mirror of
https://github.com/github/semantic.git
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119 lines
4.3 KiB
Swift
119 lines
4.3 KiB
Swift
/// A node in a syntax tree. Expressed algebraically to enable representation of both normal syntax trees and their diffs.
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public enum Syntax<Recur, A>: CustomDebugStringConvertible {
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case Leaf(A)
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case Indexed([Recur])
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case Keyed([String:Recur])
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// MARK: Functor
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public func map<T>(@noescape transform: Recur -> T) -> Syntax<T, A> {
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switch self {
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case let .Leaf(n):
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return .Leaf(n)
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case let .Indexed(x):
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return .Indexed(x.map(transform))
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case let .Keyed(d):
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return .Keyed(Dictionary(elements: d.map { ($0, transform($1)) }))
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}
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}
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// MARK: CustomDebugStringConvertible
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public var debugDescription: String {
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switch self {
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case let .Leaf(n):
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return ".Leaf(\(n))"
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case let .Indexed(x):
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return ".Indexed(\(String(reflecting: x)))"
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case let .Keyed(d):
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return ".Keyed(\(String(reflecting: d)))"
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}
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}
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}
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// MARK: - Hylomorphism
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/// Hylomorphism through `Syntax`.
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///
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/// A hylomorphism (from the Aristotelian philosophy that form and matter are one) is a function of type `A → B` whose call-tree is linear in the size of the nodes produced by `up`. Conceptually, it’s the composition of a catamorphism (see also `cata`) and an anamorphism (see also `ana`), but is implemented by [Stream fusion](http://lambda-the-ultimate.org/node/2192) and as such enjoys O(n) time complexity, O(1) size complexity, and small constant factors for both (modulo inadvisable implementations of `up` and `down`).
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///
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/// Hylomorphisms are used to construct diffs corresponding to equal terms; see also `CofreeType.zip`.
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///
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/// `hylo` can be used with arbitrary functors which can eliminate to and introduce with `Syntax` values.
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public func hylo<A, B, Leaf>(down: Syntax<B, Leaf> -> B, _ up: A -> Syntax<A, Leaf>) -> A -> B {
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return up >>> { $0.map(hylo(down, up)) } >>> down
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}
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/// Reiteration through `Syntax`.
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///
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/// This is a form of hylomorphism (from the Aristotelian philosophy that form and matter are one). As such, it returns a function of type `A → B` whose call-tree is linear in the size of the nodes produced by `up`. Conceptually, it’s the composition of a catamorphism (see also `cata`) and an anamorphism (see also `ana`), but is implemented by [Stream fusion](http://lambda-the-ultimate.org/node/2192) and as such enjoys O(n) time complexity, O(1) size complexity, and small constant factors for both (modulo inadvisable implementations of `up` and `down`).
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///
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/// Hylomorphisms are used to construct diffs corresponding to equal terms; see also `CofreeType.zip`.
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///
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/// `hylo` can be used with arbitrary functors which can eliminate to and introduce with `Annotation` & `Syntax` pairs.
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public func hylo<A, B, Leaf, Annotation>(down: (Annotation, Syntax<B, Leaf>) -> B, _ up: A -> (Annotation, Syntax<A, Leaf>)) -> A -> B {
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return up >>> { ($0, $1.map(hylo(down, up))) } >>> down
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}
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// MARK: - ArrayLiteralConvertible
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extension Syntax: ArrayLiteralConvertible {
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public init(arrayLiteral: Recur...) {
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self = .Indexed(arrayLiteral)
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}
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}
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// MARK: - DictionaryLiteralConvertible
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extension Syntax: DictionaryLiteralConvertible {
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public init(dictionaryLiteral elements: (String, Recur)...) {
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self = .Keyed(Dictionary(elements: elements))
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}
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}
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// MARK: - Equality
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extension Syntax {
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public static func equals(leaf leaf: (A, A) -> Bool, recur: (Recur, Recur) -> Bool)(_ left: Syntax<Recur, A>, _ right: Syntax<Recur, A>) -> Bool {
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switch (left, right) {
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case let (.Leaf(l1), .Leaf(l2)):
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return leaf(l1, l2)
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case let (.Indexed(v1), .Indexed(v2)):
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return v1.count == v2.count && zip(v1, v2).lazy.map(recur).reduce(true) { $0 && $1 }
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case let (.Keyed(d1), .Keyed(d2)):
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return Set(d1.keys) == Set(d2.keys) && d1.keys.map { recur(d1[$0]!, d2[$0]!) }.reduce(true) { $0 && $1 }
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default:
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return false
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}
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}
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}
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public func == <F: Equatable, A: Equatable> (left: Syntax<F, A>, right: Syntax<F, A>) -> Bool {
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return Syntax.equals(leaf: ==, recur: ==)(left, right)
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}
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// MARK: - JSON
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extension Syntax {
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public func JSON(@noescape leaf leaf: A -> Doubt.JSON, @noescape recur: Recur -> Doubt.JSON) -> Doubt.JSON {
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switch self {
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case let .Leaf(a):
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return [ "leaf": leaf(a) ]
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case let .Indexed(a):
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return [ "indexed": .Array(a.map(recur)) ]
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case let .Keyed(d):
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return [ "keyed": .Dictionary(Dictionary(elements: d.map { ($0, recur($1)) })) ]
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}
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}
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}
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import Prelude
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