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Flattening the function type of unapplied method references

Introduction

An unapplied method reference, such as Type.instanceMethod in the following example, currently produces a curried function value of type (Self) -> (Args...) -> Ret:

struct Type {
  var x: Int
  func instanceMethod(y y: Int) -> Int {
    return x + y
  }
}

let f = Type.instanceMethod // f : (Type) -> (y: Int) -> Int
f(Type(x: 1))(y: 2)         // ==> 3

In order to make unapplied method references more useful and consistent with idiomatic Swift, and to make them workable for mutating methods, we should change them to produce a function with a flat function type, (Self, Args...) -> Ret:

let f = Type.instanceMethod // f: (Type, y: Int) -> Int
f(Type(x: 1), y: 2)         // ==> 3

Swift-evolution thread: Flattening the function type of unapplied instance methods

Motivation

Currying hasn't proven itself to be used much in idiomatic Swift. Standard library collection transforms such as reduce and sort prefer "flat" function arguments, and Cocoa APIs that use blocks are imported into Swift with flat function types as well. By producing curried types, unapplied method references simply aren't very useful as-is compared to free functions or closure literals. For instance, though you can pass the global + operator readily to reduce to sum a sequence of numbers:

func sumOfInts(ints: [Int]) -> Int {
  return ints.reduce(0, combine: +)
}

you can't do the same with a binary method, such as Set.union:

func unionOfSets<T>(sets: [Set<T>]) -> Set<T> {
  // Error: `combine` expects (Set<T>, Set<T>) -> Set<T>, but
  // `Set.union` has type (Set<T>) -> (Set<T>) -> Set<T>
  return sets.reduce([], combine: Set.union)
}

Even unary methods are referenced as type (Self) -> () -> Ret, meaning they can't be readily used with transforms like map:

func sortedArrays<T: Comparable>(arrays: [[T]]) -> [T] {
  // Error: `map` expects [T] -> [T], but
  // `Array.sort` has type ([T]) -> () -> [T]
  return arrays.map(Array.sort)
}

This currying is also incompatible with mutating methods due to the semantics of inout parameters. In a chained call such as f(&x)(y), the mutation window for x only lasts as long as the first call. The second application of y is no longer allowed to mutate x. We currently miscompile unapplied references to mutating methods, capturing a dangling pointer when the reference is partially applied and leading to undefined behavior when the full application occurs.

Proposed solution

We should change the type of an unapplied method reference to produce a flattened function value, instead of a curried one. This will make unapplied methods more readily useful with real Swift libraries, and make them supportable for mutating methods.

Detailed design

When an instance method is found by name lookup into a type reference or metatype value, a function value is produced that takes the self instance followed by the method arguments of the referenced method. If the method is mutating, then the first parameter of the resulting function value is inout. For example:

struct Type {
  func instanceMethod(x: Int) -> Float {}
  mutating func mutatingMethod(x: String) -> Double {}
}

Type.instanceMethod // : (Type, Int) -> Float
Type.mutatingMethod // : (inout Type, String) -> Double

This proposal does not propose changing the behavior of method partial applications instance.instanceMethod. It should remain possible to partially bind a nonmutating method to its self parameter in this fashion.

Impact on existing code

This will break existing code that uses unapplied method references for their curried signatures today. A blunt migration would be to replace existing type references with nested closure literals, substituting:

let x = y.map(flip(Type.method))

with:

let x = y.map(flip({ instance in { arg in instance.method(arg) } }))

However, unapplied method references are currently rare in practice, due to the limited usefulness of their curried signature today.

Alternatives considered

If we do nothing else, we should close the undefined behavior hole by banning unapplied references to mutating methods:

struct Type {
  mutating func mutatingMethod() {}
}
let f: Type.mutatingMethod // This should become an error

However, as discussed above, there are good systemic reasons to change the behavior of all unapplied method references; not only would this fix the undefined behavior hole, but also makes them a more generally useful feature.