#11188: Confusing "parse error in pattern" for spurious indentation. -------------------------------------+------------------------------------- Reporter: andreas.abel | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.10.1 (Parser) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4497 Wiki Page: | -------------------------------------+------------------------------------- Changes (by osa1): * differential: => Phab:D4497 Comment: I think the problem here is not that it's suggesting adding more `do`s (it's saying "possibly", meaning the reason may be something else), but it's showing only a small part of the large expression that the parser considers as a pattern. I submitted Phab:D4497 to improve this. Previously the error message was: {{{ Main.hs:2:3: error: Parse error in pattern: putStrLn Possibly caused by a missing 'do'? | 2 | putStrLn "" $ do | ^^^^^^^^^^^ }}} this is not good enough because it doesn't show the whole part that's considered as a pattern. With Phab:D4497: {{{ Main.hs:2:3: error: Parse error in pattern: putStrLn "" $ do a <- return 3 c Possibly caused by a missing 'do'? putStrLn "" $ do a <- return 3 c | 2 | putStrLn "" $ do | ^^^^^^^^^^^^^^^^... }}} It's now clear that left hand side of second `<-` now covers the first `<-` because the whole part is shown. If the error message is still not good enough perhaps we can split the parsers for expressions and patterns (rather than using expression parser for patterns and then doing a check to see whether it's a valid pattern). In that case I think we'd still get a parse error in expression parser while parsing `return 3 c <- do ...` which would raise a better error message I think. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11188#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler