support explicit specification of the optic target

src/sugar.jl

@@ -166,6 +166,20 @@ 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

test/test_core.jl

@@ -136,6 +136,21 @@ end
     @test [@set(t[1] = 10), @set(t[2] *= 2)] == [(10, 2), (1, 4)]
 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
+    @test (@set $(x)[2] + 2 = 100) == [1, 98, 3]  # impossible without $
+    @test (@set $(x[2]) + 2 = 100) == 98  # impossible without $
+    @test x_orig === x == [1, 2, 3]
+
+    @test (@reset $(x[2]) = 100) == 100
+    @test x_orig === x == [1, 100, 3]
+    @test (@reset $(x)[2] = 200) == [1, 200, 3]
+    @test x_orig !== x == [1, 200, 3]
+end
+
 
 struct UserDefinedLens end