Editing For loop (section)

===1990: Haskell===
{{Further|Haskell}}

In Haskell98, the function ''mapM_'' maps a [[monad (functional programming)|monadic]] function over a list, as
<syntaxhighlight lang="haskell">
mapM_ print [4, 3 .. 1]
-- prints
-- 4
-- 3
-- 2
-- 1
</syntaxhighlight>

The function ''mapM'' collects each iteration result in a list:
<syntaxhighlight lang="Haskell">
result_list <- mapM (\ indx -> do{ print indx; return (indx - 1) }) [1..4]
-- prints
-- 1
-- 2
-- 3
-- 4
-- result_list is [0,1,2,3,4]
</syntaxhighlight>

Haskell2010 adds functions ''forM_'' and ''forM'', which are equivalent to ''mapM_'' and ''mapM'', but with their arguments flipped:
<syntaxhighlight lang="Haskell">
forM_ [0..3] $ \ indx -> do
    print indx
-- prints
-- 0
-- 1
-- 2
-- 3

result_list <- forM ['a'..'d'] $ \ indx -> do
    print indx
    return indx
-- prints
-- 'a'
-- 'b'
-- 'c'
-- 'd'
-- result_list is ['a','b','c','d']
</syntaxhighlight>

When compiled with optimization, none of the expressions above will create lists. But, to save the space of the [1..5] list if optimization is turned off, a ''forLoop_'' function could be defined as

<syntaxhighlight lang="Haskell">
import Control.Monad as M

forLoop_ :: Monad m => a -> (a -> Bool) -> (a -> a) -> (a -> m ()) -> m ()
forLoop_ startIndx test next f = theLoop startIndx
  where
    theLoop indx = M.when (test indx) $ do
        f indx
        theLoop (next indx)
</syntaxhighlight>
and used as
<syntaxhighlight lang="Haskell">
forLoopM_ (0::Int) (< len) (+1) $ \indx -> do
    -- statements
</syntaxhighlight>