually be applied to equality functions,
@@ -268,7 +331,7 @@ eq2 = liftEq2 (==) (==)
-- | Lifting of the 'Ord' class to binary type constructors.
--
-- @since 4.9.0.0
-class (Eq2 f) => Ord2 f where
+class (Eq2 f, forall a. Ord a => Ord1 (f a)) => Ord2 f where
-- | Lift 'compare' functions through the type constructor.
--
-- The function will usually be applied to comparison functions,
@@ -302,7 +365,7 @@ compare2 = liftCompare2 compare compare
-- For more information, refer to the documentation for the 'Read' class.
--
-- @since 4.9.0.0
-class Read2 f where
+class (forall a. Read a => Read1 (f a)) => Read2 f where
{-# MINIMAL liftReadsPrec2 | liftReadPrec2 #-}
-- | 'readsPrec' function for an application of the type constructor
@@ -385,7 +448,7 @@ liftReadListPrec2Default rp1 rl1 rp2 rl2 = list (liftReadPrec2 rp1 rl1 rp2 rl2)
-- | Lifting of the 'Show' class to binary type constructors.
--
-- @since 4.9.0.0
-class Show2 f where
+class (forall a. Show a => Show1 (f a)) => Show2 f where
-- | 'showsPrec' function for an application of the type constructor
-- based on 'showsPrec' and 'showList' functions for the argument types.
--
=====================================
libraries/base/Data/Functor/Compose.hs
=====================================
@@ -5,6 +5,7 @@
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE Trustworthy #-}
+{-# LANGUAGE StandaloneDeriving #-}
-----------------------------------------------------------------------------
-- |
@@ -32,7 +33,7 @@ import Data.Coerce (coerce)
import Data.Data (Data)
import Data.Type.Equality (TestEquality(..), (:~:)(..))
import GHC.Generics (Generic, Generic1)
-import Text.Read (Read(..), readListDefault, readListPrecDefault)
+import Text.Read ()
infixr 9 `Compose`
@@ -47,6 +48,17 @@ newtype Compose f g a = Compose { getCompose :: f (g a) }
, Monoid -- ^ @since 4.16.0.0
)
+-- Instances of Prelude classes
+
+-- | @since 4.18.0.0
+deriving instance Eq (f (g a)) => Eq (Compose f g a)
+-- | @since 4.18.0.0
+deriving instance Ord (f (g a)) => Ord (Compose f g a)
+-- | @since 4.18.0.0
+deriving instance Read (f (g a)) => Read (Compose f g a)
+-- | @since 4.18.0.0
+deriving instance Show (f (g a)) => Show (Compose f g a)
+
-- Instances of lifted Prelude classes
-- | @since 4.9.0.0
@@ -77,27 +89,6 @@ instance (Show1 f, Show1 g) => Show1 (Compose f g) where
sp' = liftShowsPrec sp sl
sl' = liftShowList sp sl
--- Instances of Prelude classes
-
--- | @since 4.9.0.0
-instance (Eq1 f, Eq1 g, Eq a) => Eq (Compose f g a) where
- (==) = eq1
-
--- | @since 4.9.0.0
-instance (Ord1 f, Ord1 g, Ord a) => Ord (Compose f g a) where
- compare = compare1
-
--- | @since 4.9.0.0
-instance (Read1 f, Read1 g, Read a) => Read (Compose f g a) where
- readPrec = readPrec1
-
- readListPrec = readListPrecDefault
- readList = readListDefault
-
--- | @since 4.9.0.0
-instance (Show1 f, Show1 g, Show a) => Show (Compose f g a) where
- showsPrec = showsPrec1
-
-- Functor instances
-- | @since 4.9.0.0
=====================================
libraries/base/Data/Functor/Product.hs
=====================================
@@ -2,6 +2,7 @@
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE Safe #-}
+{-# LANGUAGE StandaloneDeriving #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Functor.Product
@@ -28,7 +29,7 @@ import Control.Monad.Zip (MonadZip(mzipWith))
import Data.Data (Data)
import Data.Functor.Classes
import GHC.Generics (Generic, Generic1)
-import Text.Read (Read(..), readListDefault, readListPrecDefault)
+import Text.Read ()
-- | Lifted product of functors.
