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Experiments with partial function application in Swift
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func partial<A, B, T>(f: (A, B) -> T, a: A) -> (B) -> T { | |
return { f(a, $0) } | |
} | |
func bind2nd<A, B, T>(f: (A, B) -> T, b: B) -> (A) -> T { | |
return { f($0, b) } | |
} | |
func partial<A, B, C, T>(f: (A, B, C) -> T, a: A) -> (B, C) -> T { | |
return { f(a, $0, $1) } | |
} | |
func partial<A, B, C, T>(f: (A, B, C) -> T, a: A, b: B) -> (C) -> T { | |
return { f(a, b, $0) } | |
} | |
func partial<A, B, C, D, T>(f: (A, B, C, D) -> T, a: A, b: B, c: C) -> (D) -> T { | |
return { f(a, b, c, $0) } | |
} | |
func partial<A, B, C, D, T>(f: (A, B, C, D) -> T, a: A, b: B) -> (C, D) -> T { | |
return { f(a, b, $0, $1) } | |
} | |
func partial<A, B, C, D, T>(f: (A, B, C, D) -> T, a: A) -> (B, C, D) -> T { | |
return { f(a, $0, $1, $2) } | |
} | |
// pipe-forward operator used in examples | |
operator infix |> { associativity left } | |
func |> <A, B>(a: A, f: A -> B) -> B { | |
return f(a) | |
} | |
// Numeric examples | |
func add(a: Int, b: Int) -> Int { return a + b } | |
func subtract(a: Int, b: Int) -> Int { return a - b } | |
let addOneTo = partial(add, 1) | |
let subtractFromOne = partial(subtract, 1) | |
let subtractOneFrom = bind2nd(subtract, 1) | |
addOneTo(10) // 11 | |
subtractOneFrom(10) // 9 | |
subtractFromOne(10) // -9 | |
100 |> addOneTo |> addOneTo // 102 | |
// String examples | |
func join2(a: String, b: String) -> String { | |
return a + b | |
} | |
func join3(a: String, b: String, c: String) -> String { | |
return a + b + c | |
} | |
join2("Hello, ", "world!") // "Hello, world!" | |
join3("1", "2", "3") // "123" | |
let prependHello = partial(join2, "Hello, ") | |
prependHello("Dolly") // "Hello, Dolly" | |
let appendWorld = bind2nd(join2, "World!") | |
appendWorld("Goodbye, ") // "Goodbye, World!" | |
let prependFirstSecond = partial(join3, "First ", "Second ") | |
prependFirstSecond("Third") // "First Second Third" | |
let prependFirst = partial(join3, "First ") | |
prependFirst("Second ", "Third") // "First Second Third" | |
func bracket(prefix: String, suffix: String, s: String) -> String { | |
return prefix + s + suffix | |
} | |
"Hello" // "<div><p>Hello</p></div>" | |
|> partial(bracket, "<p>", "</p>") | |
|> partial(bracket, "<div>", "</div>") |
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It seems like I should be able to implement at least one of the
<A, B, C, T>
partials in terms of the<A, B, T>
partial, but I can't figure out the necessary syntax/declarations.