Added implementation of get(coll, key) -> Nullable.

This returns a Nullable value corresponding to the key (or null).
Includes specialization for various dictionary types.
Fixes JuliaLang#13055

base/collections.jl

@@ -323,6 +323,11 @@ function get{K,V}(pq::PriorityQueue{K,V}, key, deflt)
     i == 0 ? deflt : pq.xs[i].second
 end
 
+function get{K,V}(pq::PriorityQueue{K,V}, key)
+    i = get(pq.index, key, 0)
+    i == 0 ? Nullable{V}() : Nullable{V}(pq.xs[i].second)
+end
+
 
 # Change the priority of an existing element, or equeue it if it isn't present.
 function setindex!{K,V}(pq::PriorityQueue{K, V}, value, key)

base/dict.jl

@@ -6,6 +6,8 @@ const secret_table_token = :__c782dbf1cf4d6a2e5e3865d7e95634f2e09b5902__
 
 haskey(d::Associative, k) = in(k,keys(d))
 
+get{K,V}(d::Associative{K,V}, k) = haskey(d, k) ? Nullable{V}(d[k]::V) : Nullable{V}()
+
 function in(p::Pair, a::Associative, valcmp=(==))
     v = get(a,p[1],secret_table_token)
     if !is(v, secret_table_token)
@@ -698,6 +700,11 @@ function get{K,V}(default::Callable, h::Dict{K,V}, key)
     return (index < 0) ? default() : h.vals[index]::V
 end
 
+function get{K,V}(h::Dict{K,V}, key)
+    index = ht_keyindex(h, key)
+    return (index<0) ? Nullable{V}() : Nullable{V}(h.vals[index]::V)
+end
+
 haskey(h::Dict, key) = (ht_keyindex(h, key) >= 0)
 in{T<:Dict}(key, v::KeyIterator{T}) = (ht_keyindex(v.dict, key) >= 0)
 
@@ -828,6 +835,7 @@ function getindex(dict::ImmutableDict, key)
     end
     throw(KeyError(key))
 end
+
 function get(dict::ImmutableDict, key, default)
     while isdefined(dict, :parent)
         dict.key == key && return dict.value
@@ -836,6 +844,14 @@ function get(dict::ImmutableDict, key, default)
     return default
 end
 
+function get{K,V}(dict::ImmutableDict{K,V}, key)
+    while isdefined(dict, :parent)
+        dict.key == key && return Nullable{V}(dict.value::V)
+        dict = dict.parent
+    end
+    Nullable{V}()
+end
+
 # this actually defines reverse iteration (e.g. it should not be used for merge/copy/filter type operations)
 start(t::ImmutableDict) = t
 next{K,V}(::ImmutableDict{K,V}, t) = (Pair{K,V}(t.key, t.value), t.parent)

base/docs/helpdb/Base.jl

@@ -4014,6 +4014,14 @@ end
 """
 get
 
+"""
+    get(collection, key) -> Nullable
+
+Return the value stored for the given key as `Nullable(value)`,
+or if no mapping for the key is present, return null.
+"""
+get(collection, key)
+
 """
     .!=(x, y)
     .≠(x,y)

base/weakkeydict.jl

@@ -40,6 +40,7 @@ end
 
 get{K}(wkh::WeakKeyDict{K}, key, default) = lock(() -> get(wkh.ht, key, default), wkh)
 get{K}(default::Callable, wkh::WeakKeyDict{K}, key) = lock(() -> get(default, wkh.ht, key), wkh)
+get{K}(wkh::WeakKeyDict{K}, key) = lock(() -> get(wkh.ht, key), wkh)
 get!{K}(wkh::WeakKeyDict{K}, key, default) = lock(() -> get!(wkh.ht, key, default), wkh)
 get!{K}(default::Callable, wkh::WeakKeyDict{K}, key) = lock(() -> get!(default, wkh.ht, key), wkh)
 pop!{K}(wkh::WeakKeyDict{K}, key) = lock(() -> pop!(wkh.ht, key), wkh)

doc/stdlib/collections.rst

@@ -987,6 +987,12 @@ Given a dictionary ``D``, the syntax ``D[x]`` returns the value of key ``x`` (if
 
    Determine whether a collection has a mapping for a given key.
 
+.. function:: get(collection, key) -> Nullable
+
+   .. Docstring generated from Julia source
+
+   Return the value stored for the given key as ``Nullable(value)``\ , or if no mapping for the key is present, return null.
+
 .. function:: get(collection, key, default)
 
    .. Docstring generated from Julia source

test/dict.jl

@@ -256,6 +256,10 @@ let f(x) = x^2, d = Dict(8=>19)
         f(4)
     end == 16
 
+    # get (returning Nullable)
+    @test get(get(d, 8)) == 19
+    @test isnull(get(d, 13))
+
     @test d == Dict(8=>19, 19=>2, 42=>4)
 end
 
@@ -448,6 +452,10 @@ let d = ImmutableDict{String, String}(),
     @test in(k2 => 1.0, dnum, is)
     @test !in(k2 => 1, dnum, <)
     @test in(k2 => 0, dnum, <)
+    @test get(d1, "key1") === Nullable{String}(v1)
+    @test get(d4, "key1") === Nullable{String}(v2)
+    @test get(d4, "foo") === Nullable{String}()
+    @test get(d, k1) === Nullable{String}()
     @test get(d1, "key1", :default) === v1
     @test get(d4, "key1", :default) === v2
     @test get(d4, "foo", :default) === :default

test/priorityqueue.jl

@@ -107,3 +107,21 @@ for priority in values(priorities)
     heappush!(xs, priority)
 end
 @test issorted([heappop!(xs) for _ in length(priorities)])
+
+
+# test length, in, haskey, get
+let
+    pq = PriorityQueue(Dict(
+        i => i+5 for i in 1:5
+    ))
+    @test length(pq) == 5
+
+    for i in 1:5
+        @test haskey(pq, i)
+        @test (i=>1+5) in pq
+        @test get(pq, i, 0) == i + 5
+        @test get(get(pq, i)) == i+5
+    end
+    @test get(pq, 10, 0) == 0
+    @test isnull(get(pq, 10))
+end