Suppose you have some data such as:

data : List String
data = ["1", "17", "a87", "34", "3b7"]

This was supposed to be a list of integers, but for whatever reason whoever provided you with this data also left in a bunch of garbage like "a87". You're only interested in the strings that are valid integers, and you also want to work with them as integers, not strings. Let's start writing our convert function.

We can convert strings to integers with the function

String.toInt : String -> Maybe Int

and we can use our old friend List.map to apply the conversion to all the elements in the list:

List.map String.toInt data
-- [Just 1, Just 17, Nothing, Just 34, Nothing]

Now we have our integers, wrapped in Just, so all we have to do now is to remove the values that are Nothing.

"I know this one! That's List.filter, right?", you might have said prior to learning the lesson that I am getting to in this post. Let's try.

List.filter isJust (List.map String.toInt data)

There are two problems with this solution:

First of all, isJust is not provided by the core library. We could get it from elm-community/maybe-extra, or we could save ourselves a dependency and just define it ourselves:

isJust maybe =
    case maybe of
        Just _ -> True
        Nothing -> False

Now we can see if it works:

List.filter isJust (List.map String.toInt data)
-- [Just 1, Just 17, Just 34]

We got rid of the bad entries, but we have another problem. All our values are still wrapped up in Just! Each time you would want to use these values, you would have to handle the case of it being Nothing, even though we just made sure all the Nothing-values are gone. We have now introduced an impossible state 💀. What we really want is a function with the type

convert : List String -> List Int

as opposed to what we had, which was:

convert : List String -> List (Maybe Int)

Fortunately the solution is very simple! The core library provides the function which does exactly this:

List.filterMap : (a -> Maybe b) -> List a -> List b

List.filterMap takes a function that produces a Maybe, and applies that functions to all the elements of a list while unpacking the Just-values, and removing the Nothing-values.

Our final convert function can then be defined simply as:

convert = List.filterMap String.toInt

convert data
-- [1, 17, 34]

Conclusion

Error handling in Elm is easy as long as you you have the right functions in your tool belt. With List.filterMap and a few more you will be able to handle anything 🎉