Re: Command Languages Versus Programming Languages

On 2024-04-02, Stefan Ram <ram@zedat.fu-berlin.de> wrote:

ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:

( SETQ DIFF
 ( LAMBDA( X )
   ( COND
     ( ( ATOMP X )
       ( COND
         ( ( = X 'X )
           1 )
         ( T
           0 )))
     ( T
       ( COND
         ( ( =( CAR X )'SUM )
           ( LIST 'SUM( DIFF( CADR X ))( DIFF( CADDR X )))))))))

>
  In Python:
>
def diff( x ):
    return 1 if x == 'x' else 0 if type( x )is str
    else[ 'sum', diff( x[ 1 ]), diff( x[ 2 ])]
    if x[ 0 ]== 'sum' else None

n TXR Lisp:

  (defun-match dif
    ((sum @left @right) ^(sum ,(dif left) ,(dif right)))
    ((@op . @args) (error "~s: unrecognized operator: ~s" 'dif op))
    (x 1)
    (@else 0))

diff is a built in, which we don't want to be redefining without
an exceptionally good reason, so I changed to dif.

If we compile 'dif we get unused variable warnings:

  2> (compile 'dif)
  ** expr-1:1: warning: let*: variable args unused
  ** expr-1:1: warning: let*: variable else unused
  #<vm fun: 0 param + 3 optional + variadic>

To suppress that, we use @nil instead of @args or @else; @nil is a
placeholder pattern that matches any object without binding a variable.

Otherwise we need more verbiage:

  ((@op . @args) (ignore args) (error ....))
  ...
  (@else (ignore else) 0)


--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca