base: Data.Fixed: make HasResolution poly-kinded (#10055, #15622)

2 files changed, 17 insertions, 6 deletions

diff --git a/libraries/base/Data/Fixed.hs b/libraries/base/Data/Fixed.hs
index 482ec0a694..e99efea256 100644
--- a/libraries/base/Data/Fixed.hs
+++ b/libraries/base/Data/Fixed.hs
@@ -1,5 +1,7 @@
 {-# LANGUAGE Trustworthy #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE FlexibleInstances #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -37,6 +39,7 @@ module Data.Fixed
 ) where
 
 import Data.Data
+import GHC.TypeLits (KnownNat, natVal)
 import GHC.Read
 import Text.ParserCombinators.ReadPrec
 import Text.Read.Lex
@@ -58,7 +61,7 @@ mod' n d = n - (fromInteger f) * d where
     f = div' n d
 
 -- | The type parameter should be an instance of 'HasResolution'.
-newtype Fixed a = MkFixed Integer
+newtype Fixed (a :: k) = MkFixed Integer
         deriving ( Eq  -- ^ @since 2.01
                  , Ord -- ^ @since 2.01
                  )
@@ -71,17 +74,21 @@ conMkFixed :: Constr
 conMkFixed = mkConstr tyFixed "MkFixed" [] Prefix
 
 -- | @since 4.1.0.0
-instance (Typeable a) => Data (Fixed a) where
+instance (Typeable k,Typeable a) => Data (Fixed (a :: k)) where
     gfoldl k z (MkFixed a) = k (z MkFixed) a
     gunfold k z _ = k (z MkFixed)
     dataTypeOf _ = tyFixed
     toConstr _ = conMkFixed
 
-class HasResolution a where
+class HasResolution (a :: k) where
     resolution :: p a -> Integer
 
-withType :: (p a -> f a) -> f a
-withType foo = foo undefined
+-- | For example, @Fixed 1000@ will give you a 'Fixed' with a resolution of 1000.
+instance KnownNat n => HasResolution n where
+    resolution _ = natVal (Proxy :: Proxy n)
+
+withType :: (Proxy a -> f a) -> f a
+withType foo = foo Proxy
 
 withResolution :: (HasResolution a) => (Integer -> f a) -> f a
 withResolution foo = withType (foo . resolution)
@@ -170,7 +177,7 @@ convertFixed :: forall a . HasResolution a => Lexeme -> ReadPrec (Fixed a)
 convertFixed (Number n)
  | Just (i, f) <- numberToFixed e n =
     return (fromInteger i + (fromInteger f / (10 ^ e)))
-    where r = resolution (undefined :: Fixed a)
+    where r = resolution (Proxy :: Proxy a)
           -- round 'e' up to help make the 'read . show == id' property
           -- possible also for cases where 'resolution' is not a
           -- power-of-10, such as e.g. when 'resolution = 128'
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 5256730529..8cb3366dd2 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -21,6 +21,10 @@
     The type argument `r` is marked as `Inferred` to prevent it from
     interfering with visible type application.
 
+  * Make `Fixed` and `HasResolution` poly-kinded.
+
+  * Add `HasResolution` instances for `Nat`s.
+
 ## 4.13.0.0 *TBA*
   * Bundled with GHC *TBA*