HasPostgres instance for IO with Snap framework
Yes, this looks good to me.  I've done exactly the same thing to facilitate debugging in my projects.  However, I would recommend that you put the IO instance in some special module where all your debugging code lives.  You really do not want instances like this making their way into any other code because it is akin to making a global statement that IO always has a test database available at localhost on port 5432.