-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Here is return's type signature: return :: Monad m => a -> m a What you are doing with the do notation can also be expressed as ioStr
= (\str -> return str).
do notation and bind both require you to have a value that has the same monad as before. Steven Williams My PGP Key: http://pgp.mit.edu/pks/lookup?op=get&search=0xCACA6C74669A54 FA On 10/06/15 12:35, Mike Houghton wrote:
Hi,
I’ve been tryimg to write a function with signature
asString :: IO String -> String
Does someone please have the patience to explain to me what the compiler error messages really mean for these two attempts and exactly what I’m doing (!!!) If I *do not* give this function any type signature then it works i.e..
asString ioStr = do str <- ioStr return $ str
and the compiler tells me its signature is
asString :: forall (m :: * -> *) b. Monad m => m b -> m b
which, at this stage of my Haskell progress, is just pure Voodoo. Why isn’t it’s signature asString :: IO String -> String ?
Another naive attempt is asString ioStr = str where str <- ioStr
and then compiler says parse error on input ‘<-’
Many Thanks
Mike
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