56 lines
1.8 KiB
Markdown
56 lines
1.8 KiB
Markdown
### Remarks
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Just run it with no argument:
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ruby entry.rb
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(Anyway it is just a no-op program. The above command only verifies
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that entry.rb is a valid Ruby program.)
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I confirmed the following implementations/platforms:
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* ruby 2.5.0p0 (2017-12-25 revision 61468) [x64-mingw32]
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### Description
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First, look at
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https://docs.ruby-lang.org/ja/latest/doc/spec=2flexical.html#reserved
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and then, look at entry.rb.
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The source code of entry.rb consists only of reserved words of Ruby,
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and all the reserved words are used in the code, in a way that the code
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forms a valid Ruby program. No compile error, no warning, or no runtime error.
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### Internals
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Difficult (and interesting) points of the theme are:
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* Since many of the reserved words define program structures, we cannot
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use them independently. For instance, `retry` must be inside `rescue`,
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or `break`/`next`/`redo` must be inside a looping construct.
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Or, jump-out statements cannot occur at a position that requires a
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value; `if return then true end` is a "void value expression" syntax error.
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* Inserting newlines for each 6 word (to match with the spec html) is also
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an interesting challenge, since Ruby is sensitive to newlines.
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Tricks used in the code are:
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* def/alias/undef can take even reserved words as parameters.
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That is, `def class ... end` defines a method named `class`.
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The feature is crucial since otherwise `BEGIN` etc inevitably
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introduces non-reserved tokens (like `{}`).
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* `defined?` can take some reserved words too (which I didn't know
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until trying to write this program.)
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* "void value expression" can be avoided by using `or` or `and`.
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`if begin return end then true end` is a syntax error, but
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`if begin false or return end then true end` is not.
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### Limitation
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Sad to say that it's not a "perfect pangram".
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It uses 'alias' and 'undef' twice, and 'end' 4 times.
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