(this was accidentally sent off-list, reposting here a day later)
> Le 20 févr. 2017 à 12:17, Matthew Johnson <[email protected]> a écrit :
>
>> e) Generic Containers:
>> Generic containers that impose requirements on their elements will pose some
>> additional problems, for instance: `Set.insert` needs to call `hashValue`
>> and `==` on its elements, making the purity of `Set.insert` constrained by
>> the purity of those functions. Without a way to express this kind of
>> conditional purity, `Set` and `Dictionary` cannot be pure.
>
> Could we use a mechanism similar to `rethrows` to address this kind of
> transitive purity? We have already discussed something along those lines for
> handling functions passed as arguments. Maybe that mechanism could be
> designed to handle this use case as well.
Similar, yes. But more complicated too. In pseudo code, this is what you'd have
to express for `Set.insert`:
pure(where: Element.hashValue is pure, Element.== is pure)
func insert(_ element: Element) { ... }
Then the compiler can enforce that `insert` only does things that are allowed
in a pure function, but can ignore the purity of Element.hashValue and
Element.== because the caller will check for that that.
--
Michel Fortin
https://michelf.ca
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