RFC: Add get! for Dicts (addresses #1529)

[kmsquire]

This PR refactors setindex!(d::Dict, v, key) into setindex! and ht_keyindex2, where ht_keyindex2 returns either the current index of the key, or -index for the insertion location of a new key (after rehashing, etc).

(Compare with ht_keyindex, which returns -1 if the key is not in the hash table, and does not call rehash.)

This is used by get! to get the current or new location of a key.

The PR does not have any measurable impact on performance.

Related:

• Dicts with defaults, etc. #1529, ?= "get or set" operator #2108
• RFC: Add @get! #1764. I feel that a function is preferable to a macro for most users and most uses.
• RFC: Added DefaultDict JuliaCollections/DataStructures.jl#3
  • Provides a DefaultDict implementation based on the Python one.
  • Evaluates ht_keyindex more than once --> kills performance.

CC: @toivoh, @StefanKarpinski

[StefanKarpinski]

I'm for this. The case where the RHS is very expensive could be handled with a higher-order method:

get!(dict,key) do
  value
end

I'd also like to add syntax for this: dict[key] ?= value.

[toivoh]

+1 for this and for syntax. I hope the latter would only evaluate the rhs
if needed.

[kmsquire]

Okay, good.

After playing with this for DefaultDicts, I can also see the value in not evaluating the optional argument, as in Toivoh's @get or his request about the behavior of =?.

Questions:

1. is this 0.3 material, or should it wait?
2. should =? be included in this PR? (I think it could be separate?

#1764 could also be trivially reworked to use the new ht_keyindex2 code.

[toivoh]

I think that ?= is a pretty major thing that definitely deserves its own
PR. (Do you actually know how to do it?)

[kmsquire]

I think that ?= is a pretty major thing that definitely deserves its own PR. (Do you actually know how to do it?)

Agree. I don't know how to do it off-hand, but I think I could probably figure it out. But if you (or someone else) want to take a shot, feel free! It might be a couple of simple additions to julia-syntax.scm

[kmsquire]

I've added the do-block syntax of get! that @StefanKarpinski suggested, as well as tests and docs.

[kmsquire]

In response to #1764 (comment), I added do-block syntax for get as well.

Also added get! and a do-block version of get for WeakKeyDicts. Haven't had time for ObjectIdDicts yet, though I can add that if there's interest.

[StefanKarpinski]

I like it. I'm down do merge this. @JeffBezanson?

[JeffBezanson]

Ok by me.