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Defining a writer as a Struct member fails #3097

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splattael opened this issue Jul 2, 2015 · 6 comments
Closed

Defining a writer as a Struct member fails #3097

splattael opened this issue Jul 2, 2015 · 6 comments
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@headius
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core JRuby 9000 ruby 2.0-2.2
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Won't Fix

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@splattael
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Hi,

while this code works in MRI

Struct.new(:x=).new(nil).x = 23

it breaks on JRuby (1.7.20 and 9000k) with:

ArgumentError: wrong number of arguments calling `x=` (1 for 0)
    from (irb):1:in `evaluate'
    from org/jruby/RubyKernel.java:1111:in `eval'
    from org/jruby/RubyKernel.java:1511:in `loop'
    from org/jruby/RubyKernel.java:1274:in `catch'
    from org/jruby/RubyKernel.java:1274:in `catch'
    from /home/ps/.rvm/rubies/jruby-1.7.20/bin/irb:13:in `(root)'

I am not sure if it's a bug in JRuby or MRI.

BTW Rubinius fails too:

ArgumentError: given 1, expected 0
    from (irb):1
    from kernel/common/block_environment.rb:53:in `call_on_instance'
    from kernel/common/eval.rb:176:in `eval'
    from kernel/common/kernel.rb:510:in `loop'
    from kernel/bootstrap/proc.rb:20:in `call'
    from kernel/common/throw_catch.rb:30:in `catch'
    from kernel/common/throw_catch.rb:8:in `register'
    from kernel/common/throw_catch.rb:29:in `catch'
    from kernel/bootstrap/proc.rb:20:in `call'
    from kernel/common/throw_catch.rb:30:in `catch'
    from kernel/common/throw_catch.rb:8:in `register'
    from kernel/common/throw_catch.rb:29:in `catch'
    from /home/ps/.rvm/gems/rbx-2.5.2@global/gems/rubysl-irb-2.1.1/bin/irb:12:in `__script__'
    from kernel/common/kernel.rb:497:in `load'
    from /home/ps/.rvm/gems/rbx-2.5.2@global/bin/irb:23:in `__script__'
    from kernel/delta/code_loader.rb:66:in `load_script'
    from kernel/delta/code_loader.rb:152:in `load_script'
    from kernel/loader.rb:655:in `script'
    from kernel/loader.rb:809:in `main'

¯_(ツ)_/¯

Kind regards,
Peter
@headius
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headius commented Jul 2, 2015

This is the weirdest use of struct I've ever seen. Congratulations! :-)

@headius
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headius commented Jul 2, 2015

This appears to have been a change after 1.9.3, so we won't be fixing it in 1.7.20. But 9k gets the fix.

@headius headius added this to the JRuby 9.0.0.0.rc2 milestone Jul 2, 2015
@headius
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headius commented Jul 2, 2015

So it appears that this creates a write-only value. The logic in MRI for defining the struct methods only defines one called x= and so there's no way to read the value. It does appear to get assigned, though: #<struct :x==1>.

I'm not sure what use this feature is, honestly. It seems like a bug in MRI that it allows this through.

@headius
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headius commented Jul 2, 2015

I think I'm going to call this a bug in MRI and ask you to re-file it with them. I believe the :x= should be kicked out as an invalid attribute name rather than defined as a write-only attribute. I believe the latter behavior is happening only accidentally because of how MRI detects that you're defining an = method.

Let me know when you file this with MRI and I will chime in about the various oddities in this behavior that warrant patching it.

@headius headius closed this as completed Jul 2, 2015
@headius headius modified the milestones: Won't Fix, JRuby 9.0.0.0.rc2 Jul 2, 2015
@headius headius self-assigned this Jul 2, 2015
@splattael
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Heh, will do. Thanks for your help!

@splattael
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@headius I've just file an issue for MRI: https://bugs.ruby-lang.org/issues/11326

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