Replace <| operator with generic subscript

The `<|` operator was necessary when we originally implemented Argo, but Swift
has new capabilities now that reduce the need to use a custom operator.
Additionally, the operator spelling itself actually conflicts with prior-art
that we weren't aware of when we added it. Specifically, `<|` is used in other
languages (and sometimes in Swift) as a "pipe backwards" operator, with the
type `(T -> U) -> T -> U`.

Given these concerns, as well as the additional complexity around fine-tuning
the precedence so that it works in an obvious way, I think it makes sense to
remove the operator in favor of a generic subscript (which only just became
available in Swift 4).

Argo.xcodeproj/project.pbxproj

@@ -15,7 +15,6 @@
  809754DA1BADF36D00C409E6 /* Decodable.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB6A01ABC5F1300E3B0AB /* Decodable.swift */; };
  809754DB1BADF36D00C409E6 /* StandardTypes.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB6A11ABC5F1300E3B0AB /* StandardTypes.swift */; };
  809754DC1BADF36D00C409E6 /* DecodeError.swift in Sources */ = {isa = PBXBuildFile; fileRef = F84318A71B9A2D7A00165216 /* DecodeError.swift */; };
- 809754DE1BADF36D00C409E6 /* DecodeOperators.swift in Sources */ = {isa = PBXBuildFile; fileRef = EAD9FAF519D0F7900031E006 /* DecodeOperators.swift */; };
  809754DF1BADF36D00C409E6 /* (null) in Sources */ = {isa = PBXBuildFile; };
  809754E21BADF36D00C409E6 /* decode.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB69A1ABC5F1300E3B0AB /* decode.swift */; };
  809754E31BADF36D00C409E6 /* flatReduce.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB69B1ABC5F1300E3B0AB /* flatReduce.swift */; };
@@ -63,7 +62,6 @@
  D0592EBF1B77DD8E00EFEF39 /* JSON.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB69F1ABC5F1300E3B0AB /* JSON.swift */; };
  D0592EC01B77DD8E00EFEF39 /* Decodable.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB6A01ABC5F1300E3B0AB /* Decodable.swift */; };
  D0592EC11B77DD8E00EFEF39 /* StandardTypes.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB6A11ABC5F1300E3B0AB /* StandardTypes.swift */; };
- D0592EC31B77DD9300EFEF39 /* DecodeOperators.swift in Sources */ = {isa = PBXBuildFile; fileRef = EAD9FAF519D0F7900031E006 /* DecodeOperators.swift */; };
  D0592EC51B77DD9A00EFEF39 /* decode.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB69A1ABC5F1300E3B0AB /* decode.swift */; };
  D0592EC61B77DD9A00EFEF39 /* flatReduce.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB69B1ABC5F1300E3B0AB /* flatReduce.swift */; };
  D0592EC71B77DD9A00EFEF39 /* sequence.swift in Sources */ = {isa = PBXBuildFile; fileRef = F87EB69C1ABC5F1300E3B0AB /* sequence.swift */; };
@@ -83,9 +81,6 @@
  EA04D5A11BBF2021001DE23B /* FailureCoalescing.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5A01BBF2021001DE23B /* FailureCoalescing.swift */; };
  EA04D5A21BBF2021001DE23B /* FailureCoalescing.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5A01BBF2021001DE23B /* FailureCoalescing.swift */; };
