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Add a version of map that converts thrown errors to Failures. #118

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merged 6 commits into from
Dec 9, 2015
Merged

Add a version of map that converts thrown errors to Failures. #118

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e2fa2bb
Add a version of `map` that converts thrown errors to `Failure`s.
Nov 6, 2015
1e277d8
Favor `fatalError` over `as!`.
Nov 6, 2015
dee4350
Add space between parameter name and type.
Dec 2, 2015
329b819
Merge branch 'master' into tryMap
bwhiteley Dec 2, 2015
855a5d7
Rename `map` for `transform`s that throw to `tryMap`
bwhiteley Dec 2, 2015
2e5749a
In `tryMap`, switch to `as!` when converting error.
bwhiteley Dec 5, 2015
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10 changes: 10 additions & 0 deletions Result/Result.swift
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Expand Up @@ -201,4 +201,14 @@ public func >>- <T, U, Error> (result: Result<T, Error>, @noescape transform: T
}


// MARK: - ErrorTypeConvertible conformance

/// Make NSError conform to ErrorTypeConvertible
extension NSError: ErrorTypeConvertible {
public static func errorFromErrorType(error: ErrorType) -> NSError {
return error as NSError
}
}


import Foundation
24 changes: 24 additions & 0 deletions Result/ResultType.swift
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Expand Up @@ -64,6 +64,30 @@ public extension ResultType {
}
}

/// Protocol used to constrain a version of `map` for `transform`s that throw.
public protocol ErrorTypeConvertible: ErrorType {
typealias ConvertibleType = Self
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Is this necessary? Maybe just use Self?

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It is necessary because NSError is not final. From what I've seen, this is a common pattern.

static func errorFromErrorType(error:ErrorType) -> ConvertibleType
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Nit: space between error: and ErrorType.

}

public extension ResultType where Error: ErrorTypeConvertible {

/// Returns the result of applying `transform` to `Success`es’ values, or wrapping thrown errors.
public func map<U>(@noescape transform: Value throws -> U) -> Result<U, Error> {
return flatMap { value in
do {
return .Success(try transform(value))
}
catch {
guard let convertedError = Error.errorFromErrorType(error) as? Error else {
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I'm confused about this, if Error conforms to ErrorTypeConvertible, this is always going to be non-nil and of type Error, right?

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You would think so, but

ResultType.swift:82:58: error: 'Self.Error.ConvertibleType' is not convertible to 'Self.Error'; did you mean to use 'as!' to force downcast?
                                let convertedError = Error.errorFromErrorType(error) as Error
                                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
                                                                                     as!

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That seems like a bug.
What if you remove ConvertibleType and change ErrorTypeConvertible:

public protocol ErrorTypeConvertible: ErrorType {
    static func errorFromErrorType(error: ErrorType) -> Self
}

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That does permit the use of a plain as in tryMap, but then I can't make NSError conform to ErrorTypeConvertible, presumably because it's not final.

I boiled it down to a sample that you can tinker with in a Playground.

import Foundation

// For types that can be created from an arbitrary ErrorType
public protocol ErrorTypeConvertible: ErrorType {
    typealias ConvertibleType = Self
    static func errorFromErrorType(error: ErrorType) -> ConvertibleType
}

protocol MyType {
    typealias Error: ErrorType
    var error: Error { get }
}

extension MyType where Error: ErrorTypeConvertible {
    func foo() -> Error {
        return Error.errorFromErrorType(error) as! Error // ** Why does this require `!`? **
    }
}

extension NSError: ErrorTypeConvertible {
    public static func errorFromErrorType(error: ErrorType) -> NSError {
        return error as NSError
    }
}

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Probably related to https://twitter.com/jckarter/status/672931114944696321.

Given that the cast can never fail I'd just use as! and add a comment to improve this later on.

fatalError("Unable to convert error type '\(error.dynamicType)' to type '\(Error.self)'")
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Cool

}
return .Failure(convertedError)
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Since this actually behaves like flatMap (but with a restricted parameter type), calling it map seems a bit confusing 😕

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I agree, and would have preferred to call it flatMap. However, that resulted in "ambiguous use..." errors. Do you have a recommendation?

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/cc @antitypical/result

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Spitballing, but: Is there maybe a way to rewrite the current flatMap with rethrows in a way that covers both cases?

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I don't think so. The return type of the transform is quite different, and part of the reason for this PR is to be able to pass in functions that are not aware of Result.

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Yeah, I don’t think this is precisely map or flatMap.

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I originally called it tryMap (PR #114), but tried to roll it into flatMap or map based on feedback in that PR. How are we feeling about tryMap?

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👍 on tryMap

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Yeah, that was my bad, my apologies. I’m 👍 on the name tryMap.

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I renamed it to tryMap. Any other concerns?

}
}
}
}

// MARK: - Operators

infix operator &&& {
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12 changes: 11 additions & 1 deletion ResultTests/ResultTests.swift
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Expand Up @@ -89,6 +89,16 @@ final class ResultTests: XCTestCase {
XCTAssertNil(result.error)
}

func testTryMapProducesSuccess() {
let result = success.map(tryIsSuccess)
XCTAssert(result == success)
}

func testTryMapProducesFailure() {
let result = Result<String, NSError>.Success("fail").map(tryIsSuccess)
XCTAssert(result == failure)
}

// MARK: Operators

func testConjunctionOperator() {
Expand Down Expand Up @@ -132,7 +142,7 @@ func attempt<T>(value: T, succeed: Bool, error: NSErrorPointer) -> T? {
}

func tryIsSuccess(text: String?) throws -> String {
guard let text = text else {
guard let text = text where text == "success" else {
throw error
}

Expand Down