Warning due to semicolon

April 1, 2023

I run a hello world program below

let x=3;

print_string "hello world!";;

and got this warning:

File "hello.ml", line 1, characters 6-7:
1 | let x=3;
          ^
Warning 10 [non-unit-statement]: this expression should have type unit.

How can I fix it?

>Solution :

Recall from the answers to your previous questions that ; is a sequence operator, not a statement terminator. Your code will therefore be interpreted as:

let x = (3; print_string "hello world!")

which is equivalent to:

let x = print_string "hello world!"

Meaning that you’re throwing away the 3, and x will have type unit because that’s what print_string returns. You can "fix it", i.e. get rid of the warning, by using the second form above.