OCaml: How can I return the first element of a list and after that remove it from the list?

Question

I tried to use List.hd and List.tl for this task:

let takeCard fst deck =   
fst = List.hd deck   
List.tl deck

List.hd takes two arguments but I don't understand why.

Answer 1

I think there are a couple of misunderstandings here.

First, most types in OCaml are immutable . Unless you use mutable variables you can't "remove it from the list", you can only return a version of the list that doesn't have that first item. If you want to return both things you can achieve that using a tuple .

let takeCard deck = (List.hd deck, List.tl deck)

Second, List.hd only takes one element. OCaml leverages currying . When reading an OCaml type signature the first parameters are what the function takes in and the last parameter is what the function returns. So List.hd's signature 'a list -> 'a means that it takes in a list that contains ( 'a is used as a placeholder) and returns something of the type of stuff the list contains (in this case the first element).