  EA04D5A31BBF2021001DE23B /* FailureCoalescing.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5A01BBF2021001DE23B /* FailureCoalescing.swift */; };
- EA04D5A51BBF2047001DE23B /* Argo.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5A41BBF2047001DE23B /* Argo.swift */; };
- EA04D5A61BBF2047001DE23B /* Argo.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5A41BBF2047001DE23B /* Argo.swift */; };
- EA04D5A71BBF2047001DE23B /* Argo.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5A41BBF2047001DE23B /* Argo.swift */; };
  EA08313119D5EEAF003B90D7 /* TypeTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA08313019D5EEAF003B90D7 /* TypeTests.swift */; };
  EA08313319D5EEF2003B90D7 /* post_comments.json in Resources */ = {isa = PBXBuildFile; fileRef = EA08313219D5EEF2003B90D7 /* post_comments.json */; };
  EA08313519D5EFC0003B90D7 /* types.json in Resources */ = {isa = PBXBuildFile; fileRef = EA08313419D5EFC0003B90D7 /* types.json */; };
@@ -120,7 +115,6 @@
  EA6DD69D1AB383FB00CA3A5B /* PerformanceTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA6DD69B1AB383FB00CA3A5B /* PerformanceTests.swift */; };
  EA6DD69F1AB384C700CA3A5B /* big_data.json in Resources */ = {isa = PBXBuildFile; fileRef = EA6DD69E1AB384C700CA3A5B /* big_data.json */; };
  EA6DD6A01AB384C700CA3A5B /* big_data.json in Resources */ = {isa = PBXBuildFile; fileRef = EA6DD69E1AB384C700CA3A5B /* big_data.json */; };
- EA9159F61BDE74BC00D85292 /* Argo.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5A41BBF2047001DE23B /* Argo.swift */; };
  EA9159F71BDE74C700D85292 /* catDecoded.swift in Sources */ = {isa = PBXBuildFile; fileRef = F8EF432E1BBC728A001886BA /* catDecoded.swift */; };
  EA9159F81BDE74E400D85292 /* Monad.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5901BBF1F40001DE23B /* Monad.swift */; };
  EA9159F91BDE74EB00D85292 /* Functor.swift in Sources */ = {isa = PBXBuildFile; fileRef = EA04D5941BBF1F80001DE23B /* Functor.swift */; };
@@ -135,7 +129,6 @@
  EABDF6921A9CD4EA00B6CC83 /* types.plist in Resources */ = {isa = PBXBuildFile; fileRef = EABDF68E1A9CD4EA00B6CC83 /* types.plist */; };
  EABDF6941A9CD4FC00B6CC83 /* PListDecodingTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = EABDF6931A9CD4FC00B6CC83 /* PListDecodingTests.swift */; };
  EABDF6951A9CD4FC00B6CC83 /* PListDecodingTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = EABDF6931A9CD4FC00B6CC83 /* PListDecodingTests.swift */; };
- EAD9FAF619D0F7900031E006 /* DecodeOperators.swift in Sources */ = {isa = PBXBuildFile; fileRef = EAD9FAF519D0F7900031E006 /* DecodeOperators.swift */; };
  EAD9FAFE19D2113C0031E006 /* User.swift in Sources */ = {isa = PBXBuildFile; fileRef = EAD9FAFD19D2113C0031E006 /* User.swift */; };
  EAD9FB0019D211630031E006 /* Comment.swift in Sources */ = {isa = PBXBuildFile; fileRef = EAD9FAFF19D211630031E006 /* Comment.swift */; };
  EAD9FB0219D211C10031E006 /* Post.swift in Sources */ = {isa = PBXBuildFile; fileRef = EAD9FB0119D211C10031E006 /* Post.swift */; };
@@ -185,7 +178,6 @@
  F893355F1A4CE83000B88685 /* Argo.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = F89335541A4CE83000B88685 /* Argo.framework */; };
