> Le 20 févr. 2017 à 14:45, Matthew Johnson <matt...@anandabits.com> a écrit :
>
>>
>> On Feb 20, 2017, at 1:42 PM, Michel Fortin <michel.for...@michelf.ca> wrote:
>>
>>> Le 20 févr. 2017 à 14:23, Charles Srstka <cocoa...@charlessoft.com> a écrit
>>> :
>>>
>>> I’m not sure how I feel about that, since it hamstrings the ability to
>>> improve APIs in a lot of ways without breaking backwards compatibility. A
>>> quick example off the top of my head would be all the Cocoa APIs that
>>> started out having ivars representing paths backed by simple getter
>>> methods, and were later refactored to be URL-based, but with the original
>>> path properties become computed properties pointing to the URL’s “path”
>>> property. With this, properties would not be able to be refactored in this
>>> way unless the library developer had previously declared the “path”
>>> property as private(set), which is unlikely for a property that was not
>>> intended to be changed after the class was initialized.
>>
>> Version 1:
>>
>> public class A {
>> public let path: String
>> }
>>
>> Version 2:
>>
>> public class A {
>> public pure var path: String { return url.path }
>> public let path: URL
>> }
>>
>> This is assuming `let` is implicitly pure. It probably should not be. Or at
>> least it should not when crossing module boundaries. Note that internal to
>> the module it wouldn't violate any contract to allow pure code access to
>> `let` variables.
>>
>> Which makes me think of an idea: internal to the module, `pure` could be
>> inferred for everything. Only the published APIs would require the
>> annotations, and only if you want `pure` to be part of the API contract.
>> Attaching `pure` to an internal function could still be useful for your own
>> reasoning though.
>
> That’s a very interesting approach that could lighten the syntactic load. We
> could strategically annotate our code where we want purity verified, but
> otherwise omit the annotation for members that don’t need to be visible
> outside the module. This approach works especially well for closures. I
> like it a lot!
There is an important limitation to this though: a class method that mutate
something in the class is never going to be implicitly pure (per the rules of
purity for instance methods). For that the compiler would have to prove the
method is only called from unique references, and that'd be a bit weird (add a
method call somewhere and suddenly the function becomes impure).
There's also an issue with diagnostics: say you have func1 that calls func2
that calls func3. Func1 is `pure`, the other two are pure only implicitly. Then
you change something in func3, and suddenly func1 complains that it can't call
impure func2 and you are left wondering why because you haven't changed
anything in func2. Perhaps the compiler could dig in the call tree to find the
impure operation and tell you, but that's starting to look like the awful
diagnostic messages for C++ templates.
So now I'm starting to think this idea of inferring purity was perhaps a bit
reckless.
--
Michel Fortin
https://michelf.ca
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