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Swift implementation of http://adit.io/posts/2013-06-10-three-useful-monads.html
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| /* I'm still trying to wrap my head around this monad. | |
| * Examples either seem contrived or do not use multiple | |
| * types; it's not very useful for understanding when | |
| * every input and output is a string. | |
| */ |
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| /* On hold until I fully grasp Reader. */ |
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| protocol Concatable { | |
| func byConcatenating(content: Self) -> Self | |
| } | |
| extension Array: Concatable { | |
| func byConcatenating(content: Array) -> Array { | |
| var mutableArray = self | |
| mutableArray.appendContentsOf(content) | |
| return mutableArray | |
| } | |
| } | |
| protocol WriterType { | |
| associatedtype Value | |
| associatedtype Log | |
| init(value: Value, log: Log) | |
| var value: Value { get } | |
| var log: Log { get } | |
| } | |
| class Writer<Value, Log: Concatable>: WriterType { | |
| required init(value: Value, log: Log) { | |
| self.value = value | |
| self.log = log | |
| } | |
| let value: Value | |
| let log: Log | |
| func flatMap<U>(@noescape f:(Value) -> Writer<U, Log>) -> Writer<U, Log> { | |
| let newWriter = f(value) | |
| return Writer<U, Log>(value: newWriter.value, log: log.byConcatenating(newWriter.log)) | |
| } | |
| } | |
| extension Writer: CustomStringConvertible { | |
| var description: String { | |
| return "Value: \(value), Log: \(log)" | |
| } | |
| } | |
| func half(input: Int) -> Int { | |
| return input / 2 | |
| } | |
| func triple(input: Int) -> Int { | |
| return input * 3 | |
| } | |
| func stringify(input: Int) -> String { | |
| return "\(input)" | |
| } | |
| half(8) | |
| half(half(8)) | |
| stringify(half(triple(half(8)))) | |
| typealias StringArray = [String] | |
| func half2(input:Int) -> Writer<Int, StringArray> { | |
| return Writer(value: half(input), log: ["I just halved \(input)!"]) | |
| } | |
| func triple2(input: Int) -> Writer<Int, StringArray> { | |
| return Writer(value: triple(input), log: ["I just tripled \(input)!"]) | |
| } | |
| func stringify2(input: Int) -> Writer<String, StringArray> { | |
| return Writer(value: stringify(input), log: ["I just stringified \(input)"]) | |
| } | |
| half2(8) | |
| half2(8).flatMap(triple2).flatMap(half2).flatMap(stringify2) | |
| func makeWritable<T, U>(name: String, _ f: T -> U) -> (T -> Writer<U, StringArray>) { | |
| return { input in Writer(value: f(input), log: ["\(name) \(input)"]) } | |
| } | |
| let half3 = makeWritable("halved", half) | |
| let triple3 = makeWritable("tripled", triple) | |
| let stringify3 = makeWritable("stringified", stringify) | |
| half3(8) | |
| half3(8).flatMap(triple3).flatMap(half3).flatMap(stringify3) |
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