Fix delete with Elements optic - fixes #176

- Add delete methods for Elements optic that create empty collections
- Add tests for Elements delete functionality including nested cases

Author: aplavin

src/optics.jl

@@ -247,6 +247,10 @@ modify(f, obj, ::Elements) = map(f, obj)
 modify(f, obj::Set, ::Elements) = Set(f(p) for p in obj)
 modify(f, obj::Dict, ::Elements) = Dict(f(p)::Pair for p in obj)
 
+# delete all elements, creating an empty collection of the same type
+delete(obj::Union{Tuple, NamedTuple, AbstractVector, AbstractDict, Set}, ::Elements) = empty(obj)
+delete(obj::AbstractString, ::Elements) = ""
+
 """
     If(modify_condition)
 

test/test_insert_delete.jl

@@ -120,6 +120,16 @@ end
         @test delete( (a=1, b=(2, 3, 4)), @optic(first(_.b)) ) == (a=1, b=(3, 4))
         @test delete( "path/to/file", @optic(basename(_)) ) == "path/to"
         @test delete( "path/to/file", @optic(dirname(_)) ) == "file"
+        
+        # delete with Elements creates empty collections
+        @test delete((1, 2, 3), Elements()) === ()
+        @test delete([1, 2, 3], Elements()) == Int[]
+        @test delete(Set([1, 2, 3]), Elements()) == Set{Int}()
+        @test delete(Dict(1 => :a, 2 => :b), Elements()) == Dict{Int, Symbol}()
+        @test delete((a=1, b=2, c=3), Elements()) === NamedTuple()
+        # nested case: delete all elements from nested collections
+        nested_dict = Dict("a" => (1, 2, 3))
+        @test delete(nested_dict, @optic _["a"][∗]) == Dict("a" => ())
     end
 
     @testset "macro" begin