functional programming in swift

Question

please, check my snippet, the question is there (my english is too bad to be able to explain my trouble by the words :))

func flip1<A, B, C>(f : ((A, B) -> C), _ b : B, _ a : A) -> C {
    return f(a, b)
}
flip1(-, 2, 1)      // -1
flip1(/, 2.0, 3.0)  // 1.5

// partialy curried version
func flip2<A, B, C>(f : (A, B) -> C, _ i : B, _ j : A) -> C {
    return f(j, i)
}

print(flip2(- , 2, 1))      // -1
print(flip2(/,2.0,3.0))   // 1.5

compiles without trouble, but how to use it???

// full curried version
// compiles without trouble, but how to use it???
func flip3<A, B, C>(f : A -> B -> C) -> B -> A -> C {
    return { b in { a in f(a)(b) } }
}

/*
* flip3(/)(2.0)(3.0)
*
* error: ambiguous reference to member '/'
* it meands there are more than one candidate for / function
*/

// we need curried version of /, let's define it
func curry<A,B,C>(f: (A, B) -> C) -> A -> B -> C {
    return { a in { b in f(a, b) } }
}

/*
* let divideUnknownType = curry(/)
* compiler still complain, as expected :-)
* error: ambiguous use of operator '/'
*/

// and then define the type of it
let divideDoubles: Double->Double->Double = curry(/)
let divideIntegers: Int->Int->Int = curry(/)
// :-)
print(flip3(divideDoubles)(2.0)(3.0)) // 1.5
print(flip3(divideDoubles)(2)(3)) // 1.5
print(flip3(divideIntegers)(2)(3)) // 1

as you can see, it breaks my 'generic' approach. any idea how to solve it?

For the life of me I cannot see the difference between flip1 and flip2? — Milos
– Milos, Commented Apr 3, 2016 at 13:27
(/) as (Double, Double) -> Double will help you grab the correct version of /. — Sulthan
– Sulthan, Commented Apr 3, 2016 at 13:29
@milos you are right! :-), just skip it, i need some idea how make flip3 really generic, so flip3(/)(2.0)(3.0) will work .... It is even possible? — user3441734
– user3441734, Commented Apr 3, 2016 at 13:36
It should be: func flip3<A, B, C>(f: (A, B) -> C) -> B -> A -> C — Milos
– Milos, Commented Apr 3, 2016 at 13:39
@Darko He is not declaring any. (or you're making a general comment of approval?) — Milos
– Milos, Commented Apr 3, 2016 at 14:44

Milos · Accepted Answer · 2016-04-03 14:52:27Z

1

func flip3<A, B, C>(f: (A, B) -> C) -> B -> A -> C {
    return { b in { a in f(a, b) } }
}

flip3(/)(2.0)(3.0) // 1.5

answered Apr 3, 2016 at 14:52

Milos

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2 Comments

Milos Over a year ago

You are welcome. You've just made a slight error with f parameter, which has to take two arguments (A, B) -> C to match functions like /...

user3441734 Over a year ago

Yes, i see it now. My head is like hot air balloon ... :-)

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