How to get rid of IO

From HaskellWiki

Revision as of 09:19, 28 April 2009 by Lemming (Talk | contribs)

(diff) ←Older revision | Current revision (diff) | Newer revision→ (diff)

1 Question
2 Answer
- 2.1 What we didn't tell you at the beginning
3 See also

1 Question

I have something of type

IO a

, but I need something of type

a

How can I get that?

2 Answer

You can get rid of it, but we don't tell you how, since it is certainly not what need. It is the special safety belt of Haskell, that you cannot get rid of IO! Nonetheless, the biggest parts of Haskell programs are and should be non-IO functions. Applications using both IO and non-IO functions are written by plugging together these two flavors of functions

using atomic combinator functions like

(>>=)

These combinators are the great and elegant trick that allow to do something useful with IO functions while having all safety properties of a pure functional programming language. If that scares you, you can hide the combinator using the do notation, which will looks quite conveniently like:

do text <- readFile "foo"
   writeFile "bar" (someComplicatedNonIOOperation text)

Btw. using the combinators this would look like

writeFile "bar" . someComplicatedNonIOOperation =<< readFile "foo"

2.1 What we didn't tell you at the beginning

Btw. The function that answers your initial question is

unsafePerformIO

. It is however not intended for conveniently getting rid of the

IO

constructor.

It must only be used to wrap IO functions that behave like non-IO functions,

Since this property cannot be checked by the compiler, it is your task and thus the

unsafe

part of the name.

(Some library writers have abused that name component for partial functions. Don't get confused!) You will only need this in rare cases and only experienced programmers shall do this.