#10547: feature request: expanding type synonyms in error messages -------------------------------------+------------------------------------- Reporter: osa1 | Owner: Type: feature request | Status: new Priority: low | Milestone: 8.0.1 Component: Compiler (Type | Version: checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | typecheck/should_fail/ExpandSynsFail1,2,3,4 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1016, Wiki Page: | Phab:D2198 -------------------------------------+------------------------------------- Comment (by osa1): Just as a test I added `-fprint-expanded-synonyms` to every `should_fail` test in `typechecker/`. Here's the diff: {{{ --- ./tcfail016.stderr.normalised 2016-05-12 08:00:33.702460710 -0400 +++ ./tcfail016.comp.stderr.normalised 2016-05-12 08:00:33.702460710 -0400 @@ -3,5 +3,8 @@ Couldn't match type ‘(t, Expr t)’ with ‘Expr t’ Expected type: AnnExpr t -> [[Char]] Actual type: Expr t -> [[Char]] + Type synonyms expanded: + Expected type: (t, Expr t) -> [[Char]] + Actual type: Expr t -> [[Char]] Relevant bindings include g :: AnnExpr t -> [[Char]] (bound at tcfail016.hs:8:1) --- ./tcfail068.stderr.normalised 2016-05-12 08:00:38.994460634 -0400 +++ ./tcfail068.comp.stderr.normalised 2016-05-12 08:00:38.994460634 -0400 @@ -13,6 +13,9 @@ at tcfail068.hs:11:10 Expected type: GHC.ST.ST s1 (IndTree s a) Actual type: GHC.ST.ST s1 (STArray s1 (Int, Int) a) + Type synonyms expanded: + Expected type: GHC.ST.ST s1 (STArray s (Int, Int) a) + Actual type: GHC.ST.ST s1 (STArray s1 (Int, Int) a) In the first argument of ‘runST’, namely ‘(newSTArray ((1, 1), n) x)’ In the expression: runST (newSTArray ((1, 1), n) x) --- ./T9774.stderr.normalised 2016-05-12 08:01:14.797460115 -0400 +++ ./T9774.comp.stderr.normalised 2016-05-12 08:01:14.797460115 -0400 @@ -3,6 +3,9 @@ Couldn't match type ‘Char’ with ‘[Char]’ Expected type: String Actual type: Char + Type synonyms expanded: + Expected type: [Char] + Actual type: Char In the first argument of ‘putStrLn’, namely ‘(assert True 'a')’ In the expression: putStrLn (assert True 'a') In an equation for ‘foo’: foo = putStrLn (assert True 'a') }}} I think we can enable this by default as it's both stable and fast now. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10547#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler