Modifying TVar after calling always causes freeze/spin in GHCi
The attached code, if run under GHCi, causes GHCi to freeze while spinning the CPU. It works correctly when compiled, or if the ">> modifyTVar" part of f is commented out.
(For the curious: Yes, I believed always does what check does. Thankfully nothing had made its way into production code, because of this bug. The documentation should probably be clarified.)