Using In with parameter

Instead of creating a multiple-value parameter, I am limited to only regular discreet value parameters. I want to do an "in" statement in my sql record selection on the parameter and put in my parameter value - for example "a","b","c". It doesn't work to put in my record selection -
{fieldA} in {?parameter}. Also, I'm having a hard time trying to create a formula that will create an array from my parameter - I get an error message that formulas cannot be an array. The wierd thing is that I can hard code my record selection and that works, i.e. {fieldA} in ["a","b","c"]. But not with my parameter. Can someone help?