OCaml Pattern match with non-constants

Question

Is it possible to do pattern matching on variables instead of constant values:

# let x = 2 in
let y = 5 in
match 2 with
| x -> \"foo\"
| y -> \"bar\"
|         


        

Answer 1

You need when guard:

let x = 2 in
let y = 5 in
match 2 with
| z when z = x -> "foo"
| z when z = y -> "bar"
| _ -> "baz";;

The error messages are very instructive. When you use:

let x = 2 in
...
match 2 with
| x -> "foo"
| ...

the new value x shadows the value x in previous let-bound hence the first error message. Moreover, since the new x matches everything, two below patterns of y and _ are obviously redundant.

Note that matching a constant (match 2 with) is not a good idea.