Merge pull request #36 from tfausak/gh-35-fixed-rational

Convert between `Fixed` and `Rational`

src/lib/Witch/Instances.hs

@@ -1,6 +1,7 @@
 {-# OPTIONS_GHC -Wno-orphans #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 
 module Witch.Instances where
@@ -907,6 +908,16 @@ instance From.From Rational Float where
 instance From.From Rational Double where
   from = fromRational
 
+-- | Uses `fromRational` as long as there isn't a loss of precision.
+instance Fixed.HasResolution a => TryFrom.TryFrom Rational (Fixed.Fixed a) where
+  tryFrom = Utility.eitherTryFrom $ \s ->
+    let
+      t :: Fixed.Fixed a
+      t = fromRational s
+    in if toRational t == s
+      then Right t
+      else Left Exception.LossOfPrecision
+
 -- Fixed
 
 -- | Uses 'Fixed.MkFixed'. This means @from \@Integer \@Centi 2@ is @0.02@
@@ -919,6 +930,10 @@ instance From.From Integer (Fixed.Fixed a) where
 instance From.From (Fixed.Fixed a) Integer where
   from (Fixed.MkFixed t) = t
 
+-- | Uses 'toRational'.
+instance Fixed.HasResolution a => From.From (Fixed.Fixed a) Rational where
+  from = toRational
+
 -- Complex
 
 -- | Uses '(Complex.:+)' with an imaginary part of 0.

src/test/Main.hs

@@ -1448,6 +1448,15 @@ main = runTestTTAndExit $ "Witch" ~:
       , f 0.5 ~?= 0.5
       , f (-0.5) ~?= (-0.5)
       ]
+    , "TryFrom Rational (Fixed a)" ~:
+      let f = hush . Witch.tryFrom @Rational @Fixed.Deci in
+      [ hush (Witch.tryFrom @Rational @Fixed.Uni 1) ~?= Just 1
+      , hush (Witch.tryFrom @Rational @Fixed.Uni 1.2) ~?= Nothing
+      , f 0.1 ~?= Just 0.1
+      , f 1.2 ~?= Just 1.2
+      , f 12.3 ~?= Just 12.3
+      , f 0.12 ~?= Nothing
+      ]
 
     -- Fixed
 
@@ -1455,11 +1464,22 @@ main = runTestTTAndExit $ "Witch" ~:
       let f = Witch.from @Integer @Fixed.Deci in
       [ Witch.from @Integer @Fixed.Uni 1 ~?= 1
       , f 1 ~?= 0.1
+      , f 10 ~?= 1
+      , f 120 ~?= 12
       ]
     , "From (Fixed a) Integer" ~:
       let f = Witch.from @Fixed.Deci @Integer in
       [ Witch.from @Fixed.Uni @Integer 1 ~?= 1
+      , f 0.1 ~?= 1
       , f 1 ~?= 10
+      , f 12 ~?= 120
+      ]
+    , "From (Fixed a) Rational" ~:
+      let f = Witch.from @Fixed.Deci @Rational in
+      [ Witch.from @Fixed.Uni @Rational 1 ~?= 1
+      , f 0.1 ~?= 0.1
+      , f 1 ~?= 1
+      , f 12 ~?= 12
       ]
 
     -- Complex