Implement setindex for ImmutableDict

base/dict.jl

@@ -851,6 +851,30 @@ length(t::ImmutableDict) = count(Returns(true), t)
 isempty(t::ImmutableDict) = !isdefined(t, :parent)
 empty(::ImmutableDict, ::Type{K}, ::Type{V}) where {K, V} = ImmutableDict{K,V}()
 
+"""
+    setindex(d::ImmutableDict, value, key)
+
+Creates a new `ImmutableDict` similar to `d` with the key `key` set to `value`.
+If `key` is already in the keys of `d`, `value` replaces the old value.
+
+# Examples
+```jldoctest
+julia> Base.setindex(Base.ImmutableDict(:a => 1), 2, :a) == Base.ImmutableDict(:a => 2)
+true
+julia> Base.setindex(Base.ImmutableDict(:a => 1), 2, :b) == Base.ImmutableDict(:a => 1, :b => 2)
+true
+```
+"""
+function setindex(d::ImmutableDict, value, key)
+    Base.ImmutableDict(
+        (
+            k => v
+            for (k, v) in d if k != key
+        )...,
+        key => value,
+    )
+end
+
 _similar_for(c::AbstractDict, ::Type{Pair{K,V}}, itr, isz, len) where {K, V} = empty(c, K, V)
 _similar_for(c::AbstractDict, ::Type{T}, itr, isz, len) where {T} =
     throw(ArgumentError("for AbstractDicts, similar requires an element type of Pair;\n  if calling map, consider a comprehension instead"))

test/dict.jl

@@ -907,6 +907,10 @@ import Base.ImmutableDict
     @test d6[:b] == 3
 
     @test !haskey(ImmutableDict(-0.0=>1), 0.0)
+
+    d7 = ImmutableDict(:a => 1)
+    @test Base.setindex(d7, 2, :a) == ImmutableDict(:a => 2)
+    @test Base.setindex(d7, 2, :b) == ImmutableDict(:a => 1, :b => 2)
 end
 
 @testset "filtering" begin