improving error messages

Bulat Ziganshin bulat.ziganshin at
Sat Oct 21 03:46:29 EDT 2006

Hello glasgow-haskell-users,

as you may remember, in GHC survey awkward error messages was named as
one of most serious GHC drawbacks. i propose to start collecting
examples of bad error messages together with what we want to see in
these cases. as first contribution, i've added this text as 

the following program

main = do putChar 'a'

makes this error message::

    Couldn't match expected type `IO' against inferred type `(->) Char'
    Probable cause: `putChar' is applied to too few arguments
    In the expression: putChar

while the 'probable cause' is very helpful, the error message by
itself is hard to understand for novices. i propose in these cases to
work against known value types. in this case, we know that 'putChar'
has type 'Char -> IO ()' and GHC may report smth like this:

'putChar': expression of type 'Char -> IO ()' used in context that
requires expression of 'IO ()' type
Probably, you've omitted parameter of type 'Char' to this expression

comparing this with existing message shows that may be improved:

1) for me, it's still hard to understand what is "expected" and
"inferred" means. i guess that for true beginners it's even harder.
i prefer to see something more English and less Mathematic - it will
be very helpful

2) instead of obscure 'IO' and `(->) Char' types, i strongly prefer to
see more pragmatic 'IO ()' and 'Char -> IO ()' ones

3) 'Probable cause' may be more pleasant by telling position and type
of skipped argument(s)

my second example is almost the same :)

main = do putChar
          putChar 'a'

in this case error message is even worser:

    Couldn't match expected type `(->) Char' against inferred type `IO'
    Probable cause: `putChar' is applied to too many arguments
    In the expression: putChar 'a'

afaiu, here GHC thought that putChar should have 'IO ()' type. or, it
may think that operations in this monad has type 'Char -> IO ()'.
anyway, it will be great if GHC will know that operations in monad
usually has type 'IO a' or 'm a' and try to use this heuristic when
dealing with such errors (different types of statements in 'do' block)    

Best regards,
 Bulat                          mailto:Bulat.Ziganshin at

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