Difference between revisions of "How to get rid of IO"

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(Small text about that really FAQ)
(Small text about that really FAQ)
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Revision as of 09:08, 28 April 2009


I have something of type IO a, but I need something of type a How can I get that?


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 (>>=). If that scares you, you can hide that in the do notation, which will then look quite conveniently as:

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