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0060-composable-parsing-flat-map/Composable Parsers.playground/Contents.swift

@@ -0,0 +1,194 @@
+import Foundation
+
+struct Parser<A> {
+  let run: (inout Substring) -> A?
+}
+
+let int = Parser<Int> { str in
+  let prefix = str.prefix(while: { $0.isNumber })
+  guard let int = Int(prefix) else { return nil }
+  str.removeFirst(prefix.count)
+  return int
+}
+
+let double = Parser<Double> { str in
+  let prefix = str.prefix(while: { $0.isNumber || $0 == "." })
+  guard let match = Double(prefix) else { return nil }
+  str.removeFirst(prefix.count)
+  return match
+}
+
+func literal(_ literal: String) -> Parser<Void> {
+  return Parser<Void> { str in
+    guard str.hasPrefix(literal) else { return nil }
+    str.removeFirst(literal.count)
+    return ()
+  }
+}
+
+func always<A>(_ a: A) -> Parser<A> {
+  return Parser<A> { _ in a }
+}
+
+extension Parser {
+  static var never: Parser {
+    return Parser { _ in nil }
+  }
+}
+
+struct Coordinate {
+  let latitude: Double
+  let longitude: Double
+}
+
+
+extension Parser {
+  func run(_ str: String) -> (match: A?, rest: Substring) {
+    var str = str[...]
+    let match = self.run(&str)
+    return (match, str)
+  }
+}
+
+
+// map: ((A) -> B) -> (F<A>) -> F<B>
+
+// F<A> = Parser<A>
+// map: ((A) -> B) -> (Parser<A>) -> Parser<B>
+
+// map(id) = id
+
+[1, 2, 3]
+  .map { $0 }
+
+Optional("Blob")
+  .map { $0 }
+
+
+// map: (Parser<A>, (A) -> B) -> Parser<B>
+
+extension Parser {
+  func map<B>(_ f: @escaping (A) -> B) -> Parser<B> {
+    return Parser<B> { str -> B? in
+      self.run(&str).map(f)
+    }
+  }
+
+  func fakeMap<B>(_ f: @escaping (A) -> B) -> Parser<B> {
+    return Parser<B> { _ in nil }
+  }
+  func fakeMap2<B>(_ f: @escaping (A) -> B) -> Parser<B> {
+    return Parser<B> { str in
+      let matchB = self.run(&str).map(f)
+      str = ""
+      return matchB
+    }
+  }
+}
+
+int.map { $0 }
+int.fakeMap { $0 }.run("123")
+int
+  .fakeMap2 { $0 }.run("123 Hello World")
+int
+  .run("123 Hello World")
+
+let even = int.map { $0 % 2 == 0 }
+
+even.run("123 Hello World")
+even.run("42 Hello World")
+
+let char = Parser<Character> { str in
+  guard !str.isEmpty else { return nil }
+  return str.removeFirst()
+}
+
+let northSouth = Parser<Double> { str in
+  guard
+    let cardinal = str.first,
+    cardinal == "N" || cardinal == "S"
+    else { return nil }
+  str.removeFirst(1)
+  return cardinal == "N" ? 1 : -1
+}
+
+// flatMap: ((A) -> M<B>) -> (M<A>) -> M<B>
+
+
+extension Parser {
+  func flatMap<B>(_ f: @escaping (A) -> Parser<B>) -> Parser<B> {
+    return Parser<B> { str -> B? in
+      let original = str
+      let matchA = self.run(&str)
+      let parserB = matchA.map(f)
+      guard let matchB = parserB?.run(&str) else {
+        str = original
+        return nil
+      }
+      return matchB
+    }
+  }
+}
+
+
+let eastWest = Parser<Double> { str in
+  guard
+    let cardinal = str.first,
+    cardinal == "E" || cardinal == "W"
+    else { return nil }
+  str.removeFirst(1)
+  return cardinal == "E" ? 1 : -1
+}
+
+func parseLatLong(_ str: String) -> Coordinate? {
+  var str = str[...]
+
+  guard
+    let lat = double.run(&str),
+    literal("° ").run(&str) != nil,
+    let latSign = northSouth.run(&str),
+    literal(", ").run(&str) != nil,
+    let long = double.run(&str),
+    literal("° ").run(&str) != nil,
+    let longSign = eastWest.run(&str)
+    else { return nil }
+
+  return Coordinate(
+    latitude: lat * latSign,
+    longitude: long * longSign
+  )
+}
+
+print(String(describing: parseLatLong("40.6782° N, 73.9442° W")))
+
+
+"40.6782° N, 73.9442° W"
+
+let coord = double
+  .flatMap { lat in
+    literal("° ")
+      .flatMap { _ in
+        northSouth
+          .flatMap { latSign in
+            literal(", ")
+              .flatMap { _ in
+                double
+                  .flatMap { long in
+                    literal("° ")
+                      .flatMap { _ in
+                        eastWest
+                          .map { longSign in
+                            return Coordinate(
+                              latitude: lat * latSign,
+                              longitude: long * longSign
+                            )
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+coord.run("40.6782° N, 73.9442° W")
+coord.run("40.6782° Z, 73.9442° W")

0060-composable-parsing-flat-map/Composable Parsers.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

0060-composable-parsing-flat-map/README.md

@@ -0,0 +1,5 @@
+## [Point-Free](https://www.pointfree.co)
+
+> #### This directory contains code from Point-Free Episode: [Composable Parsing: Flat-Map](https://www.pointfree.co/episodes/ep60-composable-parsing-flat-map)
+>
+> The `map` function on parsers is powerful, but there are still a lot of things it cannot do. We will see that in trying to solve some of its limitations we are naturally led to our old friend the `flatMap` function.

README.md

@@ -61,3 +61,5 @@ This repository is the home of code written on episodes of
 1. [What Is a Parser?: Part 1](0056-what-is-a-parser-pt1)
 1. [What Is a Parser?: Part 2](0057-what-is-a-parser-pt2)
 1. [What Is a Parser?: Part 3](0058-what-is-a-parser-pt3)
+1. [Composable Parsing: Map](0059-composable-parsing-map)
+1. [Composable Parsing: Flat-Map](0060-composable-parsing-flat-map)