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.

forLoopState :: Monad m => a -> (a -> Bool) -> (a -> a) -> b -> (b -> a -> m b) -> m b

forLoopState start cond inc initialState f: A C-style for loop with starting value, loop condition, incrementor and a state that is threaded through the computation.

forLoopFold :: a -> (a -> Bool) -> (a -> a) -> acc -> (acc -> a -> acc) -> acc

forLoopFold start cond inc acc0 f: A pure fold using a for loop instead of a list for performance.

Care is taken that acc0 not be strictly evaluated if unless done so by f.

numLoop :: (Num a, Ord a, Monad m) => a -> a -> (a -> m ()) -> m ()

numLoop start end f: Loops over a contiguous numerical range, including end.

Does nothing when not start <= end.

It uses (+ 1) so for most integer types it has no bounds (overflow) check.

numLoopState :: (Num a, Eq a, Monad m) => a -> a -> b -> (b -> a -> m b) -> m b

numLoopState start end f initialState: Loops over a contiguous numerical range, including end threading a state through the computation.

It uses (+ 1) so for most integer types it has no bounds (overflow) check.

numLoopFold :: (Num a, Eq a) => a -> a -> acc -> (acc -> a -> acc) -> acc

numLoopFold start end acc0 f: A pure fold over a contiguous numerical range, including end.

It uses (+ 1) so for most integer types it has no bounds (overflow) check.

Care is taken that acc0 not be strictly evaluated if unless done so by f.