Unclear hint for parsing errors involving incorrect arity in complex types.

[jmbromley]

In Elm 0.19.1 the hint provided when the inferred type doesn't match the arity of the type annotation is:

Hint: It looks like it takes too many arguments. I see 1 extra.

However, when this occurs within a complex type (e.g. as one of the type parameters) this hint isn't too helpful and could even be confusing.

For example, when using the Url.Parser library, the following incorrect code (reduced from the real world problem at https://discourse.elm-lang.org/t/cryptic-type-error-with-url-parser/2214):

module MyParser exposing (..)

import Url.Parser exposing ((</>))

myParser : Url.Parser.Parser (String -> String) String
myParser =
    Url.Parser.string </> Url.Parser.string

triggers the error:

Something is off with the body of the myParser definition:

8| Url.Parser.string </> Url.Parser.string ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The body is:

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

But the type annotation on myParser says it should be:

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

Hint: It looks like it takes too many arguments. I see 1 extra.

But this hint is potentially confusing to the user who may not understand the Url.Parser library that well. In complex cases like this a more directed hint may be more useful, although it is difficult to know exactly what such a hint would look like!

The best I can come up with would be something like:

Hint: The type parameters to myParser don't match what I was expecting. Maybe you used the wrong function in creating it? Or maybe one of those functions required an extra argument?