2015/08/19

Top-level REPL commands

Another little feature I'm playing for REPL improvements.

S-expression is great to represent tree structure, but it's a bit cumbersome to give simple commands in interactive shell; If the command line is short, the relative verbosity added by open and close parentheses gets in a way. Also in many cases simple commands take string arguments (e.g. to change working directory or to load file) and that require extra double quotes.

I'm not the only one to feel that way. In Allegro CL, if you type a line begins with :, it is interpreted as special toplevel command. Scheme 48 has similar command mode, beginning with ,. I guess there are other implementations with similar features.

So I started adding Scheme 48-ish command syntax. If the first thing you type in the REPL prompt is a comma, an entire line is read by read-line and parsed as a toplevel command, instead of being read with read.

You can see current directory or move around:

gosh> ,pwd
/home/shiro/src/Gauche/src
gosh> ,cd
"/home/shiro"
gosh> ,cd src/Gauche
"/home/shiro/src/Gauche"

You can run apropos:

gosh> ,a cons
%tree-map-check-consistency    (gauche)
acons                          (gauche)
cons                           (scheme)
cons*                          (gauche)
define-constant                (gauche)
lcons                          (gauche)
lcons*                         (gauche)
gosh> ,a trim srfi-13
string-trim                    (srfi-13)
string-trim-both               (srfi-13)
string-trim-right              (srfi-13)

or describe:

gosh> ,d 31415926
31415926 is an instance of class <integer>
  (#x1df5e76, ~ 29Mi, 1970-12-30T14:38:46Z as unix-time)

You can omit the argument for describe, in which case it takes the last value of REPL. Whenever you want to see the details of the last evaluation result, just type comma-d:

gosh> (sys-stat "Makefile")
#<<sys-stat> 0x14e01e0>
gosh> ,d
#<<sys-stat> 0x14e01e0> is an instance of class <sys-stat>
slots:
  type      : regular
  perm      : 436
  mode      : 33204
  ino       : 3295167
  dev       : 2097
  rdev      : 0
  nlink     : 1
  uid       : 500
  gid       : 500
  size      : 4599
  atime     : 1440031322
  mtime     : 1440031177
  ctime     : 1440031177

I'll gradually add more commands. Let me know your ideas.

Tags: 0.9.5, repl