generics - F# Static Member Type Constraints

Question

I'm trying to define a function, factorize, which uses structural type constraints (requires static members Zero, One, +, and /) similar to Seq.sum so that it can be used with int, long, bigint, etc. I can't seem to get the syntax right, and can't find a lot of resources on the subject. This is what I have, please help.

let inline factorize (n:^NUM) =
    ^NUM : (static member get_Zero: unit->(^NUM))
    ^NUM : (static member get_One: unit->(^NUM))
    let rec factorize (n:^NUM) (j:^NUM) (flist: ^NUM list) = 
        if n = ^NUM.One then flist
        elif n % j = ^NUM.Zero then factorize (n/j) (^NUM.One + ^NUM.One) (j::flist)
        else factorize n (j + ^NUM.One) (flist)
    factorize n (^NUM.One + ^NUM.One) []

Answer 1

Here's how I'd write it:

module NumericLiteralG = begin
  let inline FromZero() = LanguagePrimitives.GenericZero
  let inline FromOne() = LanguagePrimitives.GenericOne
end

let inline factorize n = 
  let rec factorize n j flist =  
    if n = 1G then flist 
    elif n % j = 0G then factorize (n/j) j (j::flist) 
    else factorize n (j + 1G) (flist) 
  factorize n (1G + 1G) [] 

The type inferred for factorize here is way too general, but the function will work as you'd expect. You can force a more sane signature and set of constraints if you want by adding explicit types to some of the generic expressions:

let inline factorize (n:^a) : ^a list = 
  let (one : ^a) = 1G
  let (zero : ^a) = 0G
  let rec factorize n (j:^a) flist =  
    if n = one then flist 
    elif n % j = zero then factorize (n/j) j (j::flist) 
    else factorize n (j + one) (flist) 
  factorize n (one + one) []