Re: Language Design

[qrczak@knm.org.pl (Marcin 'Qrczak' Kowalczyk)]

Fri, 1 Sep 2000 12:57:13 +0100, Dave Berry <dave@kal.com> pisze:

> Given a value in a monad, e.g. IO v, how can I remove v from the Monad?

For Haskell's IO monad, you don't have a function of type IO v -> v.
IO is carried up to main, the "action of the whole program".

Other monads may or may not provide a similar function.

> Surely this would be required to seamlessly integrate stateful and
> functional code?

In a pure language it is not possible to create a function which does
I/O when applied. If an operation may do I/O, it is reflected in its
type and usage. It may be considered a good thing, and is practically
a must in a lazy language.

In some Haskell implementations there is monad called ST (state
transformer) which provides mutable references and arrays, and can
be wrapped in a pure function. Not all stateful code requires IO.

Actually in some implementations there is unsafePerformIO :: IO a -> a
but it's, well, unsafe. It's sometimes very useful for real life
problems.

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