swift - Providing specialized initializers for a generic struct -

i have simple generic struct defined - requirements stored properties comparable:

struct bounds<a: comparable> {     let lower:     let upper: } 

however, i'd provide couple of specialized initializers struct, use math operations set properties.

    init(value: double, tolerance: percentage) {         self.lower = value * ( 1 - tolerance )         self.upper = value * ( 1 + tolerance )     }      init(value: measurement, tolerance: percentage) {         self.lower = value.value * ( 1 - tolerance )         self.lower = value.value * ( 1 - tolerance )     } 

the result should 2 different structs, double or measurement.

but how do this?

i can't provide specialized init methods in definition compiler complain double not convertible a. ok...

i can't provide init methods in individual extensions constrained specific types (where == double) compiler complains:

same-type requirement makes generic parameter 'a' non-generic

maybe should using protocol both double , measurement conform in initialization, seems odd, since bounds struct should care both conform comparable.

i feel i'm either missing simple, or trying misguided generics. it, so?

not asking for, possible workaround (swift 3):

extension bounds a: floatingpoint {     init(value: a, tolerance: a) {         self.lower = value * ( a(1) - tolerance )         self.upper = value * ( a(1) + tolerance )     } }  let b = bounds(value: 4.0, tolerance: 0.1) print(b.dynamictype) // bounds<double>