Pascal Costanza wrote: > Vesa Karvonen wrote: > >> I think that we're finally getting to the bottom of things. While >> reading your reponses something became very clear to me: latent-typing and >> latent-types are not a property of languages. Latent-typing, also known as >> informal reasoning, is something that all programmers do as a normal part >> of programming. To say that a language is latently-typed is to make a >> category mistake, because latent-typing is not a property of languages. > > I disagree with you and agree with Anton. Here, it is helpful to > understand the history of Scheme a bit: parts of its design are a > reaction to what Schemers perceived as having failed in Common Lisp (and > other previous Lisp dialects). > > One particularly illuminating example is the treatment of nil in Common > Lisp. That value is a very strange beast in Common Lisp because it > stands for several concepts at the same time: most importantly the empty > list and the boolean false value. Its type is also "interesting": it is > both a list and a symbol at the same time. It is also "interesting" that > its quoted value is equivalent to the value nil itself. This means that > the following two forms are equivalent: > > (if nil 42 4711) > (if 'nil 42 4711) > > Both forms evaluate to 4711. > > It's also the case that taking the car or cdr (first or rest) of nil > doesn't give you an error, but simply returns nil as well. > > The advantage of this design is that it allows you to express a lot of > code in a very compact way. See > http://www.apl.jhu.edu/~hall/lisp/Scheme-Ballad.text for a nice > illustration. > > The disadvantage is that it is mostly impossible to have a typed view of > nil, at least one that clearly disambiguates all the cases. There are > also other examples where Common Lisp conflates different types, and > sometimes only for special cases. [1] > > Now compare this with the Scheme specification, especially this section: > http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-6.html#%25_sec_3.2 > > This clearly deviates strongly from Common Lisp (and other Lisp > dialects). The emphasis here is on a clear separation of all the types > specified in the Scheme standard, without any exception. This is exactly > what makes it straightforward in Scheme to have a latently typed view of > programs, in the sense that Anton describes. So latent typing is a > property that can at least be enabled / supported by a programming > language, so it is reasonable to talk about this as a property of some > dynamically typed languages.
If anything, I think that this example supports my and Vesa's point. The example demonstrates that languages *that are not distinguished in whether they are called latently typed* support informal reasoning about types to varying degrees. -- David Hopwood <[EMAIL PROTECTED]> -- http://mail.python.org/mailman/listinfo/python-list
