Dfyjkt

new to Ocaml. I've no clue what is happening here and I've been trying to solve this for maybe 2 hours.

Here is the code:

let hailstorm n = 
 match n with
 | (mod n 2 == 0) -> (n/2)
 | (mod n 2 == 1) -> (3*n+1);;

When I try to compile it says:

File "./x.ml", line 3, characters 11-12:Error: Syntax error: ')' expected
File "./x.ml", line 3, characters 6-7:
Error: This '(' might be unmatched

The keyword mod is a binary operator (like lsl, lsr, asr, land, lor, lxor and or) . For instance,

let zero = 2 mod 2

Binary operator can be transformed into standard function by wrapping them around parentheses,

let zero = (mod) 2 2

this is why the parser is expecting a closing parenthesis after (mod .

Then, you pattern matching is wrong because n mod 2 == 0 is an expression, not a pattern (and you should use structural equality = rather than physical equality ==):

let f n = match n mod 2 with
| 0 -> ...
| _ -> ...

or

let f n = match n mod 2 = 0 with
| true -> ...
| false -> ...

which is probably simpler with an if ... then ... else ... .