Using generics in F# to create an EnumArray type
- by Matthew
I've created an F# class to represent an array that allocates one element for each value of a specific enum. I'm using an explicit constructor that creates a dictionary from enum values to array indices, and an Item property so that you can write expressions like:
let my_array = new EnumArray<EnumType, int>
my_array.[EnumType.enum_value] <- 5
However, I'm getting the following obscure compilation error at the line marked with '// FS0670' below.
error FS0670: This code is not sufficiently generic.
The type variable ^e when ^e : enum<int> and ^e : equality
and ^e : (static member op_Explicit : ^e -> int)
could not be generalized because it would escape its scope.
I'm at a loss - can anyone explain this error?
type EnumArray< 'e, 'v when 'e : enum<int>
and 'e : equality
and ^e : (static member op_Explicit : ^e -> int) > =
val enum_to_int : Dictionary<'e, int>
val a : 'v array
new() as this =
{
enum_to_int = new Dictionary<'e, int>()
a = Array.zeroCreate (Enum.GetValues(typeof<'e>).Length)
}
then
for (e : obj) in Enum.GetValues(typeof<'e>) do
this.enum_to_int.Add(e :?> 'e, int(e :?> 'e))
member this.Item
with get (idx : 'e) : 'v = this.a.[this.enum_to_int.[idx]] // FS0670
and set (idx : 'e) (c : 'v) = this.a.[this.enum_to_int.[idx]] <- c
