Sunday, December 7, 2008

Why Lisp Packages are so Easy (Hard)

Slobodan Blazeski wrote:
> May somebody please explain me with simple words why cl packages are
> hard to use, 'couse I really don't get it?
> The best is probably to hear from somebody who just learned lisp or
> somebody who teaches lisp (do such people exist anymore?) and his/her
> students has problems with packages.

...or a good teacher, defined as someone who remembers what it was like before they knew what they were teaching.

The first problem is one of those immune system deals. Packages have a lot of the same proteins as, say, C dot aitch definition files so the recognition systems of programmers coming to Lisp from other languages (you know, everyone) does its damndest to parse them as dot aitches.

In fact, packages have nothing to with anything but symbols, neither the functions nor global values they might name. More expansively: packages are about mapping string names to first class Lisp symbol objects and dividing this up into discrete mappings called packages so the same string "Hi, Mom!" can map to multiple |Hi, Mom!| symbols in multiple packages.

The second problem is that symbols take us straight to black-belt Lisp in which one must understand the Lisp reader and fancy things like read-time and compile-time and all that. Example:

I remember getting messed over because some symbol was mysteriously appearing the base CL-USER package. I could not for the life of me see where it was being introduced. I turned the problem over to a few trees worth of monkeys and one of them came up with this rule:

"Never use a normal symbol like |Hi, Mom!| in a defpackage form coded in a source file with (in-package :cl-user) at the top which would likely be the default even without that."

Instead, use a string like "Hi, Mom!" or an uninterned symbol like #:|Hi, Mom!|.

Strings are dicey because you might want to export 'banana and be silly enough to code "banana". Lisp will take you seriously and create |banana| instead of BANANA, case sensitivity being what it is except when it is not when Lisp folds 'banana to 'BANANA which is what it does unless you are in Modern (case sensitive) mode.

Did I mention "black-belt"? The fundamental idea is simple, but noobs get thrown off mostly by the non-simular simularity with C and then in the debugging stage by case sensitivity seeming to come and go at random and the whole read-time compile-time thang.

hth, kxo

3 comments:

Joe Gutierrez said...

Absolutely right. Noobs screw this one up. Burned once not twice.

I interned a symbol in one package and was testing it from another package and I couldn't find it.

Slobodan Blazeski said...

Ok they require some different thinking but this is lisp, everything else requires a little different thinking.

Will Fitzgerald said...

As much as I love Lisp, another practical problem with packages is that there are just so many more symbols to package than in languages that have dot syntax. For example, in Ruby, in theory, a package could export just one symbol, and have all the methods come along for free. I'm not sure I've ever seen this commented on before.