Cryptic type error with Url.Parser

I’m trying to extract the first element from the path of a URL using the 0.19 Url.Parser. At first I tried this:

pathHead =
    Url.fromString ""
        |> Maybe.andThen (Url.Parser.parse Url.Parser.string)

but that gives me Nothing. So I reasoned that maybe I needed to tell the parser that I was expecting a second element to the path, so I tried…

pathHead =
    Url.fromString ""
        |> Maybe.andThen (Url.Parser.parse (Url.Parser.string </> Url.Parser.string))

This fails to compile with a Type Mismatch but then says that the type I gave it is the same as the type it requires?? Specifically:

Type Mismatch
Line 76, Column 52
The 1st argument to `parse` is not what I expect:

76|                |> Maybe.andThen (Url.Parser.parse (Url.Parser.string </> Url.Parser.string))
This argument is:

    Url.Parser.Parser (String -> String) String

But `parse` needs the 1st argument to be:

    Url.Parser.Parser (String -> String) String


  1. What do I do to actually extract the first element of the path?

  2. Is this a bug in the compiler’s error message generator?

It’s almost there:

pathHead : Maybe String
pathHead =
    Url.fromString ""
        |> Maybe.andThen (Url.Parser.parse (Url.Parser.string </> Url.Parser.s "myKey"))

should do the trick. Not sure how to make sense of the compilation error tho…

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