Add instance AsType a a (#149)

* Add instance AsType a a

Resolves #146

* Update ChangeLog.mds

* docs: expand definition of {As,Has}Type; document reflexive instances

* docs: make generic-optics/ChangeLog.md mirror generic-lens/ChangeLog.md

* format: remove trailing newlines

generic-lens/ChangeLog.md

@@ -2,6 +2,10 @@
 - Add `OverloadedLabels` support for positional lenses, e.g. `#3` as an
  abbreviation for `position @3`, starting with GHC 9.6.
 
+### Breaking API changes:
+- `AsType` now includes a reflexive case for consistency with `HasType`: every
+ type can be treated 'as' itself.
+
 ## generic-lens-2.2.2.0 (2023-04-15)
 - Support unprefixed constructor prisms on GHC 9.6 (#152)
 

generic-lens/generic-lens.cabal

@@ -69,11 +69,12 @@ test-suite inspection-tests
  type: exitcode-stdio-1.0
  hs-source-dirs: test
  main-is: Spec.hs
- other-modules: Util Test24 Test88 Test25 Test40 Test62 Test63 CustomChildren
+ other-modules: Util Test24 Test88 Test25 Test40 Test62 Test63 Test146 CustomChildren
 
  build-depends: base
  , generic-lens
  , lens
+ , mtl
  , profunctors
  , inspection-testing >= 0.2
  , HUnit

generic-lens/src/Data/Generics/Product/Typed.hs

@@ -57,7 +57,8 @@ import "generic-lens-core" Data.Generics.Internal.Void
 -- human = Human "Tunyasz" 50 "London" False
 -- :}
 
--- |Records that have a field with a unique type.
+-- |Types that contain another type, either by being a record with a field
+-- with that type, or by actually being that type.
 class HasType a s where
  -- |A lens that focuses on a field with a unique type in its parent type.
  -- Compatible with the lens package's 'Control.Lens.Lens' type.
@@ -102,6 +103,7 @@ instance Core.Context a s => HasType a s where
  typed = VL.ravel Core.derived
  {-# INLINE typed #-}
 
+-- |Every type 'has' itself.
 instance {-# OVERLAPPING #-} HasType a a where
  getTyped = id
  {-# INLINE getTyped #-}

generic-lens/src/Data/Generics/Sum/Typed.hs

@@ -64,7 +64,8 @@ import "generic-lens-core" Data.Generics.Internal.Void
 -- :}
 
 
--- |Sums that have a constructor with a field of the given type.
+-- |Types that can represent another type, either by being a sum with a
+-- constructor containing that type, or by actually being that type.
 class AsType a s where
  -- |A prism that projects a constructor uniquely identifiable by the type of
  -- its field. Compatible with the lens package's 'Control.Lens.Prism' type.
@@ -97,6 +98,11 @@ class AsType a s where
 
  {-# MINIMAL (injectTyped, projectTyped) | _Typed #-}
 
+-- |Every type can be treated 'as' itself.
+instance {-# OVERLAPPING #-} AsType a a where
+ injectTyped = id
+ projectTyped = Just
+
 instance Core.Context a s => AsType a s where
  _Typed eta = prism2prismvl Core.derived eta
  {-# INLINE _Typed #-}

generic-lens/test/Spec.hs

@@ -33,6 +33,7 @@ import Data.Generics.Labels ()
 import Test24 ()
 import Test25 ()
 import Test88 ()
+import Test146 ()
 
 import CustomChildren (customTypesTest)
 

generic-lens/test/Test146.hs

@@ -0,0 +1,17 @@
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE FlexibleContexts #-}
+
+module Test146 where
+
+import Control.Monad.Except
+import Data.Generics.Sum
+import GHC.Generics
+
+data Error = Error
+ deriving (Generic)
+
+poly :: (AsType Error e, MonadError e m) => m ()
+poly = undefined
+
+mono :: ExceptT Error IO ()
+mono = poly

generic-optics/ChangeLog.md

@@ -1,3 +1,9 @@
+## Unreleased
+
+### Breaking API changes:
+- `AsType` now includes a reflexive case for consistency with `HasType`: every
+ type can be treated 'as' itself.
+
 ## generic-optics-2.2.1.0 (2022-01-22)
 - GHC 9.2 compatibility
 

generic-optics/generic-optics.cabal

@@ -65,10 +65,11 @@ test-suite generic-optics-inspection-tests
  type: exitcode-stdio-1.0
  hs-source-dirs: test
  main-is: Spec.hs
- other-modules: Util Test24 Test88 Test25 Test40 Test62 Test63 CustomChildren
+ other-modules: Util Test24 Test88 Test25 Test40 Test62 Test63 Test146 CustomChildren
 
  build-depends: base
  , generic-optics
+ , mtl
  , optics-core
  , inspection-testing >= 0.2
  , HUnit

generic-optics/src/Data/Generics/Product/Typed.hs

@@ -59,7 +59,8 @@ import "generic-lens-core" Data.Generics.Internal.Void
 -- human = Human "Tunyasz" 50 "London" False
 -- :}
 
--- |Records that have a field with a unique type.
+-- |Types that contain another type, either by being a record with a field
+-- with that type, or by actually being that type.
 class HasType a s where
  -- |A lens that focuses on a field with a unique type in its parent type.
  --
@@ -103,6 +104,7 @@ instance Core.Context a s => HasType a s where
  typed = normaliseLens (Optic Core.derived)
  {-# INLINE typed #-}
 
+-- |Every type 'has' itself.
 instance {-# OVERLAPPING #-} HasType a a where
  getTyped = id
  {-# INLINE getTyped #-}

generic-optics/src/Data/Generics/Sum/Typed.hs

@@ -64,7 +64,8 @@ import "generic-lens-core" Data.Generics.Internal.Void
 -- :}
 
 
--- |Sums that have a constructor with a field of the given type.
+-- |Types that can represent another type, either by being a sum with a
+-- constructor containing that type, or by actually being that type.
 class AsType a s where
  -- |A prism that projects a constructor uniquely identifiable by the type of
  -- its field.
@@ -97,6 +98,11 @@ class AsType a s where
 
  {-# MINIMAL (injectTyped, projectTyped) | _Typed #-}
 
+-- |Every type can be treated 'as' itself.
+instance {-# OVERLAPPING #-} AsType a a where
+ injectTyped = id
+ projectTyped = Just
+
 instance Core.Context a s => AsType a s where
  _Typed = normalisePrism (Optic Core.derived)
  {-# INLINE _Typed #-}

generic-optics/test/Spec.hs

@@ -29,6 +29,7 @@ import Optics.Core
 -- This is sufficient at we only want to test that they typecheck
 import Test24 ()
 import Test25 ()
+import Test146 ()
 
 -- import CustomChildren (customTypesTest)
 

generic-optics/test/Test146.hs

@@ -0,0 +1,17 @@
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE FlexibleContexts #-}
+
+module Test146 where
+
+import Control.Monad.Except
+import Data.Generics.Sum
+import GHC.Generics
+
+data Error = Error
+ deriving (Generic)
+
+poly :: (AsType Error e, MonadError e m) => m ()
+poly = undefined
+
+mono :: ExceptT Error IO ()
+mono = poly