When requiring modules, should the name be a string, symbol, or keyword?

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What is the difference between the following?

(require "asdf")  ; String.
(require 'asdf)   ; Symbol.
(require :asdf)   ; Keyword.

Which should I use when requiring modules?

common-lisp
2 Answers

The argument of require is a string designator which is

a designator for a string; that is, an object that denotes a string and that is one of: a character (denoting a singleton string that has the character as its only element), a symbol (denoting the string that is its name), or a string (denoting itself).

So any of the above will work. However, require is defined to make comparisons between module names using string=, which means that case matters. So this means that (require 'asdf) is the same as (require :ASDF) is the same as (require "ASDF") but is not the same as (require "asdf"). (In fact, ASDF adds both "asdf" and "ASDF" to *modules* so both will work.)

Personally, I use keyword symbols, so (require :asdf), (provide :spotbat), which means *modules* always ends up with upper-case strings.

I find the decision to use string= to compare module names a bit annoying, but it is what it is now, and it is also compatible with various other conventions: package names, for instance, are case sensitive. And in theory it would allow, for instance, a case sensitive lowercase-preferred language to exist alongside CL in the same image ((find-package "cl") could be the package that exported all its symbols).

Admin1

This answer is specific to ASDF.

The recommended way to load ASDF is by using (require "asdf") (i.e. using a lowercase string). According to the ASDF manual, section 3 ("Loading ASDF"):

The recommended way to load ASDF is via:

(require "asdf")

The footnote says:

NB: all implementations except GNU CLISP also accept (require "ASDF"), (require 'asdf) and (require :asdf). For portability’s sake, you should use (require "asdf").

user_59169150
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