CODEWITHSUNDEEP
haskellmonads
MathematicalOrchid
MathematicalOrchid

Reputation: 62818

Does this function already exist? (Maybe monad)

I find myself writing this:

and_then :: x -> Maybe [x] -> Maybe [x]
x `and_then` mxs = do
  xs <- mxs
  return (x:xs)

Is there some simpler way to achieve the same thing?

Basically this is your standard recursive loop that processes a list - except that processing can fail (which is different from the result merely being an empty list). That means that where I would normally write

z : process xs ys

I now have to write

z `and_then` process xs ys

Since this is a rather common idiom, I was wondering whether some pre-existing function could help me out. (Several Hoogle searches fail to find any functions with types similar to the above.)

Upvotes: 1

Views: 126

Answers (1)

J. Abrahamson
J. Abrahamson

Reputation: 74354

Since your do notation is only invoking the Maybe layer we might think about the (:) operation as being independent of the Maybe's effect—it's just yet another function being passed to the value wrapped in Maybe. That leads us to generalize the code a bit

foo :: (x -> b -> b) -> x -> Maybe b -> Maybe b
foo f x m = do
  b <- m
  return (f x b)

This makes it much more clear that we're just dealing with an fmap

fmap fx m = do b <- m
               return (fx b)

So perhaps the most standard way of writing your desired function is

(z:) <$> process xs ys

Upvotes: 11

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