Functions inside case statements

[joshhornby]

If you try and use a function inside a case statement the results are potentially unexpected

decoder : Decode.Decoder String
decoder =
    Decode.at [ "example", "foo" ] Decode.string
        |> Decode.andThen
            (\string ->
                case string of
                    "test" ->
                        Decode.succeed "Working"

                    wrappedInAFunc ->
                        Decode.succeed "Working"

                    _ ->
                        Decode.fail <| "Invalid " ++ string
            )

wrappedInAFunc : String
wrappedInAFunc =
    "test"

Will throw:

The following pattern is redundant. Remove it.Any value with this shape will be handled by a previous pattern.

This is because you can't call a function as you need to use a 'pattern'.

Maybe the error message can check if a function is being used, and if so warn the user about what they are doing as well as telling them the pattern is redundant?

Example Ellie: https://ellie-app.com/jBM3qQDFMa1/0

[evancz]

It's redundant because pattern matching does not work like in Erlang or Elixir.

In the next release of the compiler, this will be caught because the compiler disallows variable name shadowing. I think that will also be helpful for folks coming from Erlang and Elixir who are expecting pattern matches to work different.

I recommend asking around on the slack channel for further clarification in the meantime. And thanks for the report!