improving error messages #1

the following program

main = do putChar 'a'
          putChar

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)

Trac metadata

Trac field	Value
Version	6.6
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Unknown
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