diff --git a/Project.toml b/Project.toml
index ae6c77687..9321d78d0 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "0.4.0"
+version = "0.5.0-DEV"
 
 [compat]
 julia = "^1.0"
diff --git a/src/ChainRulesCore.jl b/src/ChainRulesCore.jl
index db3e6e94f..3a8942e65 100644
--- a/src/ChainRulesCore.jl
+++ b/src/ChainRulesCore.jl
@@ -4,7 +4,7 @@ using Base.Broadcast: materialize, materialize!, broadcasted, Broadcasted, broad
 export frule, rrule
 export wirtinger_conjugate, wirtinger_primal, refine_differential
 export @scalar_rule, @thunk
-export extern, store!
+export extern, store!, unthunk
 export Wirtinger, Zero, One, DoesNotExist, Thunk, InplaceableThunk
 export NO_FIELDS
 
diff --git a/src/differential_arithmetic.jl b/src/differential_arithmetic.jl
index eb06b9bac..54d142812 100644
--- a/src/differential_arithmetic.jl
+++ b/src/differential_arithmetic.jl
@@ -78,12 +78,12 @@ for T in (:AbstractThunk, :Any)
 end
 
 
-Base.:+(a::AbstractThunk, b::AbstractThunk) = extern(a) + extern(b)
-Base.:*(a::AbstractThunk, b::AbstractThunk) = extern(a) * extern(b)
+Base.:+(a::AbstractThunk, b::AbstractThunk) = unthunk(a) + unthunk(b)
+Base.:*(a::AbstractThunk, b::AbstractThunk) = unthunk(a) * unthunk(b)
 for T in (:Any,)
-    @eval Base.:+(a::AbstractThunk, b::$T) = extern(a) + b
-    @eval Base.:+(a::$T, b::AbstractThunk) = a + extern(b)
+    @eval Base.:+(a::AbstractThunk, b::$T) = unthunk(a) + b
+    @eval Base.:+(a::$T, b::AbstractThunk) = a + unthunk(b)
 
-    @eval Base.:*(a::AbstractThunk, b::$T) = extern(a) * b
-    @eval Base.:*(a::$T, b::AbstractThunk) = a * extern(b)
+    @eval Base.:*(a::AbstractThunk, b::$T) = unthunk(a) * b
+    @eval Base.:*(a::$T, b::AbstractThunk) = a * unthunk(b)
 end
diff --git a/src/differentials.jl b/src/differentials.jl
index 640ee02bb..bfe6ba91d 100644
--- a/src/differentials.jl
+++ b/src/differentials.jl
@@ -164,6 +164,18 @@ end
     return element, (externed, new_state)
 end
 
+"""
+    unthunk(x)
+
+On `AbstractThunk`s this removes 1 layer of thunking.
+On any other type, it is the identity operation.
+
+In contrast to `extern` this is nonrecursive.
+"""
+@inline unthunk(x) = x
+
+@inline extern(x::AbstractThunk) = extern(unthunk(x))
+
 #####
 ##### `Thunk`
 #####
@@ -228,9 +240,9 @@ end
 # have to define this here after `@thunk` and `Thunk` is defined
 Base.conj(x::AbstractThunk) = @thunk(conj(extern(x)))
 
-
 (x::Thunk)() = x.f()
-@inline extern(x::Thunk) = extern(x())
+@inline unthunk(x::Thunk) = x()
+
 
 Base.show(io::IO, x::Thunk) = println(io, "Thunk($(repr(x.f)))")
 
@@ -252,8 +264,8 @@ struct InplaceableThunk{T<:Thunk, F} <: AbstractThunk
     add!::F
 end
 
-(x::InplaceableThunk)() = x.val()
-@inline extern(x::InplaceableThunk) = extern(x.val)
+@inline unthunk(x::InplaceableThunk) = unthunk(x.val)
+(x::InplaceableThunk)() = unthunk(x)
 
 function Base.show(io::IO, x::InplaceableThunk)
     println(io, "InplaceableThunk($(repr(x.val)), $(repr(x.add!)))")
diff --git a/test/differentials.jl b/test/differentials.jl
index 6ddad9b4f..bbe84bd0a 100644
--- a/test/differentials.jl
+++ b/test/differentials.jl
@@ -62,6 +62,11 @@
             @test extern(@thunk(@thunk(3))) == 3
         end
 
+        @testset "unthunk" begin
+            @test unthunk(@thunk(3)) == 3
+            @test unthunk(@thunk(@thunk(3))) isa Thunk
+        end
+
         @testset "calling thunks should call inner function" begin
             @test (@thunk(3))() == 3
             @test (@thunk(@thunk(3)))() isa Thunk