Collect result from two monadic actions

Question

I'm trying to tidy up the following into a nice one-liner:

(r1,r2) <- do
  r1 <- action1
  r2 <- action2
  return (r1,r2)

This snippet is itself inside a do-block. By looking at the type of (,) :: a -> b -> (a, b), I thought to use liftM2 (,) :: Monad m => m a -> m b -> m (a, b). Now I can write it as

(r1,r2) <- liftM2 (,) action1 action2

which is less repetitive, but doesn't look very intuitive. What I'm wondering, and can't seem to find, is whether there is a standard combinator to join these actions and tuple-up their results?

`liftA2 (,)` is the standard combinator. [I call it `zipA`](http://stackoverflow.com/a/36480067). — Benjamin Hodgson, Mar 16 '17 at 12:56

ZhekaKozlov · Answer 1 · 2017-03-16T13:08:39.247

4

Actually, liftM2 is intuitive but it needs some time to get used to. So, the answer is yes, liftM2 or liftA2 are the standard combinators for your case. If you really want a readable solution, there are a number of other options:

<*> operator: (,) <$> action1 <*> action2
MonadComprehensions extension: [(r1, r2) | r1 <- action1, r2 <- action2]

edited Mar 16 '17 at 13:08

answered Mar 16 '17 at 13:04

ZhekaKozlov

36,558
20
126
155

I like the MonadComprehensions approach - reads like a clean do-block. – jarmond Mar 16 '17 at 15:01

Collect result from two monadic actions

1 Answers1