From aac3a0e75509c97e3eabce83aca75393453fde8c Mon Sep 17 00:00:00 2001
From: Taylor Fausak <taylor@fausak.me>
Date: Sat, 10 Jul 2021 15:29:03 -0400
Subject: [PATCH] Add instance for converting from rational into fixed

---
 src/lib/Witch/Instances.hs | 9 +++++++++
 src/test/Main.hs           | 9 +++++++++
 2 files changed, 18 insertions(+)

diff --git a/src/lib/Witch/Instances.hs b/src/lib/Witch/Instances.hs
index a160a52..bc0b958 100644
--- a/src/lib/Witch/Instances.hs
+++ b/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 +908,14 @@ instance From.From Rational Float where
 instance From.From Rational Double where
   from = fromRational
 
+-- | TODO
+instance Fixed.HasResolution a => TryFrom.TryFrom Rational (Fixed.Fixed a) where
+  tryFrom = Utility.eitherTryFrom $ \s ->
+    let t = fromRational s :: Fixed.Fixed a
+    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@
diff --git a/src/test/Main.hs b/src/test/Main.hs
index 2818d4a..9bb1c89 100644
--- a/src/test/Main.hs
+++ b/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