Add modify! function for lookup/update/insert/delete in one go

base/abstractdict.jl

@@ -465,6 +465,63 @@ function hash(a::AbstractDict, h::UInt)
     hash(hv, h)
 end
 
+"""
+    modify!(f, d::AbstractDict{K, V}, key)
+
+Lookup and then update, insert or delete in one go without re-computing the hash.
+
+`f` is a callable object that must accept `Union{Some{V}, Nothing}` and return
+`Union{T, Some{T}, Nothing}` where `T` is a type [`convert`](@ref)-able to the value type
+`V`.  The value `Some(d[key])` is passed to `f` if `haskey(d, key)`; otherwise `nothing`
+is passed.  If `f` returns `nothing`, corresponding entry in the dictionary `d`  is removed.
+If `f` returns non-`nothing` value `x`, `something(x)` is inserted to `d`.
+
+`modify!` returns whatever `f` returns as-is.
+
+# Examples
+```jldoctest
+julia> dict = Dict("a" => 1);
+
+julia> modify!(dict, "a") do val
+           Some(val === nothing ? 1 : something(val) + 1)
+       end
+Some(2)
+
+julia> dict
+Dict{String,Int64} with 1 entry:
+  "a" => 2
+
+julia> dict = Dict();
+
+julia> modify!(dict, "a") do val
+           Some(val === nothing ? 1 : something(val) + 1)
+       end
+Some(1)
+
+julia> dict
+Dict{Any,Any} with 1 entry:
+  "a" => 1
+
+julia> modify!(_ -> nothing, dict, "a")
+
+julia> dict
+Dict{Any,Any} with 0 entries
+```
+"""
+function modify!(f, dict::AbstractDict, key)
+    if haskey(dict, key)
+        val = f(Some(dict[key]))
+    else
+        val = f(nothing)
+    end
+    if val === nothing
+        delete!(dict, key)
+    else
+        dict[key] = something(val)
+    end
+    return val
+end
+
 function getindex(t::AbstractDict, key)
     v = get(t, key, secret_table_token)
     if v === secret_table_token

base/dict.jl

@@ -391,6 +391,45 @@ function setindex!(h::Dict{K,V}, v0, key::K) where V where K
     return h
 end
 
+function modify!(f, h::Dict{K}, key0) where K
+    key = convert(K, key0)
+    if !isequal(key, key0)
+        throw(ArgumentError("$(limitrepr(key0)) is not a valid key for type $K"))
+    end
+
+    # Ideally, to improve performance for the case that requires
+    # resizing, we should use something like `ht_keyindex` while
+    # keeping computed hash value and then do something like
+    # `ht_keyindex2!` if `f` returns non-`nothing`.
+    idx = ht_keyindex2!(h, key)
+
+    age0 = h.age
+    if idx > 0
+        @inbounds vold = h.vals[idx]
+        vnew = f(Some(vold))
+    else
+        vnew = f(nothing)
+    end
+    if h.age != age0
+        idx = ht_keyindex2!(h, key)
+    end
+
+    if vnew === nothing
+        if idx > 0
+            _delete!(h, idx)
+        end
+    else
+        if idx > 0
+            h.age += 1
+            @inbounds h.keys[idx] = key
+            @inbounds h.vals[idx] = something(vnew)
+        else
+            @inbounds _setindex!(h, something(vnew), key, -idx)
+        end
+    end
+    return vnew
+end
+
 """
     get!(collection, key, default)
 

test/dict.jl

@@ -1057,8 +1057,11 @@ end
                 new{keytype(d), valtype(d)}(d)
             end
         end
+        Base.Dict(td::TestDict) = td.dict
+        Base.haskey(td::TestDict, key) = haskey(td.dict, key)
         Base.setindex!(td::TestDict, args...) = setindex!(td.dict, args...)
         Base.getindex(td::TestDict, args...) = getindex(td.dict, args...)
+        Base.delete!(td::TestDict, key) = delete!(td.dict, key)
         Base.pairs(D::TestDict) = pairs(D.dict)
         testdict = TestDict(:a=>1, :b=>2)
         map!(v->v-1, values(testdict))
@@ -1072,3 +1075,51 @@ end
         @test testdict[:b] == 1
     end
 end
+
+@testset "modify!(f, ::$constructor, key)" for constructor in [
+    Dict,
+    TestDict,
+]
+    @testset "update" begin
+        dict = constructor(Dict("a" => 1))
+
+        @test modify!(dict, "a") do val
+            Some(val === nothing ? 1 : something(val) + 1)
+        end == Some(2)
+
+        @test Dict(dict) == Dict("a" => 2)
+    end
+
+    @testset "insert" begin
+        dict = constructor(Dict())
+
+        @test modify!(dict, "a") do val
+            Some(val === nothing ? 1 : something(val) + 1)
+        end == Some(1)
+
+        @test Dict(dict) == Dict("a" => 1)
+    end
+
+    @testset "delete" begin
+        dict = constructor(Dict("a" => 1))
+        @test modify!(_ -> nothing, dict, "a") === nothing
+        @test Dict(dict) == Dict()
+    end
+
+    @testset "no-op" begin
+        dict = constructor(Dict("a" => 1))
+        @test modify!(_ -> nothing, dict, "b") === nothing
+        @test Dict(dict) == Dict("a" => 1)
+    end
+
+    @testset "mutation inside `f`" begin
+        dict = constructor(Dict())
+
+        @test modify!(dict, "a") do val
+            dict["a"] = 0
+            Some(val === nothing ? 1 : something(val) + 1)
+        end == Some(1)
+
+        @test Dict(dict) == Dict("a" => 1)
+    end
+end