F# pattern matching with tuples rule will never be matched

Question

First off I'm new to f#, so maybe the answer is obvious, but I'm not seeing it. So I have some tuples with an id, and a value. I know the id that I'm looking for and I want to select the correct tuple out of three that I'm passing in. I was going to do this with two match statements one nested in the other, but every thing is matching with the first rule. For this example I cut it down to two tuples since this shows my issue. The compiler is giving a "rule will never be matched" warning, but I don't see why.

let selectTuple tupleId tuple1 tuple2 = 
    match tuple1 with
    | (tupleId, _) -> tuple1
    | _ -> tuple2

Any help or suggestions on a better way to do this would be greatly appreciated.

Answer 1

Use a when clause:

let selectTuple tupleId tuple1 tuple2 = 
    match tuple1 with
    | (x, _) when x = tupleId -> tuple1
    | _ -> tuple2

What is happening here is that when you use tupleId as part of the match case, you introduce a new value called tupleId that you can refer to on the right side of the match case. This shadows your function argument.

Since you're effectively only giving a name to the first element of a tuple, any tuple will match the first case, and that's how you get 'rule won't be matched' warning on the second one.