comp.lang.lisp

William James

3/5/2016 8:35:00 PM

WJ wrote:

> Joel Ray Holveck wrote:
>
> > As a concrete example, suppose (as I did in a program I recently
> > wrote) that you have a string with several arbitrary variable names
> > you need to substitute in, and a hash with their values. This can be
> > done in one line in Perl:
> > # $string holds something like:
> > # 'The $dimension of the $obj is $length cm.'
> > # %vars holds something like:
> > # { $dimension => length, $obj => plank, $length => 105 }
> > $string =~ s/\$(\S+)/$vars->{$1}/g;
> >
> > Now, look at a Lisp parallel. You have a list, and you want to
> > substitute keywords with their values from an alist:
> > ;; LIST holds something like:
> > ;; (the :dimension of the :object is :value cm)
> > ;; VARS holds something like:
> > ;; ((:dimension . length) (:object . plank) (:value . 105))
> > (mapcar #'(lambda (elt)
> > (if (keywordp elt)
> > (cdr (assoc elt vars))
> > elt))
> > list)
>
> MatzLisp (Ruby):
>
> vars = Hash[*%w(dimension length obj plank length 105)]
>
> 'The $dimension of the $obj is $length cm.'.gsub(/\$\w+/){|s| vars[s[1..-1]]}
> ===>
> "The length of the plank is 105 cm."

OCaml:

let table =
["$dimension", "length"; "$obj", "plank"; "$length", "105"] ;;

Str.global_substitute
(Str.regexp "\\$[a-z]+")
(fun s ->
let x = Str.matched_string s in
try List.assoc x table with Not_found -> x)
"The $dimension of the ($dummy) $obj is $length cm." ;;

===>
"The length of the ($dummy) plank is 105 cm."

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