also see, in Chapter 6, how even using a global variable is more flexible in Common Lisp than it may be in other languages.
26
One of the coolest FORMAT directives is the ~R directive. Ever want to know how to say a really big number in English words? Lisp knows. Evaluate this:
(format nil '~r' 1606938044258990275541962092)
and you should get back (wrapped for legibility):
'one octillion six hundred six septillion nine hundred thirty-eight sextillion forty-four quintillion two hundred fifty-eight quadrillion nine hundred ninety trillion two hundred seventy-five billion five hundred forty-one million nine hundred sixty-two thousand ninety-two'
27
Windows actually understands forward slashes in filenames even though it normally uses a backslash as the directory separator. This is convenient since otherwise you have to write double backslashes because backslash is the escape character in Lisp strings.
28
The word lambda is used in Lisp because of an early connection to the lambda calculus, a mathematical formalism invented for studying mathematical functions.
29
The technical term for a function that references a variable in its enclosing scope is a closure because the function 'closes over' the variable. I'll discuss closures in more detail in Chapter 6.
30
Note that in Lisp, an IF form, like everything else, is an expression that returns a value. It's actually more like the ternary operator (?:) in Perl, Java, and C in that this is legal in those languages:
some_var = some_boolean ? value1 : value2;
while this isn't:
some_var = if (some_boolean) value1; else value2;
because in those languages, if is a statement, not an expression.
31
You need to use the name delete-rows rather than the more obvious delete because there's already a function in Common Lisp called DELETE. The Lisp package system gives you a way to deal with such naming conflicts, so you could have a function named delete if you wanted. But I'm not ready to explain packages just yet.
32
If you're worried that this code creates a memory leak, rest assured: Lisp was the language that invented garbage collection (and heap allocation for that matter). The memory used by the old value of *db* will be automatically reclaimed, assuming no one else is holding on to a reference to it, which none of this code is.
33
A friend of mine was once interviewing an engineer for a programming job and asked him a typical interview question: how do you know when a function or method is too big? Well, said the candidate, I don't like any method to be bigger than my head. You mean you can't keep all the details in your head? No, I mean I put my head up against my monitor, and the code shouldn't be bigger than my head.
34
It's unlikely that the cost of checking whether keyword parameters had been passed would be a detectible drag on performance since checking whether a variable is NIL is going to be pretty cheap. On the other hand, these functions returned by where are going to be right in the middle of the inner loop of any select, update, or delete-rows call, as they have to be called once per entry in the database. Anyway, for illustrative purposes, this will have to do.
35
Macros are also run by the interpreter—however, it's easier to understand the point of macros when you think about compiled code. As with everything else in this chapter, I'll cover this in greater detail in future chapters.
36
http://www-formal.stanford.edu/jmc/history/lisp/node3.html
