WIP

src/tangent_types/abstract_zero.jl

@@ -126,36 +126,38 @@ end
 
 @generated function zero_tangent(primal)
     fieldcount(primal) == 0 && return NoTangent()  # no tangent space at all, no need for structural zero.
-    zfield_exprs = map(fieldnames(primal)) do fname
-        fval = :(
-            if isdefined(primal, $(QuoteNode(fname)))
-                zero_tangent(getfield(primal, $(QuoteNode(fname))))
-            else
-                # This is going to be potentially bad, but that's what they get for not giving us a primal
-                # This will never me mutated inplace, rather it will alway be replaced with an actual value first
-                ZeroTangent()
-            end
-        )
-        Expr(:kw, fname, fval)
-    end
-
+
     # easy case exit early, can't hold references, can't be a reference.
     if isbitstype(primal)
+        zfield_exprs = map(fieldnames(primal)) do fname
+            fval = :(zero_tangent(getfield(primal, $(QuoteNode(fname)))))
+            Expr(:kw, fname, fval)
+        end
         return :($Tangent{$primal}($(Expr(:parameters, zfield_exprs...))))
     end
 
-    # hard case need to be prepared for cycic references to this, or that are contained within this
+    # hard case need to be prepared for references to this, or that are contained within this
     quote
-        counts = $count_references!(primal)
+        counts = $count_references(primal)
+        any_mask = $(Expr(:tuple, Expr(:parameters, map(fieldnames(primal), fieldtypes(primal)) do fname, ftype
+            # If it is is unassigned, or if it doesn't have a concrete type, or we have multiple reference to it
+            # then let it take any value for its tangent
+            fdef = :(
+                !isdefined(primal, $(QuoteNode(fname))) ||
+                !isconcretetype($ftype) || 
+                get(counts, $(QuoteNode(fname)), 0) > 1
+            )
+            Expr(:kw, fname, fdef)
+        end...)))
+
+        # Construct tangents
+
+        # Go back and fill in tangents that were not ready
     end
 
 ## TODO rewrite below
     has_mutable_tangent(primal)
-        any_mask = map(fieldnames(primal), fieldtypes(primal)) do fname, ftype
-            # If it is is unassigned, or if it doesn't have a concrete type, let it take any value for its tangent
-            fdef = :(!isdefined(primal, $(QuoteNode(fname))) || !isconcretetype($ftype))
-            Expr(:kw, fname, fdef)
-        end
+        any_mask = 
         :($MutableTangent{$primal}(
             $(Expr(:tuple, Expr(:parameters, any_mask...))),
             $(Expr(:tuple, Expr(:parameters, zfield_exprs...))),
@@ -184,7 +186,7 @@ function zero_tangent(x::Array{P,N}) where {P,N}
 end
 
 ###############################################
-count_references!(x) = count_references(IdDict{Any, Int}(), x)
+count_references(x) = count_references(IdDict{Any, Int}(), x)
 function count_references!(counts::IdDict{Any, Int}, x)
     isbits(x) && return counts  # can't be a refernece and can't hold a reference
     counts[x] = get(counts, x, 0) + 1  # Increment *before* recursing