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Avoiding IO

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(longer introduction)

Revision as of 12:00, 25 December 2008

Haskell requires an explicit type for operations involving input and output. This way it makes a problem explicit, that exists in every language: Input and output functions can have so many effects, that it is hard to combine them. It is hard to test them, because they can in principle depend on every state of the real world. Thus in order to maintain modularity you should avoid IO whereever possible.

It is too tempting to get rid of IO by

but we want to present some clean techniques to avoid IO.


1 Lazy construction

map putStr vs. putStr concat

2 State monad


3 ST monad

STRef instead of IORef, STArray instead of IOArray

4 Custom type class

example getText