The description of mkstemp and GHC/Hugs and non-GHC/Hugs code disagree (on requiring the user to provide a valid template). Attached is a patch which fixes this for Temp and Temp.ByteString. Additionally, I would create another ticket with a feature request for this, but since the patch provides the implementation I'll include it here: addition of mkdtemp. (I would very much appreciate it this or some other of mkdtemp can be included in the next release!)