improving error messages #1
|Reported by:||Bulat Ziganshin <Bulat.Ziganshin@…>||Owned by:||simonpj|
|Type of failure:||None/Unknown||Test Case:|
|Related Tickets:||Differential Revisions:|
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)
Change History (8)
comment:2 Changed 8 years ago by simonmar
- Milestone changed from 6.6.1 to 6.8
- Owner set to simonpj
- Priority changed from low to normal