F# Incomplete pattern matches on this expression when using "when"..Why?

Question

I have this simple F# function:

let compareNum x =
    let y = 10
    match x with
    | _ when x = y -> 0
    | _ when x > y -> 1
    | _ when x < y -> -1

However, F# compiler gives me "Incomplete pattern matches on this expression" warning. In this case, all cases should cover every pattern.

I also see a similar example in "Pattern Matching" section in the 1st edition of Programming F# book by Chris Smith. So something might be changed in the later version of F#?

Voluntarism

The compiler assumes that any code with when guards is an incomplete match. It is inelegant, and produces false positives like you saw. – Orwin

possible duplicate of Incomplete pattern matching a tuple in F# – Orwin

Thanks! I like how one of the answer in the above question said, "In general, it is an anti-pattern to have a when guard in the last pattern." I think that makes sense now. – Voluntarism

This is one of those cases where using "if" is not only more idiomatic--it's also a far better way to code the test. – Erudite

Answer 1

I think the answer to the previous question (and the comments -- "In general, it is an anti-pattern to have a when guard in the last pattern" -- by kimsk) explain the situation.

However, I would not say that having a guard in the last pattern is an anti-pattern - it is the easiest workaround, but I find this somewhat unfortunate, because the when pattern gives you useful information about the values you can expect - and that makes understanding the program easier. Last time I had this problem, I left it there, at least as a comment:

let compareNum x =
  let y = 10
  match x with
  | _ when x = y -> 0
  | _ when x > y -> 1
  | _ (*when x < y*) -> -1

Judi

+1, if you had the equality check -> 0 as the final pattern it might go some way towards preserving readability without the comment - but personally I think I like this better. – Carven