View source for How to get rid of IO

== Question ==

I have something of type <hask>IO a</hask>, but I need something of type <hask>a</hask>
How can I get that?

== 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.
This is achieved by plugging together IO functions and non-IO functions
using atomic combinator functions like <hask>(>>=)</hask>.
If that scares you, you can hide that in the [[do notation]],
which will then look quite conveniently as:
<haskell>
do text <- readFile "foo"
   writeFile "bar" (someComplicatedNonIOOperation text)
</haskell>
Btw. using the combinators this would look like
<haskell>
writeFile "bar" . someComplicatedNonIOOperation =<< readFile "foo"
</haskell>

=== What we didn't tell you at the beginning ===

Btw. The function that answers your initial question is <hask>unsafePerformIO</hask>.
It is however not intended for conveniently getting rid of the <hask>IO</hask> 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 <hask>unsafe</hask> part of the name.
(Some library writers have abused that name component for [[partial function]]s. Don't get confused!)
You will only need this in rare cases and only experienced programmers shall do this.

== See also ==

* [[Avoiding IO]] - Avoiding IO in the first place is a good thing, and we tell you how to achieve that
* http://www.haskell.org/wikisnapshot/ThatAnnoyingIoType.html
* http://www.haskell.org/wikisnapshot/UsingIo.html

[[Category:FAQ]]