Lisp and Java
In this article, we\'re going to steal an idea from one of the most theft-worthy languages out there: Lisp. We\'re going to pick out one of its most useful features -- the ability to treat functions as data -- and talk about how to apply th
Tutorial Details:
Lisp Code and How To Read It
(this (is what)
(lisp code
(looks)
(like (more (or less)))))
A casual observer of Lisp code will notice the dazzling collection of parentheses, without a heck of a lot else to visually break up the code. Where in a language with C-descended syntax you get parentheses, curly braces, commas, colons, semicolons, and a whole horde of other syntactical signposts to keep you on your way, in Lisp, you get pretty much nothing but ( and its friend ). All other tokens are separated by white space, and indentation, though extremely important culturally, has no significance as part of the language itself.
For example, in Lisp, if you want to, say, call a function that doubles a number, you write:
(double 7)
Side note: when discussing Lisp, it\'s common to show the result of evaluating an expression as follows:
(double 7)
=> 14
Which can be read as \"(double 7) evaluates to 14.\"
In Java, there are basically two ways you can call a function: as an instance method or as a class method. For the former, you\'d have something like:
aNumberSeven.double()
For the latter:
aClass.double(7)
For both of these cases in Java, you learn to see the word just before the opening parenthesis as \"hot\" -- it\'s the verb of the phrase you\'re reading. One nice thing about Java\'s instance method-call syntax is that makes it very clear what the subject of that verb is (by matching the English language\'s subject-verb-object order). In Lisp, the word immediately after the opening parenthesis is what you learn to read as the verb (in most cases), and there is no distinguished subject of that verb. This pattern is used for functions that are infix operators in other languages, such as plus and minus:
(+ 5 2)
=> 7
Or assignment (which has an undefined value, so we won\'t show it evaluating to anything):
(define a 11)
which is more or less equivalent to:
int a = 11;
Note that Lisp doesn\'t specify the type of variable a -- in Lisp, a variable can hold any type.
The intense regularity to the syntax can be forbidding. Parenthetical digression: the payoff comes through the ability to represent Lisp programs as Lisp data, which means that Lisp programmers can easily write programs to manipulate other programs. Although that is indeed a trick worth stealing or even spending a lifetime studying, it would take more of a book than an article to explore it fully. For such a book, check out Paul Graham\'s ANSI Common Lisp or On Lisp (available as a PDF on his web site), which contain many inspiring uses of the mighty Lisp macro. End parenthetical digression.
Moving on through the rudiments of the language: the basic data structure in Lisp is the list, which can be created in a variety of ways, one of the simplest of which is via the list function.
(define lst (list 2 3 5 7 11))
lst => (2 3 5 7 11)
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