Beginner question - comp.lang.lisp

Message from discussion Beginner question

Andreas Thiele

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More options Jan 31 2005, 5:42 am

Newsgroups: comp.lang.lisp

From: "Andreas Thiele" <nospam6...@nospam.com>

Date: Mon, 31 Jan 2005 11:42:34 +0100

Local: Mon, Jan 31 2005 5:42 am

Subject: Re: Beginner question

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"Yoel Jacobsen" <y...@emet.co.il> schrieb im Newsbeitrag
news:ctkt7f$ipm$1@news2.netvision.net.il...

> I'm experimenting CL by attempting to write some basic programs.

> I'm trying to write an /etc/passwd to LDIF converter and I'm stuck - I
> can't figure out from CLISP errors what is wrong. Here is my code:

> (require 'split-sequence)

> (defmacro list-to-plist (plist pos-list id-list field-list)
> `(progn ,@(map 'list
> #'(lambda (id pos)
> `(setf (getf ,id ,id-list) (elt ,field-list ,pos)))
> plist pos-list)))

> (defun pline-to-plist (line)
> "convert a passwd line to a plist"
> (let ((fields (split-sequence:split-sequence #\: line))
> (entry nil))
> (list-to-plist
> '(:login :passwd :uid :gid :group :home :shell)
> '(0 1 2 3 4 5 6 7)
> entry fields)
> entry))

> (format t "~S~%" (pline-to-plist
> "news:x:9:13:news:/usr/lib/news:/bin/false"))

> I tried to use a macro to shorten the following pattern:
> (SETF (GETF :LOGIN ENTRY) (ELT FIELDS 0))
> (SETF (GETF :PASSWD ENTRY) (ELT FIELDS 1))
> (SETF (GETF :UID ENTRY) (ELT FIELDS 2))
> (SETF (GETF :GID ENTRY) (ELT FIELDS 3))
> (SETF (GETF :GROUP ENTRY) (ELT FIELDS 4))
> (SETF (GETF :HOME ENTRY) (ELT FIELDS 5))
> (SETF (GETF :SHELL ENTRY) (ELT FIELDS 6))

> Now, I get diffetent errors:
> Inside SLIME -
> FUNCALL: undefined function NIL
> [Condition of type SYSTEM::SIMPLE-UNDEFINED-FUNCTION]
> When evaluating directly with CLISP:
> *** - SYSTEM::%EXPAND-FORM: invalid form (0 1 2 3 4 5 6 7)

> Where did I wrong?

Yoel,

you should use (macroexpand '(list-to-plist ...)) to see what your macro
really creates.

Let me suggest a typical lisp idiom to solve your problem:

(defun pline-to-plist (x)
(mapcan #'(lambda (key value) (list key value))
'(:login :passwd :uid :gid :group :home :shell)
(split-sequence:split-sequence #\: x)))

Another more verbose iterative solution might be easier to read:

(defun pline-to-plist2 (x)
(let ((r '()))
(mapc #'(lambda (key value)
(push key r)
(push value r))
'(:login :passwd :uid :gid :group :home :shell)
(split-sequence:split-sequence #\: x))
(nreverse r)))

Andreas

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