Question

I have checked everywhere and I can’t seem to locate the problem. The compiler is giving me this error: “error FS0039: The value or constructor 'dotProduct' is not defined“. But dotProduct is currently defined.

Visual Studio 2010 is also highlighting the second let (let rec dotProductAux list1 list2 acum =) saying that the expression is unfinished.

let dotProduct list1 list2 =
    let rec dotProductAux list1 list2 acum =
        match list1 ,list2 with
          | [],l | l,[] -> acum
          | head1 :: tail1, head2 :: tail2 -> let updated = (head1 * head2) + acum
                                              (dotProductAux tail1 tail2 updated)

This code multiplies and adds two list like this:

dotProduct [1;4;7] [3;4;1];;   //(1*3) + (4*4) + (7*1)

I’m fairly new to F# and can’t seem to get this code right. Any help?

Answer 1

The body of dotProduct contains the definition of dotProductAux, but no actual expression. You need to actually call dotProductAux (i.e. you’re missing the call dotProductAux list1 list2 0 after the let rec).

Furthermore the case | [],[]| l,[] -> accum will cause an error because the second pattern binds the variable l, while the first does not. You can fix that by replacing l with _, since you don’t actually need it. (Apparently you’ve already fixed this in an edit).