[Haskell-cafe] IO Monad
bjpop at cs.mu.OZ.AU
Wed Jul 20 02:32:38 EDT 2005
On Wed, 2005-07-20 at 00:21 +0200, yin wrote:
> Dinh Tien Tuan Anh wrote:
> > Hi,
> > Could anyone explain for me why its not possible to return a primitive
> > type (such as Integer, String) while doing some IO actions ?
> > e.g: foo :: IO() -> String
> > What does it have to do with "lazy evalution" paradigm ?
> In short, to not break functional aproach. Non-IO functions can't call
> IO functions, because IO functions are evaluated every time you call them.
I prefer to say it another way.
I think you asking for a function like this:
f :: IO a -> a
If so, with this you could write:
someChar :: Handle -> Char
= f (hGetChar handle)
where hGetChar :: Handle -> IO Char
and Handle represents the interface to a file.
This is a big problem for a purely functional language, because it means
someChar is not a function! Given the same Handle argument, successive
calls to someChar might return different results. Functions, by
definition, are not allowed to have this kind of behaviour.
Therefore f is not allowed.
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