From: Robert Feldt <feldt@...>
Date: 2002-01-14T18:25:07+09:00
Subject: Re: MetaRuby : RubySchema.rb howto?

Hi Tobias et al,

FYI, I've played with the following way of specifying pretty-printers for
RubyAst/RubyTerms/RubySchema in Rockit (its got a similar scheme to
Matju's but its not typed; will probably use Matju's scheme in future
versions):

* You specify PPDoc's by giving a string with slots that can be later
filled in. Slots are given as "#" followed by a number starting at
0. Example (for a simple if without else-clause etc):

   p1 = PPDoc["if (#0)\n  #1\nend"]
   p2 = PPDoc["#0 == #1"]

   p2[1,2].to_s #=> "1 == 2"

   p1[p2["a", 1], "puts a\na"].to_s 

   gives:

   if (a == 1)
     puts a
     a
   end

This could be nice since you get a lego set of pretty-printers that you
can plug-and-play with. The logical step would be to connect something
like that to the RubyTerms/RubySchema so that you can pretty-print them
directly. BTW, you can override indentation etc by setting options to a
Printer object that you send to PPDoc#format. To make it work you'd
probably need optionals that I haven't yet added though (for example the
behavior for if should be different depending on if its chained with other
ifs etc).

Note that there is no real pretty-printing in PPDoc's yet, such as
implemented in prettyprinter.rb in Ruby 1.7.2, ie. it won't break lines if
more than 79 chars wide. IMHO, the problem with that implementation (and
the funcprog combinator libs its based on) is that there is no way to
explicitly state that you want a linebreak (or am I wrong here)? For
example I rarely want my if's printed as

   if (condition) statement end

even if it fits on a line. But I may be wrong cause I only took a brief
look at prettyprint and saw it was based on Wadler's prettyprint libs
(which may also give it exponential alg complexity; anyone checked
that?). I may have jumped to conclusions...

Regards,

Robert