Practical Common Lisp

no arguments.

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The biggest missing piece in Common Lisp's standard I/O facilities is a way for users to define new stream classes. There are, however, two de facto standards for user-defined streams. During the Common Lisp standardization, David Gray of Texas Instruments wrote a draft proposal for an API to allow users to define new stream classes. Unfortunately, there wasn't time to work out all the issues raised by his draft to include it in the language standard. However, many implementations support some form of so-called Gray Streams, basing their API on Gray's draft proposal. Another, newer API, called Simple Streams, has been developed by Franz and included in Allegro Common Lisp. It was designed to improve the performance of user-defined streams relative to Gray Streams and has been adopted by some of the open-source Common Lisp implementations.

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One slightly annoying consequence of the way read-time conditionalization works is that there's no easy way to write a fall-through case. For example, if you add support for another implementation to foo by adding another expression guarded with #+, you need to remember to also add the same feature to the or feature expression after the #- or the ERROR form will be evaluated after your new code runs.

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Another special value, :wild-inferiors, can appear as part of the directory component of a wild pathname, but you won't need it in this chapter.

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Implementations are allowed to return :unspecific instead of NIL as the value of pathname components in certain situations such as when the component isn't used by that implementation.

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This is slightly broken in the sense that if PROBE-FILE signals an error for some other reason, this code will interpret it incorrectly. Unfortunately, the CLISP documentation doesn't specify what errors might be signaled by PROBE-FILE and probe- directory, and experimentation seems to show that they signal simple-file-errors in most erroneous situations.

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The language now generally considered the first object-oriented language, Simula, was invented in the early 1960s, only a few years after McCarthy's first Lisp. However, object orientation didn't really take off until the 1980s when the first widely available version of Smalltalk was released, followed by the release of C++ a few years later. Smalltalk took quite a bit of inspiration from Lisp and combined it with ideas from Simula to produce a dynamic object-oriented language, while C++ combined Simula with C, another fairly static language, to yield a static object-oriented language. This early split has led to much confusion in the definition of object orientation. Folks who come from the C++ tradition tend to consider certain aspects of C++, such as strict data encapsulation, to be key characteristics of object orientation. Folks from the Smalltalk tradition, however, consider many features of C ++ to be just that, features of C++, and not core to object orientation. Indeed, Alan Kay, the inventor of Smalltalk, is reported to have said, 'I invented the term object oriented, and I can tell you that C++ wasn't what I had in mind.'

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There are those who reject the notion that Common Lisp is in fact object oriented at all. In particular, folks who consider strict data encapsulation a key characteristic of object orientation—usually advocates of relatively static languages such as C++, Eiffel, or Java—don't consider Common Lisp to be properly object oriented. Of course, by that definition, Smalltalk, arguably one of the original and purest object-oriented languages, isn't object oriented either. On the other hand, folks who consider message passing to be the key to object orientation will also not be happy with the claim that Common Lisp is object oriented since Common Lisp's generic function orientation provides degrees of freedom not offered by pure message passing.

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Prototype-based languages are the other style of object-oriented language. In these languages, JavaScript being perhaps the most famous example, objects are created by cloning a prototypical object. The clone can then be modified and used as a prototype for other objects.

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T the constant value and T the class have no particular relationship except they happen to have the same name. T the value is a direct instance of the class SYMBOL and only indirectly an instance of T the class.

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