How to specify `frozen_string_literal: true`

RubyKaigi 2017 の LT での発表資料です。

Transcript

1. How to specify
   frozen_string_literal: true
   Kazuhiro NISHIYAMA
   RubyKaigi 2017 LT
   2017/09/19
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2. self.introduce
   One of Ruby committers
   Twitter, GitHub: @znz
   1/12
   

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3. What’s frozen_string_literal
   magic comment
   Specify string literals are frozen or not per fle
   Frozen string literals are faster
   Generate less objects
   Reduce GC
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4. Example
   After coding if exist
   example:
   # -*- coding: utf-8 -*-
   # frozen_string_literal: true
   p ''.frozen? #=> true
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5. Past release
   Add frozen_string_literal: false for almost
   *.rb fles when Ruby 2.3.0
   For compatibility with --enable=frozen-string-
   literal command line option of ruby
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6. Changes in this year
   Recent Ruby programs tend to specify true, but
   ruby standard libraries are false.
   So I changed to frozen_string_literal: true in
   some fles which assigned to no maintainer in doc/
   maintainers.rdoc.
   5/12
   

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7. Review points before change
   Almost RuntimeError: can't modify frozen String
   points are:
   String#<<
   bang methods (e.g. String#sub!)
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8. Overlooked modifcation
   IO#read(length, out_buffer)
   this buffer is overlooked
   7/12
   

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9. Find modifed string literal
   --debug=frozen-string-literal command line
   option is very useful
   examples:
   ruby --debug=frozen-string-literal test.rb
   make test-all TESTS='-v base64' RUBYOPT=--debug=frozen-string-literal
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10. Tests
    --debug=frozen-string-literal needs to run
    codes.
    So tests are very important.
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11. No change if can’t
    If no tests, there is no need to forcibly change it.
    I think working codes are useful than broken
    faster codes.
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12. mkmf.rb
    mkmf.rb has many tests
    But mkmf.rb is too complex to me in a short time
    So I remain false
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13. Conclusion
    No tests may overlook modifcations
    Use frozen_string_literal: true if you can
    You can use frozen_string_literal: false in
    some cases for compatibility for --enable=frozen-
    string-literal
    12/12
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