Better error message when attempting Haskell-style pattern matching

[Gaelan]

Quick Summary: When attempting to do Haskell-style pattern matching (multiple function declarations instead of one declaration with a case), the error message could be more helpful.

SSCCE

-- It's debatable whether this function is a good idea, but that's besides the point.
expect : String -> Maybe a -> a
expect _ (Just x) = x
expect msg Nothing = Debug.todo msg

This file has multiple `expect` declarations. One here:

3| expect msg Nothing = Debug.todo msg
   ^^^^^^
And another one here:

2| expect _ (Just x) = x
   ^^^^^^
How can I know which one you want? Rename one of them!

• Elm: whatever's currently on https://elm-lang.org/try

Additional Details

I learned Haskell before I learned Elm. Elm's similar enough that generally I can just write the same code I'd write in Haskell, and it works; when that's not the case, it's easy to get very confused (but you do know which one I want! they have distinct patterns!). Given that the mistake above is, I imagine, fairly common for (a certain type of) new Elm programmers, it'd be great if we got a more specific message, along the lines of "if you're trying to match on multiple patterns, use a single declaration and case" would be quite helpful.

[github-actions]

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