[swift-evolution] Proposal proposal: @pure keyword

[Andrew Bennett]

I'd like to discuss adding a @pure keyword, and see what it requires and
how possible it is to  include.

I'd like to use the annotation @pure on functions, methods and closures.

This will allow us to make more guarantees about what a protocol does, and
what it cannot do. It will also allow APIs like `.map` and `.forEach` to
have a meaningful distinction. It could also allow for something like an
assert to be removed as an optimisation with no side-effects in a release
build.

If something is pure it can be annotated with @pure, if it is not-pure this
will be a compile-time error. The compiler could automatically add this
annotation in the interface.

A function, method or closure is pure if:
 * all functions, methods and closures it calls are also pure (this
includes referencing self from a method or property)
 * it only externally references let variables of pure type, let properties
on those variables, and methods/computer-properties marked as pure.

A computed property can be marked as pure like this:

@pure var test: A


A function/method/closure signature is marked as pure like this:

(a: A, b: A -> B) @pure -> B


If the function/method/closure is only pure if an argument is pure (similar
to @rethrows), then you could do this:

(start: A, a: A -> B) @pure(b) -> B


Potentially this could be extended to allow a pure closure to be composed
of other pure closures:

func compose(a: A -> B, b: B -> C) @pure -> A @pure(a,b) -> C {

    return { b(a($0)) }

}


Of course this doesn't preclude you from requiring some of those closures
to be pure:

func compose(a: A @pure -> B, b: B -> C) @pure -> A @pure(b) -> C {

    return { b(a($0)) }

}


Impact on existing code:

No negative impact as if automatic annotation was allowed it would only
happen where it was safe to do so. If it was not allowed then libraries
could be slowly annotated to be marked as pure.

c and objective-c would not be marked as pure.

Pure functions can be safely removed by the optimiser if their result is
not used. From this perspective assert and print should not be marked as
pure (they would have to be an exception anyway).

Ideally existing libraries would be annotated, the more the better, but
this can be a gradual process.
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