Unused variable vs let () and let _?

Learning
1

Let’s say I have a bunch of nested let…in statements. I noticed that if I have

(* a bunch of let... in above *)
let test = expr1 in
(* a bunch of let ... in below *)

I get this Error (warning 26): unused variable test, yet when I change it to let _ = expr1 in ... or let () = expr1 in ... there are no error messages.

I understand let _ = is pattern matching for anything and let () = is pattern matching for anything of type unit, but does this mean that my expr1 is just being executed anywhere? Is it being called once?

2

Yes, expr1 is being evaluated, but the warning is telling you that its result, while being bound to a variable, is not being used.

As expr1 is of type unit there is no point in binding its result to a variable. The usual way is to use the sequencing operator ; in that case:

...
expr1;
...

If expr1 did return something other than unit then, indeed, let _ = expr1 or let _test = expr1 would be the way to tell the compiler that you want to explicitly ignore its return value. In general preceding a name with an underscore signals the compiler that it should not trigger an “unused” warning for that name.

Cheers,
Nicolas

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3

Cross-posted to ocaml - Unused variable vs let () and let _? - Stack Overflow

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