Add dict.pop()

[plajjan]

We discussed adding .pop() to dict / Mapping in our weekly meeting.

We said we wanted this:

protocol Mapping[A(Eq),B] (Container[A], Indexed[A,B]):
# NO EXCEPTIONS!!!
    get         : (A,?B) -> ?B
    pop         : mut(A,?B) -> ?B

And these functions should not throw any exceptions - they should return None if the key is not found.

[plajjan]

Hmm, so on second consideration, don't we want to have separate method for the get-with-default-value??

With the change as written in this issue and implemented in #1986, we end up having to None-check the result, even when we provide a default value - that seems suboptimal.

Like I feel the whole idea with a default value is that we know we get a non-optional value back. We can never statically enforce the type to be non-optional for a single get method where the default argument is optional, so I guess we need separate methods?

Something like this:

• __getitem__(A) -> B but throws KeyError if A does not exist
• get(A) -> ?B returns None if A does not exist
• getdef(A, B) -> B returns B if A does not exist

and similar for pop and popdef then !?

@sydow @nordlander @ddjoh WDYT?

[nordlander]

I agree, get with a default value is not the same as an optional get. One could merge the two by moving to the more elaborate signature

    get : [B(D)] => (A, Optional[D]) -> D

but that would require the planned support for unions as well as a lifted restriction in the constraint-solver. For later (perhaps).

The only thing about adding getdef is that def probably reads as definition to must Python programmers, not default. Maybe getopt could used instead?

[plajjan]

I wouldn't read too much into the name. Can't say I would immediately feel getopt is more intuitive than getdef. I mean, sometimes you just have to learn what something means and not what you might guess from a few characters.