diff -r 8b80aa64adec -r 3b7fcc651585 src/DifferentiableFN.jl
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/DifferentiableFN.jl	Sat Feb 20 16:26:02 2021 -0500
@@ -0,0 +1,32 @@
+module DifferentiableFN
+
+using ..LinOps
+
+export DiffF,
+       value,
+       differential,
+       adjoint_differential
+
+abstract type DiffF{X,Y,T} end
+
+function value(f :: D, x :: X) :: Y where {X, Y, T <: LinOp{X,Y}, D <: DiffF{X,Y,T}}
+    @error "`value` unimplemented"
+end
+
+# function (f :: D)(x::X) where {X, Y, T <: LinOp{X, Y}, D <: DiffF{X, Y, T}}
+#     return value(x)
+# end
+
+function differential(f :: DiffF{X,Y,T}, x :: X) :: T where {X, Y, T <: LinOp{X, Y}}
+    @error "`differential` unimplemented"
+end
+
+function adjoint_differential(f :: DiffF{X,Y,T}, x :: X) :: T where {X, Y, T <: LinOp{X, Y}}
+    @error "`adjoint_differential` unimplemented"
+end
+
+function adjoint_differential(f :: DiffF{X,Y, T}, x :: X) :: T where {X, Y, T <: AdjointableOp{X, Y}}
+    return AdjointOp(differential(f, x))
+end
+
+end