Suggests -fglasgow-exts for contexts-differ-in-length error

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index 5fcaea6..169cf7b 100644 (file)
--- a/compiler/typecheck/TcUnify.lhs
+++ b/compiler/typecheck/TcUnify.lhs
@@ -841,7 +841,8 @@ unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
 -- Acutal and expected types
 unifyTheta theta1 theta2
   = do { checkTc (equalLength theta1 theta2)
-                 (ptext SLIT("Contexts differ in length"))
+                 (vcat [ptext SLIT("Contexts differ in length"),
+                        nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
        ; uList unifyPred theta1 theta2 }
 
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