On Tue, 23 Nov 1999, Matthias Riese wrote:
> Hi!
>
> ghc 4.04 refuses to compile this piece of code:
>
> g :: [String] -> String
> g a = a!!0
>
> f :: String
> f = do
> x <- [""]
> return (g x)
>
> error message:
> Couldn't match `Char' against `String'
> Expected type: Char
> Inferred type: String
> In the application `g x'
> In the first argument of `return', namely `(g x)'
The correct version is:
f :: Monad m => m String
f = do
let x = [""]
return (g x)
The <- in do notation extracts the value from the List monad, that is:
do x <- [ "" ]
is equivalent to
let x = ""
The type of f can't be String since the return expression encapsulates the
value in a monad.
Hope that helps
/Stefan
- stefan larsson -- http://www.dtek.chalmers.se/~d95stela -- ...och h�r sen! -
Here come the Martian Martians, and they're riding on their Martian bikes
and we have to find out right now, what kind of ice cream do the Martians like?
