• Ömer Sinan Ağacan's avatar
    Slighly improve infix con app pattern errors · cb6d8589
    Ömer Sinan Ağacan authored
    Given this program:
    
        main = do
          f $ do
            a <- return 3
              c <- do
              return 5
    
    GHC previously gave this error message:
    
        Main.hs:2:7: error:
            Parse error in pattern: do a <- return 3 c
            Possibly caused by a missing 'do'?
          |
        2 |   f $ do
          |       ^^...
    
    What happened is GHC considered the whole `f $ do a <- return 3 c` as a
    pattern. When parsed as an expression it becomes an infix application of
    `($)`, and GHC checks left and right hand sides before checking if `($)`
    is a valid infix constructor name, and shows the first error it got.
    
    If instead we first check if the infix op is valid in pattern context,
    the error message becomes much clearer:
    
        Main.hs:2:3: error:
            Parse error in pattern: f $ do a <- return 3 c
            Possibly caused by a missing 'do'?
          |
        2 |   f $ do
          |   ^^^^^^...
    
    This may not entirely fix #11188 but I think it's an improvement.
    
    Reviewers: bgamari
    
    Reviewed By: bgamari
    
    Subscribers: rwbarton, thomie, carter
    
    GHC Trac Issues: #11188
    
    Differential Revision: https://phabricator.haskell.org/D4497
    cb6d8589
all.T 5.29 KB