2013/02/23

Land of Lisp on Gauche

I've been working on translating Conrad Barski's "Land of Lisp" to Japanese, and it finally hit the bookstores in Japan (published from O'Reilly Japan). To celebrate that, I ported the code in the book to Gauche.

• https://github.com/shirok/Gauche-LoL

Since Gauche adopts quite a few Common Lisp idioms, the port is pretty straightforward. The code may serve as an example to show how to translate CL-style code into Gauche.

Here are some observations.

• Most of CL's loop use cases can be written using srfi:42.
  • The trick is that, in CL, you think "OK, I loop over this (list, vector, integer, etc.) and I want to get this (list, sum, etc.)", while in srfi-42, you first think what you want (ok, I want list so I use list-ec / ok, I want to stop iteration when any of the element satisfies the condition, so I use any?-ec), then you think about what to iterate.
• CL has many idioms that take advantage of () being boolean false, and car/cdr of nil is still nil. It is not a good idea to carry over that idiom to Scheme, or you'll be frustrated feeling that Scheme's distinction of #f and () gets in your way. It just needs a different mindset (complaining about it is like a C programmer complaining 0 isn't false in some languages).
  • For example, in CL, you can access to the value associated to the key in alist by the following code, without worrying the case when key isn't found:

    (cdr (assoc key alist))
    

    You'll be annoyed that in Scheme, assoc returns #f when key isn't found, and cdr barfs. In Gauche, recommended way is as follows:

    (assoc-ref alist key :optional default)
    

  • Another common CL idiom is to build a list conditionally:

    (append (list up down)
            (unless (zerop (mod pos *board-size*))
              (list (1- up) (1- pos)))
            (unless (zerop (mod (1+ pos) *board-size*))
              (list (1+ pos) (1+ down))))
    

    This idiom uses the fact that unless returns nil when the condition is satisfied. In Gauche, unless returns #<undef> so this doesn't work. Instead of falling back to more verbose if, you can use cond-list.

    (cond-list 
     [#t @ (list up down)]
     [(not (zero? (mod pos *board-size*))) @ (list (- up 1) (- pos 1))]
     [(not (zero? (mod (+ pos 1) *board-size*))) @ (list (+ pos 1) (+ down 1))])
    

• The lazy evaluation stuff built in Chapter 18 (lazy.lisp) comes free in Gauche, as util.stream module. The lazy.scm file in Gauche version only shows the correspondence of two APIs.
• I have a plan to make format CL-compatible. It's not done yet, so I had to rewrite some code using advanced formatting directives with the plain Scheme code.
• The CL's defstruct code is translated to define-record-type straightforwardly, except that our record type can't set the default value when the constructor argument is omitted. (R6RS records can do that using protocol. I prefer easier way, though.)

Tag: CommonLisp