Pattern matching with guard clauses

[Sathish Kumar]

Hi all,

Does Elm allow pattern matching with a guard clause?

I could not find something like this in syntax (http://elm-lang.org/docs/syntax)

take k xs =

  case (k, xs) of

    (k, xs) if k == 0 -> Ok []

    (k, xs) if k < 0 -> Err ("negative index")

    (k, []) -> Err "insufficient elements)

    (k, xs) -> ....

[Joey Eremondi]

No, Elm does not currently allow this.

In particular, Elm requires all matches to be exhaustive, and such clauses would make it much more difficult to analyze exhaustiveness.

There have been some requests to have it implemented, but it's definitely not on the top of the list for future Elm features.

Good luck!

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[Max Goldstein]

You either want to place the if-statement as the result of the pattern match, or to call out to a function.