OCaml Non-Literals in Match

Question

Match against non-literal values

I in Ocaml (reasonml), I'm able to to match against integer values like,

switch (x) {
| 0 | 1 => "small"
| _ => "large"
}

However, say I now switch my number type to something like Zarith. How do I match against values, like in the above?

Is there a nicer way other than using | x when x == SomeNumberModule.of_int(0) || x == SomeNumberModule.of_int(1) => ...?

Answer 1

Zarith types are abstract. One consequence is that their implementation is not visible by the compiler (outside of the module defining them). Therefore, it is not possible to pattern match on them because this would require to peek at the inner structure of the abstract type. Depending on your use case, you could project the Zarith type to a non-abstract type and pattern match on this projection:

switch (Z.to_int(x)) {
| 0 | 1 => "small"
| exception Overflow => "large"
| _ => "large"
}

or if you are using OCaml with version ≥ 4.07, it is possible to merge the exception and any cases with an or-pattern:

switch (Z.to_int(x)) {
| 0 | 1 => "small"
| exception Overflow | _ => "large"
}

Answer 2

Pattern matching is a compile-time construct. The compiler needs to know all the possible values in order to provide exhaustiveness checking. It can also optimize it really well with all that information at hand.

You can still use runtime values with match/switch, as you have discovered, but have to use when. You can then still use patterns with literals before when and runtime values after, and will get exhaustiveness checking for the pattern part of it. But if you don't use any patterns at all, just guards, it might be better to use an if expression since that better signifies the intent:

if (x == SomeNumberModule.of_int(0) || x == SomeNumberModule.of_int(1)) {
  ...
}