swift - Why does reduce not work on Struct? -

i have following code

struct tab {   var origin:cgfloat   var width:cgfloat   init(width:cgfloat) {     origin = 0     self.width = width   } }  let subtabs = [tab(width:1), tab(width:2)] let w =  reduce(subtabs, cgfloat(0), { $0.width + $1.width }) 

and expect reduce sum widths in struct. raises compiler error (you can test in playground).

the function reduce needs takes 2 arguments: accumulated value , next item in sequence. function expecting 2 sequence items (tab structs). since returning float, accumluted value float, , not tab expect. correct call is:

let w =  reduce(subtabs, cgfloat(0), { $0 + $1.width })