Can someone explain the traverse function in Haskell?

Question

I am trying and failing to grok the traverse function from Data.Traversable. I am unable to see its point. Since I come from an imperative backgrou

Answer 1

It's kind of like fmap, except that you can run effects inside the mapper function, which also changes the result type.

Imagine a list of integers representing user IDs in a database: [1, 2, 3]. If you want to fmap these user IDs to usernames, you can't use a traditional fmap, because inside the function you need to access the database to read the usernames (which requires an effect -- in this case, using the IO monad).

The signature of traverse is:

traverse :: (Traversable t, Applicative f) => (a -> f b) -> t a -> f (t b)

With traverse, you can do effects, therefore, your code for mapping user IDs to usernames looks like:

mapUserIDsToUsernames :: (Num -> IO String) -> [Num] -> IO [String]
mapUserIDsToUsernames fn ids = traverse fn ids

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There's also a function called mapM:

mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b)

Any use of mapM can be replaced with traverse, but not the other way around. mapM only works for monads, whereas traverse is more generic.

If you just want to achieve an effect and not return any useful value, there are traverse_ and mapM_ versions of these functions, both of which ignore the return value from the function and are slightly faster.