Implement iforM and iforM_ (#262)

[fumieval]

Added iforM and iforM_, indexed variants of forM and forM_

[Shimuuar]

LGTM

Also I vaguely remember that GHC required fully saturated application to inline. So iforM xs won't be inlined since only one parameter is supplied. Consequently it won't be specialized and everything will be slow. I'll try to invent test case for that

[fumieval]

Right, I copypasted the definition of forM and forM_, assuming that they are written this way deliberately. I can update them (including existing forM) easily

[Shimuuar]

LGTM

Sorry. I take that back. :) You forgot Data.Vector.Primitive

[fumieval]

Good catch!

[Shimuuar]

Amended PR LGTM. @lehins @Bodigrim could any of you take a look it just in case. I already almost missed Primitive reexport

No it seems GHC is good with current definition. Toy benchmark sees no difference:

import qualified Data.Vector as V
import Criterion.Main

fun1 :: (Int -> IO ()) -> IO ()
{-# NOINLINE fun1 #-}
fun1 = V.forM_ (V.generate 4000 id)

fun2 :: (Int -> IO ()) -> IO ()
{-# NOINLINE fun2 #-}
fun2 = \f -> V.forM_ (V.generate 4000 id) f

main :: IO ()
main = defaultMain
  [ bench "1" $ whnfIO (fun1 (\i -> return ()))
  , bench "2" $ whnfIO (fun2 (\i -> return ()))
  ]

-O0

benchmarking 1
time                 14.95 μs   (14.88 μs .. 15.04 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 14.96 μs   (14.91 μs .. 15.04 μs)
std dev              210.7 ns   (150.5 ns .. 332.3 ns)
variance introduced by outliers: 10% (moderately inflated)

benchmarking 2
time                 12.20 μs   (12.18 μs .. 12.21 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 12.15 μs   (12.13 μs .. 12.17 μs)
std dev              62.67 ns   (54.52 ns .. 73.09 ns)

-O

benchmarking 1
time                 11.84 μs   (11.83 μs .. 11.85 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 11.85 μs   (11.84 μs .. 11.86 μs)
std dev              45.18 ns   (35.97 ns .. 58.34 ns)

benchmarking 2
time                 13.23 μs   (13.21 μs .. 13.25 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 13.21 μs   (13.20 μs .. 13.23 μs)
std dev              52.67 ns   (40.70 ns .. 70.79 ns)

[Bodigrim]

Great stuff! Sorry for succumbing to unholy nitpicking.

[lehins]

Looks great. All comments are addressed. @fumieval Thank you!

[fumieval]

Thank you guys too for swift feedbacks!