I guess I should also include the example where the user actually wanted the oldValue to be "x":
if case let .two(newValue, value) = example, value == oldValue { ... } No surprises there, even if another conditional is required. — Pyry > On 1 Mar 2017, at 22.53, Pyry Jahkola via swift-evolution > <swift-evolution@swift.org> wrote: > > Erica, > > Instead of going into all these lengths introducing new syntax, why not > simply turn it into a warning when a `case let` binding shadows an existing > variable with the exact same type (which supposedly also conforms to > Equatable)? > > Examples, including how to silence the said warning: > > enum Value<T> { case one(T), two(T, T), three(T, T, T) } > > let example: Value<String> = .two("a", "b") > let oldValue = "x" > // (Besides, you probably intended `oldValue` to be a `Character` in your > // example. Well, it's a `String` in mine.) > > if case let .two(newValue, oldValue) = example { ... } > // ~~~~~~~~ > // warning: 'oldValue' shadows an existing variable of same type 'String' > > if case let .two(newValue, (oldValue)) = example { assert(oldValue == > "b") } > // Ok, adding extra parentheses silences the warning. > > if case let .one(example) = example { ... } > // Ok, because there's no way the author would equate the `example: > String` > // in the LHS to the `example: Value<String>` of the RHS. > > let maybe: Optional = "perhaps" > if case let maybe? = maybe { ... } > if case let .some(maybe) = maybe { ... } > // Again, none of these examples would be affected by the warning, because > // the `maybe: String` bound in the `case let` has a different type than > // the `maybe: String?` in the outer scope. > > Personally, I do like the convenience that I can bind several variables with > one `let` keyword in a case expression. And I can't see the appeal to making > `~=`, let alone a completely new special-syntax assignment operator, more > prominent in Swift. > > — Pyry > >> On 28 Feb 2017, at 21.01, Erica Sadun via swift-evolution >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >> >> The following draft proposal addresses one matter of substance (eliminating >> edge case errors by adopting at-site conditional binding) and one of style >> (using the pattern match operator consistently). Its discussion was deferred >> from Phase 1 and remains in a fairly early stage. Your feedback will help me >> decide whether this is a proposal I want to keep developing or one that I >> should set aside and focus on other matters. Thank you. -- E >> >> The work-in-progress gist is here: >> https://gist.github.com/erica/06dad9bbe1a70290fe6b89a64f73bc0c >> <https://gist.github.com/erica/06dad9bbe1a70290fe6b89a64f73bc0c> >> >> Simplifying case syntax >> >> Proposal: TBD >> Author: Erica Sadun <https://github.com/erica> >> Status: TBD >> Review manager: TBD >> >> <https://gist.github.com/erica/06dad9bbe1a70290fe6b89a64f73bc0c#introduction>Introduction >> >> This proposal re-architects case syntax grammar to reduce potential errors >> and simplify unwrapping enumerations. >> >> Swift-evolution thread: [Pitch] Reimagining guard case/if case >> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20161024/tbd.html> >> >> <https://gist.github.com/erica/06dad9bbe1a70290fe6b89a64f73bc0c#motivation>Motivation >> >> In its current design, Swift case binding suffers from two weaknesses. >> >> Mixed external and internal let/var binding may introduce errors from >> uncommon edge cases. >> Real-world users may not consider the parallel construction between if >> case/guard case with switchstatements or naturally connect the two layouts. >> >> <https://gist.github.com/erica/06dad9bbe1a70290fe6b89a64f73bc0c#internal-case-binding>Internal >> Case Binding >> >> When pattern matching, it's common to bind a variable or constant. It's >> uncommon but legal to use a bound value as an argument. Adopting an "always >> explicit, always within the parentheses" rule adds consistency and safety to >> Swift. >> >> Consider the following enumeration and values: >> >> // An enum with one, two, or three associated values >> enum Value<T> { case one(T), two(T, T), three(T, T, T) } >> >> // An example with two associated values >> let example2: Value<Character> = .two("a", "b") >> >> // A bound symbol >> let oldValue = "x" >> This code's goal is to conditionally bind newValue and pattern match the >> value stored in the oldValue symbol. The first example succeeds. The second >> example compiles and runs but does not match the coder's intent. Using an >> external letcreates a new oldValue shadow instead of pattern matching >> oldValue's stored value. >> >> // Safe >> if case .two(let newValue, oldValue) = example2 { >> ... >> } >> >> // Syntactically legal but incorrect >> if case let .two(newValue, oldValue) = example2 { >> ... >> } > (…) > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> -- Pyry Jahkola pyry.jahk...@iki.fi
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