is it possible to create single if (without else) ? It is very usable to can use one if
Read the control structures : conditional section §6.7.2 of the Ocaml manual.
It is only possible to avoid the else when the then part (hence the entire if expression) is of unit type. For example
let x = 3 in
( if x > 0 then Printf.printf "x is %d\n" x );
x + 5
;;
should print x is 3, and return as value 8.
The general rule is that if κ then τ is equivalent to if κ then τ else () hence the "then part" τ has to be of unit type and the "else part" is defaulted to () so the entire if is of unit type.
let x = 3 in ( if x > 0 then "abc" ); x + 7 (*faulty example*)
won't even compile since "abc" is not of unit type (like () is)
You might sometimes use the ignore function (from Pervasives) on the then part to force it to be of unit type (but that is worthwhile only when it has significant side-effects; if you replace "abc" by ignore "abc" then my faulty example would compile, but remains useless).
However, don't forget that Ocaml has only expressions (but no statements at all). Side-effecting expressions are usually of unit type (but you could, but that is usually frowned upon, define a function which computes some non-unit result and has a useful side-effect).