data Product f g a = Pair (f a) (g a)
@@ -37,6 +38,15 @@ data Product f g a = Pair (f a) (g a)
, Generic1 -- ^ @since 4.9.0.0
)
+-- | @since 4.18.0.0
+deriving instance (Eq (f a), Eq (g a)) => Eq (Product f g a)
+-- | @since 4.18.0.0
+deriving instance (Ord (f a), Ord (g a)) => Ord (Product f g a)
+-- | @since 4.18.0.0
+deriving instance (Read (f a), Read (g a)) => Read (Product f g a)
+-- | @since 4.18.0.0
+deriving instance (Show (f a), Show (g a)) => Show (Product f g a)
+
-- | @since 4.9.0.0
instance (Eq1 f, Eq1 g) => Eq1 (Product f g) where
liftEq eq (Pair x1 y1) (Pair x2 y2) = liftEq eq x1 x2 && liftEq eq y1 y2
@@ -59,25 +69,6 @@ instance (Show1 f, Show1 g) => Show1 (Product f g) where
liftShowsPrec sp sl d (Pair x y) =
showsBinaryWith (liftShowsPrec sp sl) (liftShowsPrec sp sl) "Pair" d x y
--- | @since 4.9.0.0
-instance (Eq1 f, Eq1 g, Eq a) => Eq (Product f g a)
- where (==) = eq1
-
--- | @since 4.9.0.0
-instance (Ord1 f, Ord1 g, Ord a) => Ord (Product f g a) where
- compare = compare1
-
--- | @since 4.9.0.0
-instance (Read1 f, Read1 g, Read a) => Read (Product f g a) where
- readPrec = readPrec1
-
- readListPrec = readListPrecDefault
- readList = readListDefault
-
--- | @since 4.9.0.0
-instance (Show1 f, Show1 g, Show a) => Show (Product f g a) where
- showsPrec = showsPrec1
-
-- | @since 4.9.0.0
instance (Functor f, Functor g) => Functor (Product f g) where
fmap f (Pair x y) = Pair (fmap f x) (fmap f y)
=====================================
libraries/base/Data/Functor/Sum.hs
=====================================
@@ -2,6 +2,7 @@
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE Safe #-}
+{-# LANGUAGE StandaloneDeriving #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Functor.Sum
@@ -25,7 +26,7 @@ import Control.Applicative ((<|>))
import Data.Data (Data)
import Data.Functor.Classes
import GHC.Generics (Generic, Generic1)
-import Text.Read (Read(..), readListDefault, readListPrecDefault)
+import Text.Read ()
-- | Lifted sum of functors.
data Sum f g a = InL (f a) | InR (g a)
@@ -34,6 +35,15 @@ data Sum f g a = InL (f a) | InR (g a)
, Generic1 -- ^ @since 4.9.0.0
)
+-- | @since 4.18.0.0
+deriving instance (Eq (f a), Eq (g a)) => Eq (Sum f g a)
+-- | @since 4.18.0.0
+deriving instance (Ord (f a), Ord (g a)) => Ord (Sum f g a)
+-- | @since 4.18.0.0
+deriving instance (Read (f a), Read (g a)) => Read (Sum f g a)
+-- | @since 4.18.0.0
+deriving instance (Show (f a), Show (g a)) => Show (Sum f g a)
+
-- | @since 4.9.0.0
instance (Eq1 f, Eq1 g) => Eq1 (Sum f g) where
liftEq eq (InL x1) (InL x2) = liftEq eq x1 x2
@@ -64,22 +74,6 @@ instance (Show1 f, Show1 g) => Show1 (Sum f g) where
liftShowsPrec sp sl d (InR y) =
showsUnaryWith (liftShowsPrec sp sl) "InR" d y
--- | @since 4.9.0.0
-instance (Eq1 f, Eq1 g, Eq a) => Eq (Sum f g a) where
- (==) = eq1
--- | @since 4.9.0.0
-instance (Ord1 f, Ord1 g, Ord a) => Ord (Sum f g a) where
- compare = compare1
--- | @since 4.9.0.0
-instance (Read1 f, Read1 g, Read a) => Read (Sum f g a) where
- readPrec = readPrec1
-
- readListPrec = readListPrecDefault
- readList = readListDefault
--- | @since 4.9.0.0
-instance (Show1 f, Show1 g, Show a) => Show (Sum f g a) where
- showsPrec = showsPrec1
-
-- | @since 4.9.0.0
instance (Functor f, Functor g) => Functor (Sum f g) where
fmap f (InL x) = InL (fmap f x)
=====================================
libraries/base/changelog.md
=====================================
@@ -31,6 +31,11 @@
as well as [the migration guide](https://github.com/haskell/core-libraries-committee/blob/main/guides/export-lifta2-prelude.md)
* Update to Unicode 15.0.0.
* Add `Eq` and `Ord` instances for `Generically1`.
+ * Relax instances for Functor combinators; put superclass on Class1 and Class2
+ to make non-breaking. See [CLC
+ #10](https://github.com/haskell/core-libraries-committee/issues/10) for the
+ related discussion, as well as [the migration
+ guide](https://github.com/haskell/core-libraries-committee/blob/main/guides/functor-combinator-instances-and-class1s.md).
## 4.17.0.0 *August 2022*
=====================================
utils/haddock
=====================================
@@ -1 +1 @@
-Subproject commit b5e40b15228fdca5ce7d4e2f2241156d0b085261
+Subproject commit 7e4326f999056fb7b0b955ccadf5eab86b755a0d
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b3b50765a2267fb648e79dcb45319e2a6c9a4f75
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b3b50765a2267fb648e79dcb45319e2a6c9a4f75
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