I seem to remember running into this way back, but not being able to figure out which instance was broken. Glad to see someone tracked it down. On Aug 3, 2015 3:10 PM, "Edward Kmett" wrote:

At the very least, it does seem like we're going to need to do a broader survey of the instances out there, as well as fix a lot more Applicative instances to have a better (*>) first as well as thoroughly document what to do, if we want to proceed on this front.

If we ultimately want to remove mapM from the class to get it a more permissive type signature, and get mapM_ from Foldable to have the more general signature to boot, then we'll need to figure out how to address these concerns.

It still strikes me as the right general direction to go in, but this is troubling.

-Edward

On Mon, Aug 3, 2015 at 12:33 PM, Ben Gamari wrote:

...

Edward Kmett writes:

...

On Tue, May 12, 2015 at 3:58 AM, Simon Marlow wrote:

...

Yes, I'm not really concerned that mapM is a method of Traversable

rather

...

than just being an alias for traverse, but I'm wondering why we define it in the list instance rather than using the default.

We were pretty paranoid about introducing space or time regressions and didn't have a proof that we wouldn't introduce them by changing something there, so we left it alone.

On a related note, D924 [1] proposed that mapM_ be redefined in terms of traverse_. Unfortunately at least one monad in GHC itself was adversely affected [2] by this change, resulting in non-linear complexity in a previously well-behaved function (a minimal demonstration of this can be found below).

We discussed this in the GHC weekly meeting and felt that we should ensure that the libraries group was aware of this issue.

Cheers,

- Ben

[1] https://phabricator.haskell.org/D924 [2] https://ghc.haskell.org/trac/ghc/ticket/10711 [3] Demonstration of regression in complexity of mapM_ when expressed in terms of `traverse_`,

{{{ module Main where

import Control.Monad hiding (mapM_) import Prelude hiding (mapM_)

-- | Testcase derived from Assembler monad in ByteCodeAsm data Assembler a = Thing Int (Int -> Assembler a) | Pure a

instance Functor Assembler where fmap = liftM

instance Applicative Assembler where pure = return (<*>) = ap

instance Monad Assembler where return = Pure Pure x >>= f = f x Thing i k >>= f = Thing i (k >=> f)

-- This is traverse_ mapA_ :: (Foldable t, Monad f) => (a -> f b) -> t a -> f () mapA_ f = foldr ((*>) . f) (pure ())

-- This is the current definition mapM_ :: (Foldable t, Monad f) => (a -> f b) -> t a -> f () mapM_ f = foldr ((>>) . f) (pure ())

test = map (\i->Thing i (const $ return 2)) [0..10000]

doTestM = mapM_ id test doTestA = mapA_ id test

run :: Assembler a -> a run (Thing i f) = run (f i) run (Pure r) = r {-# NOINLINE run #-}

main :: IO () main = print $ run doTestM }}}

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