feat(type-infer): 💄 Improve output of NotFunctionType error

#29

src/Elara/TypeInfer/Error.hs

@@ -77,6 +77,7 @@ data TypeInferenceError where
         (Context SourceRegion) ->
         TypeInferenceError
     NotFunctionType ::
+        SourceRegion ->
         SourceRegion ->
         (Type SourceRegion) ->
         TypeInferenceError
@@ -277,4 +278,14 @@ instance ReportableError TypeInferenceError where
                 [(sourceRegionToDiagnosePosition loc, Where "Referenced here")]
                 []
     report (KindInferError e) = report e
+    report (NotFunctionType loc appLoc a) = do
+        writeReport $
+            Err
+                Nothing
+                (vsep ["Type error: The following type is not a function type:", pretty a])
+                [ (sourceRegionToDiagnosePosition loc, Where "This should be a function type, but isn't")
+                , (sourceRegionToDiagnosePosition appLoc, Where "Applied here")
+                ]
+                [ Hint "Perhaps you applied too many arguments to a function?"
+                ]
     report e = writeReport $ Err Nothing (showColored e) [] []

src/Elara/TypeInfer/Infer.hs

@@ -1480,7 +1480,7 @@ inferApplication Type.Function{..} e = do
 
     pure (e', output)
 inferApplication Type.VariableType{..} _ = throw (NotNecessarilyFunctionType location name)
-inferApplication _A _ = throw (NotFunctionType (location _A) _A)
+inferApplication _A _B = throw (NotFunctionType (location _A) (_B ^. sourceRegion) _A)
 
 {- | This corresponds to the judgment:
 > Γ ⊢ e ⇐ A • e ⇒⇒ C ⊣ Δ
@@ -1521,7 +1521,7 @@ inferPatternApplication Type.Function{..} e = do
 
     pure (e', output)
 inferPatternApplication Type.VariableType{..} _ = throw (NotNecessarilyFunctionType location name)
-inferPatternApplication _A _ = throw (NotFunctionType (location _A) _A)
+inferPatternApplication _A _B = throw (NotFunctionType (location _A) (_B ^. sourceRegion) _A)
 
 -- Helper functions for displaying errors