After upgrading to the newest version of stack (prompted by @chrisdone in the thread for #665), I ran stack ghci in the turtle project directory. Rather than failing (which was the previous undesirable behavior), it started building the dependencies for the test and bench suites. While this is clearly an improvement, I expect to be able to run ghci without necessarily installing the dependencies for the test and bench suites, and I think that should be the default behavior.