support explicit specification of the optic target

src/sugar.jl

@@ -158,6 +158,20 @@ function lower_index(collection::Symbol, index, dim)
 end
 
 function parse_obj_optics(ex)
+    dollar_exprs = foldtree([], ex) do exs, x
+        x isa Expr && x.head == :$ ?
+            push!(exs, only(x.args)) :
+            exs
+    end
+    if !isempty(dollar_exprs)
+        length(dollar_exprs) == 1 || error("Only a single dollar-expression is supported")
+        # obj is the only dollar-expression:
+        obj = esc(only(dollar_exprs))
+        # parse expr with an underscore instead of the dollar-expression:
+        _, optics = parse_obj_optics(postwalk(x -> x isa Expr && x.head == :$ ? :_ : x, ex))
+        return obj, optics
+    end
+
     if @capture(ex, (front_ |> back_))
         obj, frontoptic = parse_obj_optics(front)
         backoptic = try
@@ -289,7 +303,7 @@ function setmacro(optictransform, ex::Expr; overwrite::Bool=false)
         f = :($_UpdateOp($op,$val))
         :($modify($f, $obj, ($optictransform)($optic)))
     end
-    return overwrite ? :($obj = $ret) : ret
+    return _macro_expression_result(obj, ret; overwrite=overwrite)
 end
 
 """
@@ -322,9 +336,18 @@ function insertmacro(optictransform, ex::Expr; overwrite::Bool=false)
     obj, optic = parse_obj_optic(ref)
     val = esc(val)
     ret = :($insert($obj, ($optictransform)($optic), $val))
-    return overwrite ? :($obj = $ret) : ret
+    return _macro_expression_result(obj, ret; overwrite=overwrite)
 end
 
+_macro_expression_result(obj, ret; overwrite) =
+    if overwrite
+        @assert Meta.isexpr(obj, :escape)
+        only(obj.args) isa Symbol || throw(ArgumentError("Rebinding macros can only be used with plain variables as targets. Got expression: $obj"))
+        return :($obj = $ret)
+    else
+        return ret
+    end
+
 """
     @optic
 

test/test_core.jl

@@ -45,6 +45,8 @@ end
     nt = (a=1,)
     @reset nt.a = 5
     @test nt === (a=5,)
+
+    @test_throws Exception eval(:(@reset func(x, y) = 100))
 end
 
 @testset "@set" begin
@@ -137,6 +139,23 @@ end
     @test @set(p[2] = 1.23) === (1 => 1.23)
 end
 
+@testset "explicit target" begin
+    x = [1, 2, 3]
+    x_orig = x
+    @test (@set $(x)[2] = 100) == [1, 100, 3]
+    @test (@set $(x[2]) = 100) == 100
+    # these are impossible with @set without $:
+    @test (@set $(x)[2] + 2 = 100) == [1, 98, 3]
+    @test (@set $(x[2]) + 2 = 100) == 98
+    @test (@set first($x, 2) = [10, 20]) == [10, 20, 3]
+
+    @test_throws Exception eval(:(@reset $(x[2]) = 100))
+    @test_throws Exception eval(:(@reset $(x)[2] = 200))
+
+    # ensure the object itself didn't change:
+    @test x_orig === x == [1, 2, 3]
+end
+
 
 struct UserDefinedLens end