On Mon, Oct 6, 2008 at 14:58, Cale Gibbard <[EMAIL PROTECTED]> wrote: > 2008/10/6 Don Stewart <[EMAIL PROTECTED]>: >> dagit: >>> data and newtype vary in one more subtle way, and that's how/when they >>> evaluate to bottom. Most of the time they behave identically, but in the >>> right cases they act sightly differently. newtype is usually regarded as >>> more efficient than data. This is because the compiler can choose to >>> optimize away the newtype so that it only exists at type check time. I >>> think this is also possible with data in some, but not all, uses. >> >> The compiler *must* optimise away the use. They're sort of 'virtual' >> data, guaranteed to have no runtime cost. > > I'm not sure that I'd want to be that emphatic about what an > implementation *must* do regarding something so operational. > > The informal semantics of pattern matching in the Report says: > > Matching the pattern con pat against a value, where con is a > constructor defined by newtype, depends on the value: > * If the value is of the form con v, then pat is matched against v. > * If the value is _|_, then pat is matched against _|_. > That is, constructors associated with newtype serve only to change the > type of a value. > > This clearly has an implementation which introduces no overhead, which > one can expect from good implementations of the language. There are > obviously implementations of these semantics which do introduce > overhead as well though, so I would be hesitant to make any > requirement like that.
And this requirement is there why? Is it specifically put in so that one is able to create this overhead-less implementation? Given: data A = A Int newtype B = B Int ta (A x) = True tb (B x) = True This happens (not surprisingly given your above comments): *Main GOA> :load test.hs [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main GOA> ta undefined *** Exception: Prelude.undefined *Main GOA> tb undefined True Why is the x evaluated in ta? cheers, Arnar _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
