RFC: Add @get!

base/dict.jl

@@ -7,6 +7,24 @@ const _jl_secret_table_token = :__c782dbf1cf4d6a2e5e3865d7e95634f2e09b5902__
 has(t::Associative, key) = !is(get(t, key, _jl_secret_table_token),
                                _jl_secret_table_token)
 
+# @get! works like get, but evaluates the default value only if the key is not
+# present; it then assigns the default value to the key and returns the
+# (converted) value
+macro get!(d, k, default)
+    quote
+        d::Associative, k = $(esc(d)), $(esc(k))
+        p, index = _assoc_keyindex(d, k)
+        V = valtype(d)
+        ( p ? _get_at(d, index) : (d[k] = convert(V,$(esc(default)))) )::V
+    end
+end
+
+# Used by @get!.
+# If you provide _assoc_keyindex for a type,
+# you must provide a matching _get_at.
+_assoc_keyindex(d::Associative, k) = (has(d, k), k)
+_get_at(d::Associative, index)     = d[index]
+
 function show{K,V}(io, t::Associative{K,V})
     if isempty(t)
         print(io, typeof(t),"()")
@@ -423,6 +441,9 @@ function get{K,V}(h::Dict{K,V}, key, deflt)
     return (index<0) ? deflt : h.vals[index]::V
 end
 
+_assoc_keyindex(d::Dict, k) = (i=ht_keyindex(d,k);(i<0) ? (false,0) : (true,i))
+_get_at(d::Dict, index)     = d.vals[index]
+
 has(h::Dict, key) = (ht_keyindex(h, key) >= 0)
 
 function key{K,V}(h::Dict{K,V}, key, deflt)

base/export.jl

@@ -1294,6 +1294,7 @@ export
     unsetenv,
 
 # Macros
+    @get!,
     @v_str,
     @unexpected,
     @assert,

test/corelib.jl

@@ -181,6 +181,17 @@ d4[1001] = randstring(3)
 @test !isequal({1 => 2}, {"dog" => "bone"})
 @test isequal(Dict{Int, Int}(), Dict{String, String}())
 
+#@get!
+let
+    d = {8=>19}
+    def = {}
+    f = x->(push(def,x); x)
+    @test @get!(d,  8, f(5)) == 19
+    @test @get!(d, 19, f(2)) == 2
+    @test d == {8=>19, 19=>2}
+    @test def == [2]
+end
+
 # ############# end of dict tests #############
 
 # #################### set ####################