Help me to explain the F# Matrix transpose function

Question

There is a Matrix transpose function:

let rec transpose = function
    | (_::_)::_ as M -> List.map List.head M :: transpose (List.map Li         


        

Answer 1

(_::_)::_ is pattern matching. _ is simply an unused variable. This would be equivalent:

(a::b)::c as M -> List.map List.head M :: transpose (List.map List.tail M)