  F893356E1A4CE8FC00B88685 /* Argo.h in Headers */ = {isa = PBXBuildFile; fileRef = F893356D1A4CE8FC00B88685 /* Argo.h */; settings = {ATTRIBUTES = (Public, ); }; };
  F893356F1A4CE8FC00B88685 /* Argo.h in Headers */ = {isa = PBXBuildFile; fileRef = F893356D1A4CE8FC00B88685 /* Argo.h */; settings = {ATTRIBUTES = (Public, ); }; };
- F89335761A4CE93600B88685 /* DecodeOperators.swift in Sources */ = {isa = PBXBuildFile; fileRef = EAD9FAF519D0F7900031E006 /* DecodeOperators.swift */; };
  F8ABEBD11D42C67100B4363C /* Runes.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = F8ABEBD01D42C67100B4363C /* Runes.framework */; };
  F8ABEBD31D42C67800B4363C /* Runes.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = F8ABEBD21D42C67800B4363C /* Runes.framework */; };
  F8ABEBD51D42C67E00B4363C /* Runes.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = F8ABEBD41D42C67E00B4363C /* Runes.framework */; };
@@ -293,7 +285,6 @@
  EA04D5981BBF1FA4001DE23B /* Applicative.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Applicative.swift; sourceTree = "<group>"; };
  EA04D59C1BBF1FB9001DE23B /* Alternative.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Alternative.swift; sourceTree = "<group>"; };
  EA04D5A01BBF2021001DE23B /* FailureCoalescing.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = FailureCoalescing.swift; sourceTree = "<group>"; };
- EA04D5A41BBF2047001DE23B /* Argo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Argo.swift; sourceTree = "<group>"; };
  EA08313019D5EEAF003B90D7 /* TypeTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TypeTests.swift; sourceTree = "<group>"; };
  EA08313219D5EEF2003B90D7 /* post_comments.json */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.json; path = post_comments.json; sourceTree = "<group>"; };
  EA08313419D5EFC0003B90D7 /* types.json */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.json; path = types.json; sourceTree = "<group>"; };
@@ -317,7 +308,6 @@
  EAD9FACF19D0EAB50031E006 /* Argo.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Argo.framework; sourceTree = BUILT_PRODUCTS_DIR; };
  EAD9FAD319D0EAB50031E006 /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
  EAD9FADA19D0EAB60031E006 /* ArgoTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = ArgoTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
- EAD9FAF519D0F7900031E006 /* DecodeOperators.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DecodeOperators.swift; sourceTree = "<group>"; };
  EAD9FAFD19D2113C0031E006 /* User.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = User.swift; sourceTree = "<group>"; };
  EAD9FAFF19D211630031E006 /* Comment.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Comment.swift; sourceTree = "<group>"; };
  EAD9FB0119D211C10031E006 /* Post.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Post.swift; sourceTree = "<group>"; };
@@ -477,7 +467,6 @@
  isa = PBXGroup;
  children = (
  F87EB69D1ABC5F1300E3B0AB /* Types */,
- EAD9FAE919D0F6480031E006 /* Operators */,
  F87EB6981ABC5F1300E3B0AB /* Functions */,
  F8CBE6651A6451F800316FBC /* Extensions */,
  );
@@ -506,15 +495,6 @@
