Converting Either a (IO b) to IO (Either a b)

Question

Once again, through some bad, bad programming, I have managed to get myself into an awful situation with types.

Is there any way to convert Either a (IO b) to IO (Either a b)? I know, it's not very good programming to get myself into this situation, so I'd also be open to suggestions on how to avoid situations like this.

Why don't you show us how you [keep getting](http://stackoverflow.com/questions/26051376/how-to-print-the-type-either-string-io-string) in this situation, and maybe we can show you how to avoid it. — AndrewC, Sep 29 '14 at 00:12

rampion · Accepted Answer · 2014-09-29T00:19:59.953

6

Sure!

convert :: Either a (IO b) -> IO (Either a b)
convert = either (return . Left) (fmap Right)

Now, if you wanted to convert IO (Either a b) to Either a (IO b), you'd be in trouble, but you don't, so you're ok.

Let's see how convert works:

either :: (a -> c) -> (b -> c) -> Either a b -> c, so it takes care of pattern matching for us, figuring out whether we have an a or an IO b in your case.
if we have an a, we just need to convert it to a IO (Either a b). The constructor Left :: a -> Either a b does the first part, and IO is a monad, so we can use return to do Either a b -> IO (Either a b).
if we have an IO b, we need to convert it to a IO (Either a b). We could do this with do notation:
```
given iob = do
  b <- iob
  return . Right $ b
```
using return . Right as b -> IO (Either a b). But this is exactly the situation that mapM :: Monad m => (a -> b) -> m a -> m b is for in Monads: given iob = mapM Right iob. Most people don't use mapM, though, as it's just the specialization of fmap to Monads, so we'll use fmap : given iob = fmap Right iob, or, pointfree : given = fmap Right.

edited Sep 29 '14 at 00:19

answered Sep 29 '14 at 00:11

rampion

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``convert`` here is actually a special case of ``sequenceA``, exported from ``Data.Traversable``. – Josh Kirklin Sep 29 '14 at 01:10
1

Good point. With `liftM` used instead of `fmap` it would be a special case of `Data.Traversable.sequence` ;) – David Unric Sep 29 '14 at 02:04
Note that the type is restricted without a good reason. This works for any monad in place of IO. – Ingo Sep 29 '14 at 19:17
3

@scrambledeggs You should really turn that into an answer, even if this answer has already been accepted. – Gabriella Gonzalez Sep 29 '14 at 19:29

Converting Either a (IO b) to IO (Either a b)

1 Answers1