From dd1b3d4d7c8dd7bbffd2e1f5c20c4d018b55ba74 Mon Sep 17 00:00:00 2001
From: Evan Czaplicki <info@elm-lang.org>
Date: Fri, 21 Feb 2014 16:21:00 -0500
Subject: [PATCH] Restrict list interpolation to numbers!

---
 compiler/Type/Constrain/Expression.hs | 8 ++++----
 compiler/Type/Type.hs                 | 5 +++++
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/compiler/Type/Constrain/Expression.hs b/compiler/Type/Constrain/Expression.hs
index 21389e5..aa5df70 100644
--- a/compiler/Type/Constrain/Expression.hs
+++ b/compiler/Type/Constrain/Expression.hs
@@ -36,10 +36,10 @@ constrain env (A region expr) tipe =
           | otherwise           -> return (name <? tipe)
 
       Range lo hi ->
-          exists $ \x -> do
-            clo <- constrain env lo x
-            chi <- constrain env hi x
-            return $ and [clo, chi, list x === tipe]
+          existsNumber $ \n -> do
+            clo <- constrain env lo n
+            chi <- constrain env hi n
+            return $ and [clo, chi, list n === tipe]
 
       ExplicitList exprs ->
           exists $ \x -> do
diff --git a/compiler/Type/Type.hs b/compiler/Type/Type.hs
index 023ecd3..55ec193 100644
--- a/compiler/Type/Type.hs
+++ b/compiler/Type/Type.hs
@@ -141,6 +141,11 @@ exists f = do
   v <- liftIO $ var Flexible
   ex [v] <$> f (VarN v)
 
+existsNumber :: Error e => (Type -> ErrorT e IO TypeConstraint) -> ErrorT e IO TypeConstraint
+existsNumber f = do
+  v <- liftIO $ var (Is Number)
+  ex [v] <$> f (VarN v)
+
 
 instance Show a => Show (UF.Point a) where
   show point = unsafePerformIO $ fmap show (UF.descriptor point)