  path = Resources;
  sourceTree = "<group>";
  };
- EAD9FAE919D0F6480031E006 /* Operators */ = {
- isa = PBXGroup;
- children = (
- EA04D5A41BBF2047001DE23B /* Argo.swift */,
- EAD9FAF519D0F7900031E006 /* DecodeOperators.swift */,
- );
- path = Operators;
- sourceTree = "<group>";
- };
  EAD9FAFC19D2110D0031E006 /* Models */ = {
  isa = PBXGroup;
  children = (
@@ -992,7 +972,6 @@
  809754DB1BADF36D00C409E6 /* StandardTypes.swift in Sources */,
  EA9159F91BDE74EB00D85292 /* Functor.swift in Sources */,
  809754DA1BADF36D00C409E6 /* Decodable.swift in Sources */,
- 809754DE1BADF36D00C409E6 /* DecodeOperators.swift in Sources */,
  809754D81BADF36D00C409E6 /* Decoded.swift in Sources */,
  809754E41BADF36D00C409E6 /* sequence.swift in Sources */,
  809754E21BADF36D00C409E6 /* decode.swift in Sources */,
@@ -1002,7 +981,6 @@
  809754E61BADF36D00C409E6 /* RawRepresentable.swift in Sources */,
  809754D91BADF36D00C409E6 /* JSON.swift in Sources */,
  F8C2561C1C3C855C00B70968 /* NSNumber.swift in Sources */,
- EA9159F61BDE74BC00D85292 /* Argo.swift in Sources */,
  EA9159FB1BDE74F800D85292 /* Alternative.swift in Sources */,
  );
  runOnlyForDeploymentPostprocessing = 0;
@@ -1049,12 +1027,10 @@
  F84318AA1B9A2D7A00165216 /* DecodeError.swift in Sources */,
  D0592EBF1B77DD8E00EFEF39 /* JSON.swift in Sources */,
  F8EF43311BBC729F001886BA /* catDecoded.swift in Sources */,
- D0592EC31B77DD9300EFEF39 /* DecodeOperators.swift in Sources */,
  EA1200CE1BAB621C006DDBD8 /* RawRepresentable.swift in Sources */,
  EA04D5931BBF1F40001DE23B /* Monad.swift in Sources */,
  EA04D5971BBF1F80001DE23B /* Functor.swift in Sources */,
  D0592EC71B77DD9A00EFEF39 /* sequence.swift in Sources */,
- EA04D5A71BBF2047001DE23B /* Argo.swift in Sources */,
  D0592EC61B77DD9A00EFEF39 /* flatReduce.swift in Sources */,
  );
  runOnlyForDeploymentPostprocessing = 0;
@@ -1076,11 +1052,9 @@
  F87EB6A81ABC5F1300E3B0AB /* sequence.swift in Sources */,
  F84318A81B9A2D7A00165216 /* DecodeError.swift in Sources */,
  F8CBE6671A64521000316FBC /* Dictionary.swift in Sources */,
- EAD9FAF619D0F7900031E006 /* DecodeOperators.swift in Sources */,
  F87EB6B01ABC5F1300E3B0AB /* StandardTypes.swift in Sources */,
  EA04D5911BBF1F40001DE23B /* Monad.swift in Sources */,
  EA04D5951BBF1F80001DE23B /* Functor.swift in Sources */,
- EA04D5A51BBF2047001DE23B /* Argo.swift in Sources */,
  F8EF432F1BBC728A001886BA /* catDecoded.swift in Sources */,
  );
  runOnlyForDeploymentPostprocessing = 0;
@@ -1128,12 +1102,10 @@
  F84318A91B9A2D7A00165216 /* DecodeError.swift in Sources */,
  F8CBE6681A64526300316FBC /* Dictionary.swift in Sources */,
  F8EF43301BBC729F001886BA /* catDecoded.swift in Sources */,
- F89335761A4CE93600B88685 /* DecodeOperators.swift in Sources */,
  EA1200CD1BAB621C006DDBD8 /* RawRepresentable.swift in Sources */,
  EA04D5921BBF1F40001DE23B /* Monad.swift in Sources */,
  EA04D5961BBF1F80001DE23B /* Functor.swift in Sources */,
  F87EB6B11ABC5F1300E3B0AB /* StandardTypes.swift in Sources */,
- EA04D5A61BBF2047001DE23B /* Argo.swift in Sources */,
  );
  runOnlyForDeploymentPostprocessing = 0;
  };

