David Hopwood wrote: > Marshall wrote: >>David Hopwood wrote: >>>Marshall wrote: >>> >>>>The real question is, are there some programs that we >>>>can't write *at all* in a statically typed language, because >>>>they'll *never* be typable? >>> >>>In a statically typed language that has a "dynamic" type, all >>>dynamically typed programs are straightforwardly expressible.
Correction: as the paper on gradual typing referenced below points out in section 5, something like the "typerec" construct of <http://citeseer.ist.psu.edu/harper95compiling.html> would be needed, in order for a system with a "dynamic" type to be equivalent in expressiveness to dynamic typing or gradual typing. ("typerec" can be used to implement "dynamic"; it is more general.) >>So, how does this "dynamic" type work? > > <http://citeseer.ist.psu.edu/abadi89dynamic.html> > >>It can't simply be the "any" type, because that type has no/few >>functions defined on it. > > It isn't. From the abstract of the above paper: > > [...] even in statically typed languages, there is often the need to > deal with data whose type cannot be determined at compile time. To handle > such situations safely, we propose to add a type Dynamic whose values are > pairs of a value v and a type tag T where v has the type denoted by T. > Instances of Dynamic are built with an explicit tagging construct and > inspected with a type safe typecase construct. > > "Gradual typing" as described in > <http://www.cs.rice.edu/~jgs3847/pubs/pubs/2006/siek06:_gradual.pdf> is > another alternative. The difference between gradual typing and a > "dynamic" type This should be: the difference between gradual typing and a system with "typerec"... > is one of convenience rather than expressiveness -- > gradual typing does not require explicit tagging and typecase constructs. -- David Hopwood <[EMAIL PROTECTED]> -- http://mail.python.org/mailman/listinfo/python-list
