Control.Loop

Provides a convenient and fast alternative to the common forM_ [1..n] idiom, which in many cases GHC cannot fuse to efficient code.

Notes on fast iteration:

• For Int, (+1) is almost twice as fast as succ because succ does an overflow check.
• For Int, you can get around that while still using Enum using toEnum . (+ 1) . fromEnum.
• However, toEnum . (+ 1) . fromEnum is slower than succ for Word32 on 64-bit machines since toEnum has to check if the given Int exceeds 32 bits.
• Using (+1) from Num is always the fastest way, but it gives no overflow checking.
• Using forLoop you can flexibly pick the way of increasing the value that best fits your needs.
• The currently recommended replacement for forM_ [1..n] is forLoop 1 (<= n) (+1).

Documentation

forLoop :: Monad m => a -> (a -> Bool) -> (a -> a) -> (a -> m ()) -> m ()

forLoop start cond inc f: A C-style for loop with starting value, loop condition and incrementor.