Question on subst(), deferred evaluation and trap()

[Jack Brennen <jfb@brennen.net>]

Using GP, I'm trying to set up a vector full of expressions on the 
variable x, and then evaluate the vector with each non-computable 
element replaced by the number 0.  (Could be any value, 0 works for my 
purposes...)

Basically, I'd like to do something like this:

  V=[trap(,0,1/(x-1)),trap(,0,1/(x-2)),trap(,0,1/(x-3))];

  N=3;subst(V,x,N) -> [1/2, 1, 0]    ...  doesn't do the right thing.

However, that doesn't work because the trap() functions are evaluated 
immediately rather than stored in the vector.


So far, the best I've come up with is:

  V=[1/(x-1),1/(x-2),1/(x-3)];

And then instead of a simple subst() on V, I use:

  N=3;vector(#V,j,trap(,0,subst(V[j],x,N)))


However, for cases where the trap is not taken, my method is about 10 
times slower than just doing subst(V,x,N).  Doing it the hard way only 
if the easy way traps is an obvious and useful optimization:

  N=3;trap(,vector(#V,j,trap(,0,subst(V[j],x,N))),subst(V,x,N))

However, in many (most?) cases, I expect that at least some of the 
vector elements will be non-computable.  (My expressions are more 
complicated than simple reciprocals -- the reciprocal expressions are 
just an easy way to demonstrate the problem...)

Is there any way to do this with better performance in GP?  I think I 
could resort to using the PARI library and basically writing my own 
version of vector() with the desired behavior, but I'd really prefer to 
do something which is usable in a vanilla PARI/GP interpreter.