Should an if statement be associated with else statement in ocaml? - if-statement

Question

In ocaml, I want to have many nested if statements and a return value on each of the conditions. The code is becoming complicated like this.
let func arg1 arg2 =
if condition1 then arg1+arg2
else
(
//code1
if condition2 then arg1*arg2
else
(
//code2
if condition3 then arg1+arg2
else
(
//code3
)
)
)
Instead of such nested statements can I have code like this?
let func arg1 arg2 =
if condition1 then arg1+arg2
//code1
if condition2 then arg1*arg2
//code2
if condition3 then arg1+arg2
//code3

Answer 1

You can use an if statement without a else if it returns a value of type unit (basically when it only does something).
if condition then print_int 3
However, in your case, you want to return a value of type int and as such the else branch is mandatory. It can nonetheless be shortened using else if statements.
if condition1 then arg1+arg2
else if condition2 then arg1*arg2
else if condition3 then arg1+arg2
else arg1
Note that once again, you need to use else at the end.
Pattern-matching can also be extended to verify some conditions using when clauses:
match 3 with
| 0 -> 0
| 1 -> 1
| x when x mod 2 = 0 -> x/2
| x when x mod 3 = 0 -> x/3
| x -> x

Answer 2

OCaml is a strongly and statically typed language, which means that the type of every expression is checked at compile-time.
Take a look at the following snippet.
if condition then true_value else false_value
During compilation, the type-checker will check for the following:
condition must have type bool;
true_value must have the same type as false_value;
The whole expression has the same type as true_value and false_value.
If any one of these statements is not true, then the compilation fails with a type error.
Now, let's take a look at a if statement, without a else.
if condition then true_value
If the condition is false, then the expression evaluates to (), the only value of type unit. Using statements 2 and 3 from before, the only type that true_value can have here is unit. That means that you can't use int or string or anything as the true_value.
Usually, deeply nested if-else statements is considered a code smell: it may indicate that your code needs refactoring. For instance, OCaml offers pattern-matching. Depending on what your actual code looks like, it may be the way to go.

Answer 3

Just fyi, you don't need all those parentheses. Anything that comes after an else is considered to be a single expression in the 'else' branch. You could write your code like
let func arg1 arg2 =
if condition1 then arg1+arg2 else
(* code1 *)
if condition2 then arg1*arg2 else
(* code2 *)
if condition3 then arg1+arg2 else
(* code2 *)