Pre-Pitch: Explicit protocol fulfilment with the 'conformance' keyword

Evolution Pitches
8

Thanks @xwu. The pitch does not rely on any code structure, such as fulfillments grouped in dedicated extensions, for several reasons:

Indeed it is not always possible to declare a fulfillment in an extension. A stored property that fulfills a protocol requirement, for example, can not be declared in an extension. But even if this would becomes possible eventually, I wouldn't rely on extensions anyway.

First, even if extension grouping could help with near-misses (it does not), it would not help with the second described problem, due to protocol evolution. I'm talking about functions that becomes useless because one requirement of a protocol has been deleted, and functions that need to update their signature. Should we start to require that extension T: P { ... } can only contain P fulfillments? Even people who group protocol conformances into a dedicated extension happen to declare more related methods inside it - and I'm not sure this would be considered "bad style".

Next, the pitch does not favor one particular programming style. Grouping fulfillments in extensions is frequent, in some social groups, but not universal:

  • Some code crunching tools (I think of jazzy) don't always produce good results when conformances are declared in an extension. I've seen myself declaring all conformances upfront, at the location of the type declaration, and provide fulfillments elsewhere. It's not a personal preference, it's a pragmatic decision based on my constraints at the moment I was writing this code:

    struct T: P { ... }
// "Detached" P conformance
extension T { ... }

  • The language does not mandate any programming style. The language comes first, style comes second, and can vary. Grouping fulfillments in an extension is not required by the language. I do not know of another language feature that requires a particular style to be enabled.

  • Not all users have the same skills, or time, to polish their code. Requiring a particular style (grouping fulfillments in an extension) for enabling language features could be seen as elitist, or ableist, or pedantic for no good reason.

  • If the compiler would help solving the problems described in the pitch by emitting warnings or errors, based on extension grouping, then a simple move of code (copy and paste) would create or resolve this diagnostic. I'm afraid this would bring more confusion than good: how can the same identical function be correct here, and incorrect there? Make you your mind, compiler!

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