Suggestions on how to infer types for prolog/clp variables?

Hello, friends:
Currently I am doing a transformation from prolog into constraint logic
programming(clpfd).
One problem I am facing now is how to differentiate between variables: for
instance, some variables will bound to a integer while others might bound to
characters; If I insert domain definition (e.g. "Var in inf..sup "), there
will be an type error if the variable Var is not meant to bound to a
variable. 
One idea I came up with is to infer the types of such variable through the
context: after I read in a line, if the variable detected is involved in
arithmatic operation(e.g. "Var1 is Var2 - 1" or "Var1 > Var2"), we can make
the assumption that both Var1 and Var2 are bound to integers(we ignore the
complications of float numbers at this moment) and safely insert the domain
definition. 
I am still not sure about how to implement this (libraries?) and whether
this is a legit approach. Any ideas or concerns will be highly appreciated.

Thanks,
Yu Fu
School of Information Sciences and Technology
Pennsylvania State University, USA



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