haskelllambdaeither

Mapping an Either list to integers

I am trying to do the following:

processRights :: [Either a Int] -> Int
processRights xs = map (\Right x -> x, \Left x -> 0) xs

So, xs is a [Either a Int], and I wish to produce a mapped list of the same length where for each int there is the same int, 0 otherwise.

How can I acomplish that?

Solution

  • You can use the either, id and const functions:

    processRights :: [Either a Int] -> [Int]
processRights = map $ either (const 0) id

    either runs the first function for any Left, the second function for any Right.

    id returns its argument.

    const ignores its second argument and returns its first argument, its intended use is that e.g. const 0 becomes a function that ignores its argument and just returns 0.