OCaml: How does List.fold_left work?

Question

I'm trying to find the average of a list of floats.

let avg l =
    List.fold_left ( +. ) 0 l /. List.length l;;

How does List.fold_left work? (Applies the first argument to a list (the third arg)... but then what is the second argument?)

Toplevel returns this error:

Characters 43-44:
        List.fold_left ( +. ) 0 l /. List.length l;;
                             ^
Error: This expression has type int but is here used with type float

What is the preferred way to iterate through a list in OCaml?

Answer 1

The second argument is the initial value of the accumulator. For a left fold, you can visually place it to the left of the list. So for a left fold on [1;2;3;4;5] with an initial value of 0, it works out to:

((((0 + 1) + 2) + 3) + 4) + 5

So in this case, the innermost parenthesis works out to (0 +. 1.0), which won't work in OCaml because 0 is an int and the language doesn't automatically convert it to a float for you.

Answer 2

As Chuck mentioned, there is no automatic conversion between ints and floats. Also, to be a float literal it must have either a decimal point or be in exponential notation.

So to fix your code, you need to add a decimal point to the 0 to make it a float constant; and you also need to convert the length from an int to a float:

let avg l =
    List.fold_left ( +. ) 0. l /. float_of_int (List.length l);;