in OCaml how can I pattern match a function argument while keeping the variable name

I have some code:

let some_func some_arg = (* ... *)

where some_arg really needs to be [| arg1; arg2 |]. I want to pattern match that in the function argument position, like:

let some_func [| arg1; arg2 |] = (* ... *)

but I also want to keep the variable name some_arg in case I want to do something with it directly. How can I do that?

>Solution :

Function arguments are patterns, so you can use the as construct to name parts (or all) of the pattern:

let some_func ([| arg1; arg2 |] as some_arg) = (* . . . *)

However this pattern isn’t exhaustive as it only matches arrays of length 2. So it’s a brittle function definition, and you’ll get a warning from the compiler.

It might be better just to use a match so you can specify the desired behavior when the array is some length other than 2. Or you can use a type that always has exactly 2 components (such as a tuple).