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From: kevin@dime.cs.umass.edu (kevin gallagher)
Newsgroups: comp.lang.lisp
Subject: Re: string to symbol conversion
Summary: Speak softly and carry a small stick
Message-ID: <32421@dime.cs.umass.edu>
Date: 21 Jun 91 19:32:10 GMT
References: <80761@eerie.acsu.Buffalo.EDU> <dfs.677520608@spock>
Reply-To: kevin@dime.cs.umass.edu (Kevin Gallagher)
Organization: University of Massachusetts, Amherst
Lines: 37


An excellent programming principle is to use the least powerful
method for acheiving your objective.  This is especially important in
lisp because there are always several ways to do something, each with
widely different costs.  Using a big stick when a small one will do will
make your program run slower, probably lead to unexpected bugs, and
contribute to the misperception that `lisp is slow.'

In the case of converting a string to a symbol use INTERN.  INTERN looks
for an existing symbol with the specified name and creates one if no
such symbol exists.  It returns the symbol that was found or created.
(If you expect the symbol to already exist you should use FIND-SYMBOL
which returns nil no symbol with the specified name is found.)

As was pointed out in other messages, intern doesn't do case conversion
or anything else.

    (intern "Foo") => |Foo|
    (intern "FOO") => FOO

The function READ-FROM-STRING is too big a stick.  It invokes the reader
which is a large program by itself.  (On my Explorer (intern string) is
6-7 times faster than (read-from-string string) and it doesn't cons at
all.)  Perhaps even worse, INTERN will *always* return a string.
READ-FROM-STRING can return (and in fact *do*) anything.

    (intern "(cons 1 2)") => |(cons 1 2)|

    (read-from-string "(cons 1 2)" => (CONS 1 2)

    (catch 'exit (read-from-string "#.(throw 'exit 'dumb)")) => DUMB

EVAL, READ, and READ-xxx are the functions that are most overused when a
smaller stick will do.  Also, FORMAT and the other printing functions
can often be replaced with less powerful (and less costly) functions.

Kevin Gallagher