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

问题

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

回答1:

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.

回答2:

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

回答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 *)

来源：https://stackoverflow.com/questions/57041443/should-an-if-statement-be-associated-with-else-statement-in-ocaml

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if-statement

return

ocaml