Is Lisp Still Unique? Or at Least Different?
In 1991 Lisp offered a combination of features that could not be found in any other language. This combination made Lisp almost a necessity for certain kinds of work (in AI and other applications) and made Lisp the language of choice for a large community of users. Since then Lisp has maintained and added to features, but other languages have caught up. Let's look at whether Lisp is still unique, or at least different from other languages in 2002.On [p. 25] of PAIP I list eight important factors that make Lisp a good language for AI applications, and remarked that mostly these allow a programmer to delay making decisions. Let's look at how Lisp compares to, say, Java and Python in terms of these eight factors:
- Built-in Support for Lists. Java has the Vector type, which allows for sequences of dynamically varying length. Lists are better suited for functional programming (but have the disadvantage of linear access time), while Vectors are better suited for Object-Oriented programming (but have the disadvantage of linear time for non-destructive push). Support for Vectors in Java is not nearly as complete as support for lists and sequences in Lisp, mostly because Lisp endorses first-class functions as arguments to functions. Java may soon have a Collection class which cleans up some of the faults with enumerators, vectors and hashtables. Python too has a vector class (which it calls list, and Python's support is similar to Lisp's.
- Automatic Storage Management. Java and Python support this. Lisp implementations tend to be more mature and perform better.
- Dynamic Typing. Java attaches run-time type information to instances of the class Object, but not to primitive data elements. However, Java requires a type declaration for every variable. This has some advantages for production code, but has disadvantages for rapid prototyping and evolution of programs. Java does not have a generic/template system that would allow types life Vector<String>, and suffers greatly because of it. Python's object model is the same as Lisp's, but Python does not allow optional type declarations as Lisp does.
- First-Class Functions. Java has anonymous classes, which serve some of the purposes of closures, although in a less versatile way with a more clumsy syntax. In Lisp, we can say (lambda (x) (f (g x))) where in Java we would have to say
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