Get dict value as a nullable

[dbeach24]

I need a good way to both test for a value in a dictionary and get that value, without resorting to one of the following fallbacks, all of which are somewhat undesirable:

1. get(dict, key, specialnullval) -- problematic because I must either use a separate type to represent null (inefficient due to the type-variadic return), or I must have some special way to represent null within the value type of the dictionary.
2. haskey(dict, key) followed by dict[key] -- problematic because it looks up the key twice when it exists.
3. try k = dict[key] catch ... -- problematic because it relies on exceptions when the key is not present.

It seems that all of this can be solved my making use of Julia v0.4's Nullable type, with something like the following:

function getnull{K,V}(h::Dict{K,V}, key)
    index = ht_keyindex(h, key)
    return (index<0) ? Nullable{V}() : Nullable{V}(h.vals[index]::V)
end

I am proposing that this be included as part of base/dict.jl, possibly under a different/better name.

Is there some other/better way to do this?

[JeffBezanson]

Very reasonable idea.

[yuyichao]

Ref #12157. Not necessarily a dup of that but should be solved by the proposal there.

[dbeach24]

@yuyichao Thanks for the heads up. I read through #12157, but personally I would side with Stefan: I don't care for the complexity and the syntax. IMO, this seems like a more direct way to solve this (admittedly more narrow) problem.

[ScottPJones]

Does it have to be one or the other (this or #12157)? I think this would be a very nice capability to have, no matter what.

[davidagold]

If ? could be made to parse slightly differently, then perhaps the getnull method proposed above could be named get? and suffixing method names with an ? could indicate in general that the method returns a Nullable object.

[tkelman]

Don't think that would play nicely with ternary syntax. Though resolving #4233 (nicest approach proposed so far would be a general macro @if for compile-time conditionals and replacing @unix? a : b with that) would help free up ? a bit.

[davidagold]

Indeed, hence conditioning the suggestion on getting ? to parse differently. I think that if at least one whitespace character were required between the condition and the ternary operator, then names could include ? at the end (or elsewhere in them, for that matter) without parsing as conditionals.

Fun aside, it is currently possible to use the ? character itself as a name, but the only way I've found the result to be at all functional is if one defines ? to be a callable object, since evidently the parser does not interpret ? as the ternary operator if it is the very first symbol in an expression:

julia>  ? = x -> x + 5
(anonymous function)

julia>  ?(5)
10

[tkelman]

#6286 #1910

[mbauman]

I think this could be spelled get with just two arguments (and no default value).

julia> methods(get, Tuple{Associative,Any})
0-element Array{Any,1}

julia> methods(get, Tuple{Associative,Vararg{Any}})
5-element Array{Any,1}:
 get(t::ObjectIdDict, key::ANY, default::ANY) at dict.jl:326
 get{K,V}(h::Dict{K,V}, key, default) at dict.jl:722
 get{K}(wkh::WeakKeyDict{K,V}, key, default) at dict.jl:834
 get(::Base.EnvHash, k::AbstractString, def) at env.jl:80
 get{K,V}(pq::Base.Collections.PriorityQueue{K,V,O<:Base.Order.Ordering}, key, deflt) at collections.jl:227

julia> methods(get, Tuple{Any,Any})
1-element Array{Any,1}:
 get{T}(x::Nullable{T}, y) at nullable.jl:33

[davidagold]

Ahah, thank you for these resources. I'll be curious to see if requiring whitespace for the ternary operator and thereby freeing up ? for use in identifiers ever becomes a thing.

[toivoh]

I think when the original idea to allow ? in identifiers came up, Stefan said that "you can petty the ternary operator out of my cold, dead hands". So for better or worse, I don't think that will ever be a thing.

[davidagold]

Hmm. Is requiring whitespace between a condition and the ternary operator tantamount to nixing the latter altogether? I understand that such a requirement may be a particularly annoying gotcha, but beyond that I must be missing something.

[johnmyleswhite]

I think @StefanKarpinski may be softening his stance given his growing appreciation of the centrality of nullable types in many practical settings.

[ScottPJones]

Too bad, the ternary operator is one of the things that is always confusing to new programmers, and uses up two ASCII characters (a very limited resource).
I think at least requiring spaces around the ? and : is not such a terrible thing.

[jakebolewski]

Death to the ternary operator.