lens support for only

src/Accessors.jl

@@ -3,11 +3,6 @@ using MacroTools
 using MacroTools: isstructdef, splitstructdef, postwalk
 using Requires: @require
 
-include("setindex.jl")
-include("optics.jl")
-include("sugar.jl")
-include("functionlenses.jl")
-include("testing.jl")
 
 
 if !isdefined(Base, :only)
@@ -22,4 +17,10 @@ function __init__()
     @require StaticArrays = "90137ffa-7385-5640-81b9-e52037218182" include("staticarrays.jl")
 end
 
+include("setindex.jl")
+include("optics.jl")
+include("sugar.jl")
+include("functionlenses.jl")
+include("testing.jl")
+
 end

src/functionlenses.jl

@@ -10,6 +10,11 @@ insert(obj, ::typeof(first), val) = insert(obj, IndexLens((firstindex(obj),)), v
 set(obj, ::typeof(identity), val) = val
 set(obj, ::typeof(inv), new_inv) = inv(new_inv)
 
+function set(obj, ::typeof(only), val)
+    only(obj) # error check
+    set(obj, first, val)
+end
+
 ################################################################################
 ##### eltype
 ################################################################################

test/test_core.jl

@@ -5,6 +5,9 @@ using Accessors: test_getset_laws, test_modify_law
 using Accessors: compose, get_update_op
 using ConstructionBase: ConstructionBase
 using StaticNumbers: StaticNumbers, static
+if !isdefined(Base, :only)
+    using Accessors: only
+end
 
 struct T
     a
@@ -126,6 +129,9 @@ end
 
     t = (1, 2)
     @test (@set t |> first = 10) === (10, 2)
+
+    @test @set(only((1,)) = 2 ) === (2,)
+    @test_throws ArgumentError @set(only((1,2)) = 2 )
 end