[Hat] Possibly incorrect Hat file

[Thomas Davie]

Hi guys,
   I'm not sure, but I think I've found a bug in hat-trans.  I have an  
instance of map being applied to a single element list, however, only  
one element is ever demanded from the result, so the empty list is  
never evaluated.  This results in these nodes in the hat file (the  
result pointer of the application of map is 0xa76d):

0xec6:  AtomConstructor     	(Module 0xeb2) position 0:0-0:0, infixr  
5, arity=2, :
0xa76d:  ExpHidden           	parent=(Exp 0xa544) result=(Exp 0xa784)  
children=(ListCons 0xa7c7)
...
0xa77f:  ExpForward          	result=(Exp 0xa76d)
0xa784:  ExpValueApp         	parent=(Exp 0xa76d) fun=(Atom 0xec6)  
arity=2, args (Exp 0xa77a)(Exp 0xa77f)

It makes sense that the result is a hidden node, and as expected the  
result of that is an application of cons.  But look at the result of  
the cons -- it's the same hidden node as before, shouldn't this point  
to an unevaluated node?

If I try to resurrect an expression from result of map, I get an  
infinite list, which is obviously rather odd for applying map to a  
single element list.  On the other hand, hat-observe appears to figure  
out that the list argument of cons is unevaluated, but I can't see  
exactly what the condition here is that allows hat-observe to tell  
that this is indeed an unevaluated structure rather than an infinite  
list.

Bob