ruby--ruby/sample/trick2018/01-kinaba/remarks.markdown

Remarks

Just run it with no argument:

ruby entry.rb

(Anyway it is just a no-op program. The above command only verifies that entry.rb is a valid Ruby program.)

I confirmed the following implementations/platforms:

• ruby 2.5.0p0 (2017-12-25 revision 61468) [x64-mingw32]

Description

First, look at

https://docs.ruby-lang.org/ja/latest/doc/spec=2flexical.html#reserved

and then, look at entry.rb.

The source code of entry.rb consists only of reserved words of Ruby, and all the reserved words are used in the code, in a way that the code forms a valid Ruby program. No compile error, no warning, or no runtime error.

Internals

Difficult (and interesting) points of the theme are:

• Since many of the reserved words define program structures, we cannot use them independently. For instance, retry must be inside rescue, or break/next/redo must be inside a looping construct. Or, jump-out statements cannot occur at a position that requires a value; if return then true end is a "void value expression" syntax error.
• Inserting newlines for each 6 word (to match with the spec html) is also an interesting challenge, since Ruby is sensitive to newlines.

Tricks used in the code are:

• def/alias/undef can take even reserved words as parameters. That is, def class ... end defines a method named class. The feature is crucial since otherwise BEGIN etc inevitably introduces non-reserved tokens (like {}).
• defined? can take some reserved words too (which I didn't know until trying to write this program.)
• "void value expression" can be avoided by using or or and. if begin return end then true end is a syntax error, but if begin false or return end then true end is not.

Limitation

Sad to say that it's not a "perfect pangram". It uses 'alias' and 'undef' twice, and 'end' 4 times.