On Wed, 2007-09-05 at 19:50 +0200, Twan van Laarhoven wrote:
Bulat Ziganshin wrote:
Hello Simon,
Wednesday, September 5, 2007, 11:19:28 AM, you wrote:
when you come across a case where GHC produces an unhelpful message, send it in, along with the program that produced it,
i have put such tickets about year ago :) basically, it was about just changing wording: instead of "inferred" write:
Expected type: ... Actual type: ...
This doesn't help enough. What is an 'expected' type? How is it not 'actual'? I want it to be immediatly clear which type is which.
Say I write
"x" ++ 'y' Right now the error is Couldn't match expected type `[Char]' against inferred type `Char' In the second argument of `(++)', namely 'y'
What always confuses me is which of these two types is the parameter I gave, and which is the one expected by the function? Changing 'infered' to 'actual' is an improvement, but it is not enough.
I would suggest:
(++) expects second argument to be of type '[Char]' but was given 'y' of type 'Char'
Anothing thing that would be useful is *why* (++) expects a certian type, say I enter
"x" ++ [1::Int] Instead of the above, the following would be more useful:
the function (++) has type: [a] -> [a] -> [a] the first argument suggests: a = Char the second argument suggests: a = Int
Maybe: In the expression "x" ++ 'y': (++) :: [a] -> [a] -> [a] "x" :: String 'y' :: Char (I expect the whole thing to have type String) or In the expression "x" ++ [1]: (++) :: [a] -> [a] -> [a] "x" :: String [1] :: [Int] (I expect the whole thing to have a type similar to [a]) jcc