Sources/Argo/Functions/decode.swift

@@ -79,7 +79,7 @@ public func decode<T: Decodable>(_ object: Any) -> T? where T == T.DecodedType {
  - returns: A `Decoded<T>` value where `T` is `Decodable`
 */
 public func decode<T: Decodable>(_ dict: [String: Any], rootKey: String) -> Decoded<T> where T == T.DecodedType {
- return JSON(dict as Any) <| rootKey
+ return JSON(dict as Any)[rootKey]
 }
 
 /**

Sources/Argo/Types/JSON.swift

@@ -44,6 +44,53 @@ public extension JSON {
  }
 }
 
+extension JSON {
+ /**
+ Attempt to extract a value at the specified key path and transform it into
+ the requested type.
+
+ This method is used to decode a mandatory value from the `JSON`. If the
+ decoding fails for any reason, this will result in a `.Failure` being
+ returned.
+
+ - parameter keyPath: The key path for the object to decode, represented by
+ a variadic list of strings.
+
+ - returns: A `Decoded` value representing the success or failure of the
+ decode operation
+ */
+ public subscript<T: Decodable>(keyPath: String...) -> Decoded<T> where T == T.DecodedType {
+ return flatReduce(keyPath, initial: self, combine: decodedJSON)
+ .flatMap(T.decode)
+ }
+
+ /**
+ Attempt to extract an optional value at the specified key path and
+ transform it into the requested type.
+
+ This method is used to decode an optional value from the `JSON`. If any of
+ the keys in the key path aren't present in the `JSON`, this will still return
+ `.success`. However, if the key path exists but the object assigned to the
+ final key is unable to be decoded into the requested type, this will return
+ `.failure`.
+
+ - parameter keyPath: The key path for the object to decode, represented by
+ a variadic list of strings.
+ - returns: A `Decoded` optional value representing the success or failure
+ of the decode operation
+ */
+ public subscript<T: Decodable>(optional keyPath: String...) -> Decoded<T?> where T == T.DecodedType {
+ switch flatReduce(keyPath, initial: self, combine: decodedJSON) {
+ case .failure:
+ return .success(.none)
+
+ case let .success(x):
+ return T.decode(x)
+ .flatMap { .success(.some($0)) }
+ }
+ }
+}
+
 extension JSON: Decodable {
  /**
  Decode `JSON` into `Decoded<JSON>`.

Tests/ArgoTests/Models/Booleans.swift

@@ -8,7 +8,7 @@ struct Booleans: Argo.Decodable {
 
  static func decode(_ json: JSON) -> Decoded<Booleans> {
  return curry(Booleans.init)
- <^> json <| "realBool"
- <*> json <| "numberBool"
+ <^> json["realBool"]
+ <*> json["numberBool"]
  }
 }

Tests/ArgoTests/Models/Comment.swift

@@ -11,8 +11,8 @@ struct Comment {
 extension Comment: Argo.Decodable {
  static func decode(_ json: JSON) -> Decoded<Comment> {
  return curry(self.init)
- <^> json <| "id"
- <*> json <| "text"
- <*> json <| ["author", "name"]
+ <^> json["id"]
+ <*> json["text"]
+ <*> json["author", "name"]
  }
 }

Tests/ArgoTests/Models/Post.swift

@@ -12,10 +12,10 @@ struct Post {
 extension Post: Argo.Decodable {
  static func decode(_ json: JSON) -> Decoded<Post> {
  return curry(self.init)
- <^> json <| "id"
- <*> json <| "text"
- <*> json <| "author"
- <*> json <| "comments"
+ <^> json["id"]
+ <*> json["text"]
+ <*> json["author"]
+ <*> json["comments"]
  }
 }
 
@@ -30,11 +30,11 @@ struct LocationPost {
 extension LocationPost: Argo.Decodable {
  static func decode(_ json: JSON) -> Decoded<LocationPost> {
  return curry(self.init)
- <^> json <| "id"
- <*> json <| "text"
- <*> json <| "author"
- <*> json <| "comments"
- <*> json <|? "location"
+ <^> json["id"]
+ <*> json["text"]
+ <*> json["author"]
+ <*> json["comments"]
+ <*> json[optional: "location"]
  }
 }
 
@@ -47,8 +47,8 @@ struct Location {
 extension Location: Argo.Decodable {
  static func decode(_ json: JSON) -> Decoded<Location> {
  return curry(self.init)
- <^> json <| "lat"
- <*> json <| "lng"
- <*> json <| "title"
+ <^> json["lat"]
+ <*> json["lng"]
+ <*> json["title"]
  }
 }