I'm trying to create a functor that makes a polynomial ring out of a ring. My underlying type, Ring_elt, has the following signature:
module type Ring_elt = sig
type t
val add : t -> t -> t
val mul : t -> t -> t
val zer : t
val one : t
val neg : t -> t
end;;
My polynomial functor looks like:
module Make_Poly2(Underlying:Ring_elt) = struct
type t = Poly of Underlying.t list
let rec create lst =
match List.rev lst with
| Underlying.zer :: tl -> create List.rev tl
| _ -> Poly of lst
end;;
(so the 'create' function should take a list, remove the leading zeros, and then return the polynomial of the result). However, I get a syntax error and utop underlines the "zer" after "Underlying."
By comparison, the following code (for making integer polynomials) works:
module Make_int_poly = struct
type t = Poly of int list
let rec create lst =
match List.rev lst with
| 0 :: tl -> create (List.rev tl)
| _ -> Poly lst
end;;
Any idea what's going on?
Jeffrey's answer is good but instead of correcting it with an if construction, what you should do is the following : use algebraic data types
Instead of writing
val zer : t
val one : t
You could write
module type Ring_elt = sig
type t = Zer | One | Num of t
val add : t -> t -> t
val mul : t -> t -> t
val neg : t -> t
end
module Make_int_poly = struct
type t = Poly of int list
let rec create lst =
match List.rev lst with
| Underlying.Zer :: tl -> create (List.rev tl)
| _ -> Poly lst
end
It's a much better way of doing it since you can easily pattern match on it and even add some constants to your type t without problems.