Is it better to put the defpackage in a separate file when creating packages

Question

The example below is given in Paul Grahams ANSI Common Lisp as an example of doing encapsulation:

(defpackage "CTR"
  (:use "COMMON-LISP")
  (:export "COUNTER" "INCREMENT" "CLEAR"))

(in-package ctr)

;function definitions here

However in Peter Seibels Practical Common Lisp, link here, he says:

Because packages are used by the reader, a package must be defined before you can LOAD or COMPILE-FILE a file that contains an IN-PACKAGE expression switching to that package. Packages also must be defined before other DEFPACKAGE forms can refer to them... The best first step toward making sure packages exist when they need to is to put all your DEFPACKAGEs in files separate from the code that needs to be read in those packages

So he recommends creating two files for every package, one for the defpackage and one for the code. The files containing defpackages should start with (in-package "COMMON-LISP-USER").

To me it seems like putting the defpackage in the same file, before the in-package and code, is a good way to ensure that the package is defined before used. So the first method, collecting everything into one file seems easier. Are there any problems with using this method for package creation?

Answer 1

I think that using a separate file for defpackage is a good habit because:

• You don't « pollute » your files with defpackage.
• It makes it easier to find the exported/shadowed/... symbols, you know you just have to look at package.lisp.

• You don't have to worry about the order when you use ASDF.

  (defsystem :your-system
    :components ((:file "package")
                 ... the rest ...))`
  

• Peter Seibel says so ;)

EDIT: I forgot to mention quickproject which facilitates the creation of new CL projects.

REPL> (quickproject:make-project "~/src/lisp/my-wonderful-project/"
                                 :depends-on '(drakma cl-ppcre local-time))`

This command will create a directory "~/src/lisp/my-wonderful-project/" and the following files:

• package.lisp
• my-wonderful-project.asd (filled)
• my-wonderful-project.lisp
• README.txt

And thus, I think it's good to use the same convention.

Answer 2

I tend to use multiple source code files, a single "packages.lisp" file and a singe "project.asd" system definition file for most of my projects. If the project requires multiple packages, they're all defined in "packages.lisp", with the relevant exports in place exported.