How to get rid of IO
Question
I have something of type IO a, but I need something of type a
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.
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"
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.
See also
- Introduction to IO
- Avoiding IO - Avoiding IO in the first place is a good thing, and we tell you how to achieve that
- Tackling the awkward squad
- http://www.haskell.org/wikisnapshot/ThatAnnoyingIoType.html
- http://www.haskell.org/wikisnapshot/UsingIo.html