Cannot match on negative integer literals

[mgritter]

There doesn't seem to be a way to match on a negative integer. Here's a small reproduction:

module ExampleMatch

let sign = (z:int{z = 0 \/ z = (-1) \/ z = 1})

let example_1 (s:sign) : string =
 match s with
 | (-1) -> "negative"
 | 1 -> "positive"
 | 0 -> "zero"

There is a syntax error in the -1 case, with or without parentheses.

I initially ran into this case while trying to use an optional sign so Some (-1) has the same problem.

[mgritter]

Workaround:

let example_3 (s:sign) : string =
 match s with
 | v when v = (-1) -> "negative"
 | 1 -> "positive"
 | 0 -> "zero"

I'm not sure in what contexts the "match ... when" syntax is supported. I've gotten the error "When clauses are not yet supported in --verify mode; they will be some day" when I attempted to use it previously, so it's not clear what is OK and what isn't.

For example, we can't match on a option sign in this way, that triggers the "not yet supported" error.

[aseemr]

It is a parser issue, negative int literals are not supported as patterns in the parser.

As a workaround, you could also use if then else:

let example_1 (s:sign) : string =
  if s = -1 then "n"
  else if s = 1 then "p"
  else begin
    assert (s == 0); //not necessary, just for illustration
    "z"
  end

let example_2 (s:option sign) : string =
  match s with
  | None -> "none"
  | Some s -> example_1 s

when clauses are not yet supported for all practical purposes. (The typechecker has an internal "lax" mode of operation that does not verify code, that's where when may be used.)