defined-or.md

Default Values in Elm

Using Maybe

In Elm, optional values are handled with Maybe. The following functions show a simple implementation of optional parameters:

combineOpts : Maybe String -> Maybe String -> String -> String -> String
combineOpts = prefix suffix s1 s2 =
    (Maybe.withDefault "" prefix) ++ s1 ++ s2 ++ (Maybe.withDefault "" suffix)

combine : String -> String -> String
combine =
    combineOpts Nothing Nothing

combineOpts is a general version of combine with prefix and suffix as optional parameters. combine is the same as combineOpts except with the optional parameters defined as Nothing.

A more concise approach

While the code in combineOpts works, it's a bit verbose and difficult to read. So let's create an operator, defined-or, like the one used in Perl, //, which will do the same as Maybe.withDefault.

We want to be able to use it like:

combineOpts : Maybe String -> Maybe String -> String -> String -> String
combineOpts = prefix suffix s1 s2 =
    (prefix // "") ++ s1 ++ s2 ++ (suffix // "")

That's far more concise. Now all we have to do is write the code for the // operator.

But before we do, let's rewrite a code fragment in prefix order:

    (//) prefix ""

Contrast this with using withDefault:

    Maybe.withDefault "" prefix

Notice how the first 2 parameters are swapped or flipped. So all we have to do is flip the parameters on withDefault and we're done.

Defined-or operator implementation

(//) : Maybe a -> a -> a
(//) =
    flip Maybe.withDefault

Now all you have to do is add this function to your projects and you can have concise default values.