OCaml - For Loop inside If Statement

Question

Coming from a Java and C background, grasping some concepts of Ocaml has been quite interesting. One such is, getting a/multiple statement to run inside a for loop.

let test_method (x:vector list)(vec:vector) = 
    if List.length x != 0 then
          {a=0.;b=0.} (* Return a vector of 0,0 *)
       else 
          for i = 0 to List.length x do
                        let key =  {a=(List.nth x i).a;b=(List.nth x i).b} 
                done;;

the error that i get is done;; is an unexpected token. Why is this happening? this happens even if i do this

let test_method (x:vector list)(vec:vector) = 
    if List.length x != 0 then
          {a=0.;b=0.} (* Return a vector of 0,0 *)
       else 
          for i = 0 to List.length x do
                        let key =  {a=(List.nth x i).a;b=(List.nth x i).b} in
                        let value = vec_neg key;
                done;;

where vec_neg is a method that works fine that negates the vector type.

Any help would be greatly appreciated.

Answer 1

let expressions must have a body. (i.e. let var = val in body) (except let definitions at the top level of a module, which implicitly use the rest of the module as the body) let creates a local binding which is in scope inside the body. What's the point of doing that if you're not using it (i.e. don't have a body)? Plus every expression in the language has to evaluate to a value. A let expression evaluates to whatever the body evaluates to. That's why it needs a body.