F# 通用和类型约束

Question

What is the proper way to write type constraints for a generic type so that we can have its instance passed to the Array.sum function?

This does not work:

let mySum< 'a when 'a : (static member get_Zero: unit -> 'a) and 'a : (static member (+) : 'a -> 'a -> 'a) > (xs: 'a[]) = Array.sum xs

The error message I get for the above is A type parameter is missing a constraint 'when ^a : (static member get_Zero : -> ^a)'

Although I would need a generic type, not a statically resolved one, I did try the following, withouth success:

let mySum< ^a when ^a : (static member get_Zero: unit -> ^a) and ^a : (static member (+) : ^a -> ^a -> ^a) > (xs: ^a[]) = Array.sum xs

(I get the same error message here again). I also tried to inline the function and it did not work either.

Answer 1

This works:

let inline mySum< 'a when 'a : (static member Zero :   ^a) and 'a : (static member (+) : 'a -> 'a -> 'a) > (xs: 'a[]) = Array.sum xs

but don't forget to mark the function inline.