FYI.
I contributed a package based on GMP for supporting integers of unlimited
precision:
https://github.com/githwxi/ATS-Postiats/blob/master/contrib/atshwxi/intinf/
There are some examples in
https://github.com/githwxi/ATS-Postiats/blob/master/contrib/atshwxi/intinf/TEST
which are pretty good for beginners to try/learn linear types.