Question

I’m trying to get the max product of 4 adjacent numbers in an array
this is what I got now:

let max4 line =
    let rec loop acc = function
        |a :: b :: c :: [] -> acc
        |a :: b :: c :: d :: tl -> loop (max(acc, a*b*c*d)) tl
        |_ -> 0
    loop 0 line

I get a compilation error on the max(,) saying:

error FS0001: Type mismatch. Expecting a
‘a but given a
‘a * ‘b -> ‘a * ‘b The resulting type would be infinite when unifying ”a’ and ”a * ‘b -> ‘a * ‘b’

---

Anybody knows what’s wrong in this code ? (or another solution)

Answer 1

Suppose the input is a list of integers:

let max4 line =
    let rec loop acc = function
        | x1::(x2::x3::x4::_ as xs) -> loop (max acc (x1*x2*x3*x4)) xs
        |_ -> acc
    loop System.Int32.MinValue line

You made some mistakes:

• The built-in max function is in the curry form max: 'a -> 'a -> 'a.
• The next case to address in your function should be b::c::d::tl, not tl only.
• The product could be negative, so 0 is not a good starting point. Beware that integer overflow could happen (which I still haven’t addressed in my function).