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unhelpful-ghc-error-messages

Unhelpful GHC error messages

“Or perhaps you meant to enable BlockArguments?”

Here’s a simple Haskell program with a silly error. The bar = 3 line is indented one space too few.

main = do
  let foo = 1
     bar = 3

  print (foo + bar)

Under GHC 8.4 I get the helpful error message

bad-block-args.hs:3:6: error: parse error on input ‘bar’
  |
3 |      bar = 3
  |      ^^^

but under GHC 8.6 I get the completely baffling and useless error message

bad-block-args.hs:1:8: error:
    Unexpected do block in function application:
        do let foo = 1
    You could write it with parentheses
    Or perhaps you meant to enable BlockArguments?
  |
1 | main = do
  |        ^^...

No, I didn’t mean to enable BlockArguments. I never saw the point of them in the first place, and now they are providing me with negative value when I’m not even trying to use them. I have had this occur to me a few times in the wild, always in an unwieldy do block that is difficult to understand even when GHC isn’t complaining about a syntax error. I would like GHC’s help here not a reference to an unrelated small syntax extension.

Under GHC 8.10 I get an error message in the correct place, but also the unhelpful and misleading error about enabling BlockArguments. Ah well, that’s better than nothing.

badd.hs:1:8: error:
    Unexpected do block in function application:
        do let foo = 1
    You could write it with parentheses
    Or perhaps you meant to enable BlockArguments?
  |
1 | main = do
  |        ^^...

badd.hs:3:10: error:
    parse error on input ‘=’
    Perhaps you need a 'let' in a 'do' block?
    e.g. 'let x = 5' instead of 'x = 5'
  |
3 |      bar = 3
  |          ^