Add some lossy instances

src/lib/Witch/Instances.hs

@@ -643,6 +643,9 @@ instance TryCast.TryCast Double Rational where
   tryCast = maybeTryCast $ \s ->
     if isNaN s || isInfinite s then Nothing else Just $ toRational s
 
+instance Cast.Cast Double Float where
+  cast = realToFrac
+
 -- Ratio
 
 instance Integral a => Cast.Cast a (Ratio.Ratio a) where
@@ -652,6 +655,12 @@ instance (Eq a, Num a) => TryCast.TryCast (Ratio.Ratio a) a where
   tryCast = maybeTryCast $ \s ->
     if Ratio.denominator s == 1 then Just $ Ratio.numerator s else Nothing
 
+instance Cast.Cast Rational Float where
+  cast = fromRational
+
+instance Cast.Cast Rational Double where
+  cast = fromRational
+
 -- Fixed
 
 instance Cast.Cast Integer (Fixed.Fixed a) where

src/test/Main.hs

@@ -1349,6 +1349,12 @@ main = Hspec.hspec . Hspec.describe "Witch" $ do
       test $ f (1 / 0) `Hspec.shouldSatisfy` Either.isLeft
       test $ f (-1 / 0) `Hspec.shouldSatisfy` Either.isLeft
 
+    Hspec.describe "Cast Double Float" $ do
+      let f = Witch.cast @Double @Float
+      test $ f 0 `Hspec.shouldBe` 0
+      test $ f 0.5 `Hspec.shouldBe` 0.5
+      test $ f (-0.5) `Hspec.shouldBe` (-0.5)
+
     -- NonEmpty
 
     Hspec.describe "TryCast [a] (NonEmpty a)" $ do
@@ -1376,6 +1382,18 @@ main = Hspec.hspec . Hspec.describe "Witch" $ do
       test $ f 0 `Hspec.shouldBe` Right 0
       test $ f 0.5 `Hspec.shouldSatisfy` Either.isLeft
 
+    Hspec.describe "Cast Rational Float" $ do
+      let f = Witch.cast @Rational @Float
+      test $ f 0 `Hspec.shouldBe` 0
+      test $ f 0.5 `Hspec.shouldBe` 0.5
+      test $ f (-0.5) `Hspec.shouldBe` (-0.5)
+
+    Hspec.describe "Cast Rational Double" $ do
+      let f = Witch.cast @Rational @Double
+      test $ f 0 `Hspec.shouldBe` 0
+      test $ f 0.5 `Hspec.shouldBe` 0.5
+      test $ f (-0.5) `Hspec.shouldBe` (-0.5)
+
     -- Fixed
 
     Hspec.describe "Cast Integer (Fixed a)" $ do