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 the type signature says more or less that almost everything must be expected. 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
You can avoid a series of output functions by constructing a complex data structure with non-IO code and output it with one output function.
-- import Control.Monad (replicateM_) replicateM_ 10 (putStr "foo")
putStr (concat $ replicate 10 "foo")
do h <- openFile "foo" WriteMode replicateM_ 10 (hPutStr h "bar") hClose h
can be shortened to
writeFile "foo" (concat $ replicate 10 "bar")
in case of failure.
Since you have now an expression for the complete result as string, you have a simple object that can be re-used in other contexts.E.g. you can also easily compute the length of the written string using
without bothering the file system, again.
2 State monad
3 ST monad
STRef instead of IORef, STArray instead of IOArray
4 Custom type class