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Implement a Decoded type for error messaging
This introduces a `Decoded` type which is similar to a `Result` type. It has a success state and 2 failure states. One failure state occurs if there is a type mismatch and the other is for a missing key in the JSON. This custom type allows us to retain some information about why a decoding has failed. It also allows us to abstract the return type that the user wants. We have defined a global `decode` function for the user to use. It can return an optional type, ignoring the failure data, or return the `Decoded` type if the user would like to retain the error information. Other return types like `Result` could also easily be implemented in a particular project. The 2 different error states will function a bit differently. If the user wants an optional value from the decoding, indicated by `<|?` or `<||?` the `Decoded` type will allow failure only if the key is missing and not if the type is a mismatch.
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Tony DiPasquale
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Mar 13, 2015
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,22 @@ | ||
| import Box | ||
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| public enum Decoded<T> { | ||
| case Success(Box<T>) | ||
| case TypeMismatch(String) | ||
| case MissingKey(String) | ||
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| public var value: T? { | ||
| switch self { | ||
| case let .Success(box): return box.value | ||
| default: return .None | ||
| } | ||
| } | ||
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| public static func optional<A>(p: Decoded<A>) -> Decoded<A?> { | ||
| switch p { | ||
| case let .Success(box): return .Success(Box(.Some(box.value))) | ||
| case let .MissingKey(string): return .Success(Box(.None)) | ||
| case let .TypeMismatch(string): return .TypeMismatch(string) | ||
| } | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,15 @@ | ||
| public func decode<T: JSONDecodable where T == T.DecodedType>(object: AnyObject) -> T? { | ||
| return decode(object).value | ||
| } | ||
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| public func decode<T: JSONDecodable where T == T.DecodedType>(object: AnyObject) -> [T]? { | ||
| return decode(object).value | ||
| } | ||
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| public func decode<T: JSONDecodable where T == T.DecodedType>(object: AnyObject) -> Decoded<T> { | ||
| return T.decode(JSON.parse(object)) | ||
| } | ||
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| public func decode<T: JSONDecodable where T == T.DecodedType>(object: AnyObject) -> Decoded<[T]> { | ||
| return decodeArray(JSON.parse(object)) | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,7 +1,7 @@ | ||
| import Runes | ||
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| func flatReduce<S: SequenceType, U>(sequence: S, initial: U, combine: (U, S.Generator.Element) -> U?) -> U? { | ||
| return reduce(sequence, initial) { accum, x in | ||
| func flatReduce<S: SequenceType, U>(sequence: S, initial: U, combine: (U, S.Generator.Element) -> Decoded<U>) -> Decoded<U> { | ||
| return reduce(sequence, pure(initial)) { accum, x in | ||
| accum >>- { combine($0, x) } | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,7 +1,13 @@ | ||
| import Runes | ||
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| func sequence<T>(xs: [T?]) -> [T]? { | ||
| return reduce(xs, []) { accum, elem in | ||
| func sequence<T>(xs: [Decoded<T>]) -> Decoded<[T]> { | ||
| return reduce(xs, pure([])) { accum, elem in | ||
| return curry(+) <^> accum <*> (pure <^> elem) | ||
| } | ||
| } | ||
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| func sequence<T>(xs: [String: Decoded<T>]) -> Decoded<[String: T]> { | ||
| return reduce(xs, pure([:])) { accum, elem in | ||
| return curry(+) <^> accum <*> ({ [elem.0: $0] } <^> elem.1) | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,30 @@ | ||
| import Runes | ||
| import Box | ||
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| public func >>-<A, B>(a: Decoded<A>, f: A -> Decoded<B>) -> Decoded<B> { | ||
| switch a { | ||
| case let .Success(box): return f(box.value) | ||
| case let .MissingKey(string): return .MissingKey(string) | ||
| case let .TypeMismatch(string): return .TypeMismatch(string) | ||
| } | ||
| } | ||
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| public func <^><A, B>(f: A -> B, a: Decoded<A>) -> Decoded<B> { | ||
| switch a { | ||
| case let .Success(box): return .Success(Box(f(box.value))) | ||
| case let .MissingKey(string): return .MissingKey(string) | ||
| case let .TypeMismatch(string): return .TypeMismatch(string) | ||
| } | ||
| } | ||
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| public func <*><A, B>(f: Decoded<A -> B>, a: Decoded<A>) -> Decoded<B> { | ||
| switch f { | ||
| case let .Success(box): return box.value <^> a | ||
| case let .MissingKey(string): return .MissingKey(string) | ||
| case let .TypeMismatch(string): return .TypeMismatch(string) | ||
| } | ||
| } | ||
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| public func pure<A>(a: A) -> Decoded<A> { | ||
| return .Success(Box(a)) | ||
| } |
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