swift - Why is `infix` the default -

i'm reading advanced operators in swift.

a basic function:

func add(a: unicorn, b: unicorn) -> unicorn { return + b } add(u1, u2) // 2 unicorns added 

an infix operator:

func + (a: unicorn, b: unicorn) -> unicorn { return + b } c = u1 + u2 

postfix:

postfix func + (a: unicorn) -> unicorn { return + 2 } // nonsense example u1+ 

i don't understand how infix can presumed default. doesn't different me normal function declaration.

what rule here--is single character legal name infix operation? mean single char functions not otherwise allowed? can any two-argument function in swift called infix style?

i don't understand how infix can presumed default.

if implement operator 2 arguments compiler can sure want infix operator, because infix operators operate on 2 values.

if implement operator 1 value compiler isn't sure whether either prefix or postfix operator , complain.

infix operators take 2 values, postfix , prefix one.

does mean single char functions not otherwise allowed? can two-argument function in swift called infix style?

no. function begins of these characters operator: /, =, -, +, !, *, %, <, >, &, |, ^, ?, or ~¹. operator can consist of multiple characters, must begin 1 of these characters.

remember new operator must declared first, instance this:

prefix operator / {} prefix func / (a: int) -> int {     return / 42 }  /56 

however declaration won't work, because a not 1 of characters operator must start:

prefix operator {} 

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¹ there more characters. read here.