type IsString + test unification error

tst/Core/HindleyMilnerSpec.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE QuasiQuotes #-}
+{-# LANGUAGE QuasiQuotes, OverloadedStrings #-}
 module Core.HindleyMilnerSpec
     ( spec
     )
@@ -6,7 +6,7 @@ module Core.HindleyMilnerSpec
 ----------------------------------------------------------------------------------
 import Core.Syntax
 import Core.TH                  (coreExpr)
-import Core.HindleyMilner       (infer)
+import Core.HindleyMilner       (infer, TypeError(..), HMError)
 import Test.Hspec
 ----------------------------------------------------------------------------------
 
@@ -14,6 +14,14 @@ import Test.Hspec
 spec :: Spec
 spec = do
     it "should infer `id 3` :: Int" $
-        let g = [ ("id", TyVar "a" :-> TyVar "a") ]
+        let g = [ ("id", "a" :-> "a") ]
         in infer g [coreExpr|id 3|] `shouldBe` Right TyInt
 
+    it "should not infer `id 3` when `id` is specialised to `a -> a`" $
+        let g = [ ("id", ("a" :-> "a") :-> "a" :-> "a") ]
+        in infer g [coreExpr|id 3|] `shouldSatisfy` isUntypedVariableErr
+
+isUntypedVariableErr :: HMError a -> Bool
+isUntypedVariableErr (Left (TyErrCouldNotUnify _ _)) = True
+isUntypedVariableErr _                               = False
+