Ambiguity in pattern matching syntax

Question

I came across an oddity in the F# pattern matching syntax today, which can lead to apparent failures in the exhaustivity check.

type Thing =
    | This
    | That
    | Other

let useThing =
    function
    | This -> "A"
    | That -> "A"
    | That -> "B" // compiler complains
    | Other -> "B"

In the above scenario the compiler helpfully tells me that the second That rule will never be matched. However, if I had tried to make the code a bit more compact and had written

let useThing =
    function
    | This | That -> "A"
    | That | Other -> "B"    

I do not get any help from the compiler. I think the reason is that | This | That ->. "A" is not a shortcut for | This -> "A" | That -> "A", even though it looks very much like it is (and I've seen many code samples that treat it as such). Instead, from what I can find, the pipe symbol is used both to separate individual patterns, and also as OR pattern.

This is not a big issue for most DUs, but I encountered the problem when mapping a DU with a large number of cases into another DU with a small number of cases. My attempt to use the shortcut syntax caused a bug.

So my questions are:

1. Is my interpretation correct?
2. Is there any workaround apart from listing each pattern on a separate line?

Answer 1

Your interpretation is correct.

By leaving out the actions for the first This and second That you are creating an OR pattern as described in Pattern Matching (F#)

To me this is slightly confusing, too, since the logical 'or' is || in F#. And while it is easy to see the first bar as new alternative and second bar as or in your formatting it becomes less obvious in

let useThing =
    function
    | This
    | That -> "A"
    | That
    | Other -> "B"

However the compiler can tell whether a whole pattern is useless but it cannot simplify a pattern. That | Other has a valid match and is therefore not redundant as considered by the compiler.
You can think of much more involved patterns where it would be not at all clear if parts can be left out or how to simplify them.