[swift-users] another issue with tuples

Neil Faiman neil.swift at faiman.org
Wed Jul 6 09:34:00 CDT 2016

The language reference says that “Assignment is performed from each part of the value to the corresponding part of the expression. A String value can be assigned to a String, and a () -> Int value can be assigned to a () throws -> Int, so one would reasonably expect that both tuple assignments would work. 

My hunch would be that the compiler has two different code paths for an assignment whose RHS is an identifier and an assignment whose RHS is a  parenthesized-expression; that in the first case starts about by asking whether the RHS type is the same or a subtype of the LHS type, and reporting an error when it isn’t, but that in the second case, it starts with an element-by-element type comparison, which succeeds.



> On Jul 6, 2016, at 9:55 AM, Shane S via swift-users <swift-users at swift.org> wrote:
> Aaron this works for me in both Swift 2.2 and Swift 3 provided that you remove the ‘throws’ keyword.
> What seems odd to me is not the first assignment, but rather the second that _allows_ the use of ‘throws’ when t.1 (i.e. f) does not throw - is your concern the same?
> Shane
>> On Jul 5, 2016, at 10:48 PM, Aaron Bohannon via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> Yesterday, it was pointed out that a variable name referring to a tuple cannot be used as a pattern.  I have noticed another sort of inconsistency in how tuples are treated when they are referenced by name:
>> func f() -> Int { return 5 }
>> let t = ("a", f)
>> let _: (String, () throws -> Int) = t  // type error
>> let _: (String, () throws -> Int) = (t.0, t.1)  // OK
>> This situation leads to a different sort of type error; however, the error seems equally unwarranted.  I can't see any good reason for a well-typed program to become ill-typed when `(t.0, t.1)` is replaced with `t` (assuming `t` is a pair).
>> Should I file a separate bug for the specific example above?
>> - Aaron
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