Use any binary function as infix operator in Swift

infix-functions.swift

infix operator « { precedence 131 associativity left }
func «<A,B,C>(a:A, f:(A,B) -> C) -> B -> C { return { f(a,$0) } }

infix operator » { precedence 131 associativity left }
func »<B,C>(f:(B) -> C, b:B) -> C { return f(b) }

infix operator < { precedence 131 associativity left }
func <<A,B,C>(a:A, f:(A,B) -> C) -> B -> C { return { f(a,$0) } }

infix operator > { precedence 131 associativity left }
func ><B,C>(f:(B) -> C, b:B) -> C { return f(b) }


func add(a:Int,b:Int) -> Int { return a + b }
func mul(a:Int,b:Int) -> Int { return a * b }

// working with two options for the same thing:
1«add»2
1<add>2

5«add»3 == 2<mul>4 // -> true

1 < add > 2 // -> 3


// because not a real operator, can't set custom precedences
1«add»2«mul»3 // -> 9, not 7 as (1+2*3) would have given you


// (a <f> b) doesn't work because < is treated as prefix and > as postfix.
// so either no spaces at all or spaces everywhere

// or using the same character:
infix operator ◊ { precedence 131 associativity left }
func ◊<A,B,C>(a:A, f:(A,B) -> C) -> B -> C { return { f(a,$0) } }
func ◊<B,C>(f:(B) -> C, b:B) -> C { return f(b) }

1◊add◊2
1 ◊ add ◊ 2