Syntax error shows in the wrong position (#984) · Issues · Glasgow Haskell Compiler / GHC

Syntax error shows in the wrong position

This bug concerns the behaviour of GHC on erroneous programs. GHC recognizes the illegal syntax, but shows an unrelated error.

This problem is best illustrated in the following code:

f _ = do
        x <- computation
        case () of
                _ ->
                        result <- computation
                        case () of () -> undefined

The error in this code is the missing "do" keyword on line 5, and the compiler should suspect this when seeing the "<-" on line 6. However, GHC gives the following error:

bug.hs:3:8: Parse error in pattern

This is misleading, since the line number is incorrect, and the syntax error (as well is line 3) is unrelated to patterns.

Trac metadata

Trac field	Value
Version	6.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Linux
Architecture	x86