CODEWITHSUNDEEP
functional-programmingocaml
Jackson Tale
Jackson Tale

Reputation: 25812

Semicolon and nested if expressions in OCaml

Well, I have problems in using semicolon (single and double) and nested if else in OCaml.

For example

let union u p q = 
  let rec unionfy id_ary i =
    if i < Array.length id_ary then begin 
      if id_ary.(i) = p then begin 
        id_ary.(i) <- id_ary.(q);
        print_array id_ary 0;
      end
      unionfy id_ary (i + 1);
    end 
    else print_string "end of union";
  in
  unionfy u.id_ary 0;;

The compiler said line 18, characters 29-95: Error: This expression is not a function; it cannot be applied

The line which has problem is if id_ary.(i) = p then begin, but I don't see why.

Also, can anyone tell me more about semicolon thing and nested if else?

here are some questions in my mind:

  1. When do I use single semicolon? If I use it for more than one expressions, will I have to add a double semicolon after the last expression?

  2. Can I use multiple begin end inside nested if?

  3. It seems I don't need to add else if the result is unit and do nothing?

Upvotes: 0

Views: 4669

Answers (1)

nlucaroni
nlucaroni

Reputation: 47934

The problem is end. The entire if expression should return unit in this case, so you need a semi-colon at the end of the expression. The other end doesn't need it since the if expression is continuing with the else clause. Below, I've removed the unnecessary semi-colons and added the remaining one,

let union u p q = 
  let rec unionfy id_ary i =
    if i < Array.length id_ary then begin 
      if id_ary.(i) = p then begin 
        id_ary.(i) <- id_ary.(q);
        print_array id_ary 0
      end;
      unionfy id_ary (i + 1)
    end 
    else print_string "end of union"
  in
  unionfy u.id_ary 0;;

EDIT: The 'rule' is really the definition of the semi-colon in OCaml. It separates sequential expressions that return unit. The content between begin ... end is a singular expression. The entire if expression is also an expression, but made up of multiple expressions. So the two statements contained in the first if statement are,

  if id_ary.(i) = p then begin ... end;
  unionfy id_ary (i + 1)

Upvotes: 5

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