how rails depends on treetop, and how I found that out

I’m working on a project where I think I’ll be using a parser library so I’ve been looking at the options. One thing I’ve noticed is that treetop is installed when Rails is installed. I didn’t know why, though, so I looked around.

First I looked at the Gemfile.lock. Had I known the format I would have found out my answer more quickly. I didn’t, though, and so when I found my first result for treetop, I stopped. It showed treetop below specs in the hierarchy.

GEM
  remote: http://rubygems.org/
  specs:
    # ...snip...
    thor (0.14.6)
    tilt (1.3.3)
    treetop (1.4.10)
      polyglot
      polyglot (>= 0.3.1)
    tzinfo (0.3.30)
    uglifier (1.0.3)

The next thing I did was run find . -iname '*.treetop' in ~/.rbenv. It found the following results:

(mbp) ~/.rbenv/versions/1.9.2-p290 $ find . -iname '*.treetop'
./lib/ruby/gems/1.9.1/gems/erector-0.8.3/lib/erector/erect/rhtml.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/address_lists.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/content_disposition.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/content_location.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/content_transfer_encoding.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/content_type.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/date_time.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/envelope_from.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/message_ids.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/mime_version.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/phrase_lists.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/received.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/rfc2045.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/rfc2822.treetop
./lib/ruby/gems/1.9.1/gems/mail-2.3.0/lib/mail/parsers/rfc2822_obsolete.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/examples/lambda_calculus/arithmetic.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/examples/lambda_calculus/lambda_calculus.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/lib/treetop/compiler/metagrammar.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/compiler/test_grammar.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/compiler/test_grammar_do.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/composition/a.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/composition/b.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/composition/c.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/composition/d.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/composition/f.treetop
./lib/ruby/gems/1.9.1/gems/treetop-1.4.10/spec/composition/subfolder/e_includes_c.treetop
(mbp) ~/.rbenv/versions/1.9.2-p290 $

Aha, so there are numerous treetop files in actionmailer! I have my answer. Seems like a good use of a parser, plus those may be worth using as examples.

Then I took another look at a Gemfile.lock from a rails project, and saw that it was plainly listed there. I just didn’t see it and didn’t keep looking after I found one.

GEM
  remote: http://rubygems.org/
  specs:
    XMLCanonicalizer (1.0.1)
      log4r (>= 1.0.4)
    actionmailer (3.1.1)
      actionpack (= 3.1.1)
      mail (~> 2.3.0)
  # ...snip...

I noticed something: Gemfile.lock doesn’t show an arbitrarily nested hierarchy; instead it shows a list of gems and their dependencies, where the list of gems includes all gems. Then, separately at the end of the file, it shows the top-level gems from the Gemfile.

To see a deeply nested, a graph could be constructed from the Gemfile.lock, using the two nesting levels under specs as an adjacency list.

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