If I use an STRef to a record, will a new record be created each time I want to update a single field? Or can I expect GHC to optimize it and have the field of the record updated in place?
You'll get a new record for each update. This might not be so bad though, depending on the number of fields in your record.
Right now I'm using a record of STRefs, like: data E s = E{ refi :: STRef s Int, refc :: STRef s Char }
but it can get a little messy, since thread s propagates to every datatype that uses the record type in it's definition.
Here's another trick if you use this route: data E s = E{ refi :: !STRef s Int, refc :: !STRef s Char } and compile with -funbox-strict-fields. This will eliminate the boxing of the STRefs. Cheers, Simon