"Neil Mitchell"
A pattern match failure in a "do" will call "fail" (not "error" as other pattern match failures do). This is the "fail" of the IO monad, which calls "ioError".
I believe according to Haskell 98, fail in the IO monad calls error - its only recent extension to GHC/Hugs which have resulted in ioError and exceptions. Perhaps for the purposes of hat stuff, error should be called instead? At least until a proper exception story can be developed.
The problem is, that would require type information about which monad the 'do' block belongs to. For IO, it might be fine for a pattern-match failure to call 'error', but it would be totally incorrect for, say, the Maybe monad, or the list monad, where fail might be defined as e.g. 'const Nothing' or 'const []'. Regards, Malcolm