annotate type of operation field in Node type

src/composition/learning_networks/nodes.jl

@@ -27,9 +27,9 @@ See also [`node`](@ref), [`Source`](@ref), [`origins`](@ref),
 [`sources`](@ref), [`fit!`](@ref).
 
 """
-struct Node{T<:Union{Machine, Nothing}} <: AbstractNode
+struct Node{T<:Union{Machine, Nothing},Oper} <: AbstractNode
 
-    operation   # eg, `predict` or a static operation, such as `exp`
+    operation::Oper   # eg, `predict` or a static operation, such as `exp`
     machine::T  # is `nothing` for static operations
 
     # nodes called to get args for `operation(model, ...) ` or
@@ -43,9 +43,11 @@ struct Node{T<:Union{Machine, Nothing}} <: AbstractNode
     # order consistent with extended graph, excluding self
     nodes::Vector{AbstractNode}
 
-    function Node(operation,
-                  machine::T,
-                  args::AbstractNode...) where T<:Union{Machine, Nothing}
+    function Node(
+        operation::Oper,
+        machine::T,
+        args::AbstractNode...,
+        ) where {T<:Union{Machine, Nothing}, Oper}
 
         # check the number of arguments:
         # if machine === nothing && isempty(args)
@@ -70,7 +72,7 @@ struct Node{T<:Union{Machine, Nothing}} <: AbstractNode
                 vcat(nodes_, (nodes(n) for n in machine.args)...) |> unique
         end
 
-        return new{T}(operation, machine, args, origins_, nodes_)
+        return new{T,Oper}(operation, machine, args, origins_, nodes_)
     end
 end