mirror of
https://github.com/mruby/mruby
synced 2026-06-08 16:11:16 +00:00
Hiroshi Mimaki
250 lines
4.8 KiB
Markdown
250 lines
4.8 KiB
Markdown
# Limitations and Differences
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The philosophy of mruby is to be a lightweight implementation of
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the Ruby ISO standard. These two objectives are partially contradicting.
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Ruby is an expressive language with complex implementation details which
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are difficult to implement in a lightweight manner. To cope with this,
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limitations to the "Ruby Compatibility" are defined.
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This document is collecting these limitations.
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## Integrity
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This document does not contain a complete list of limitations.
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Please help to improve it by submitting your findings.
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## ```1/2``` gives ```0.5```
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Since mruby does not have ```Bignum```, bigger integers are represented
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by ```Float``` numbers. To enhance interoperability between ```Fixnum```
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and ```Float```, mruby provides ```Float#upto``` and other iterating
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methods for the ```Float``` class. As a side effect, ```1/2``` gives ```0.5```
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not ```0```.
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## ```Array``` passed to ```puts```
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Passing an Array to ```puts``` results in different output.
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```ruby
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puts [1,2,3]
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```
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#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
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```
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1
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2
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3
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```
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#### mruby [2.0.1 (2019-4-4)]
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```
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[1, 2, 3]
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```
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## ```Kernel.raise``` in rescue clause
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```Kernel.raise``` without arguments does not raise the current exception within
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a rescue clause.
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```ruby
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begin
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1 / 0
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rescue
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raise
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end
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```
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#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
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```ZeroDivisionError``` is raised.
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#### mruby [2.0.1 (2019-4-4)]
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No exception is raised.
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## Fiber execution can't cross C function boundary
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mruby's ```Fiber``` is implemented in a similar way to Lua's co-routine. This
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results in the consequence that you can't switch context within C functions.
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Only exception is ```mrb_fiber_yield``` at return.
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## ```Array``` does not support instance variables
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To reduce memory consumption ```Array``` does not support instance variables.
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```ruby
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class Liste < Array
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def initialize(str = nil)
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@feld = str
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end
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end
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p Liste.new "foobar"
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```
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#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
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``` [] ```
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#### mruby [2.0.1 (2019-4-4)]
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```ArgumentError``` is raised.
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## Method visibility
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For simplicity reasons no method visibility (public/private/protected) is
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supported.
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```ruby
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class VisibleTest
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def public_method; end
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private
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def private_method; end
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end
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p VisibleTest.new.respond_to?(:private_method, false)
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p VisibleTest.new.respond_to?(:private_method, true)
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```
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#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
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```
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false
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true
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```
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#### mruby [2.0.1 (2019-4-4)]
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```
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true
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true
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```
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## defined?
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The ```defined?``` keyword is considered too complex to be fully
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implemented. It is recommended to use ```const_defined?``` and
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other reflection methods instead.
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```ruby
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defined?(Foo)
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```
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#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
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```
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nil
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```
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#### mruby [2.0.1 (2019-4-4)]
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```NameError``` is raised.
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## ```alias``` on global variables
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Aliasing a global variable works in CRuby but is not part
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of the ISO standard.
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```ruby
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alias $a $__a__
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```
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#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
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``` nil ```
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#### mruby [2.0.1 (2019-4-4)]
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Syntax error
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## Operator modification
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An operator can't be overwritten by the user.
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```ruby
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class String
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def +
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end
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end
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'a' + 'b'
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```
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#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
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```ArgumentError``` is raised.
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The re-defined ```+``` operator does not accept any arguments.
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#### mruby [2.0.1 (2019-4-4)]
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``` 'ab' ```
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Behavior of the operator wasn't changed.
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## Kernel#binding is not supported
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`Kernel#binding` method is not supported.
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#### Ruby [ruby 2.5.1p57 (2018-03-29 revision 63029)]
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```
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$ ruby -e 'puts Proc.new {}.binding'
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#<Binding:0x00000e9deabb9950>
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```
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#### mruby [2.0.1 (2019-4-4)]
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```
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$ ./bin/mruby -e 'puts Proc.new {}.binding'
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trace (most recent call last):
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[0] -e:1
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-e:1: undefined method 'binding' (NoMethodError)
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```
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## Keyword arguments
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mruby keyword arguments behave slightly different from CRuby 2.5
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to make the behavior simpler and less confusing. Maybe in the
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future, the simpler behavior will be adopted to CRuby as well.
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#### Ruby [ruby 2.5.1p57 (2018-03-29 revision 63029)]
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```
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$ ruby -e 'def m(*r,**k) p [r,k] end; m("a"=>1,:b=>2)'
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[[{"a"=>1}], {:b=>2}]
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```
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#### mruby [mruby 2.0.1]
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```
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$ ./bin/mruby -e 'def m(*r,**k) p [r,k] end; m("a"=>1,:b=>2)'
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trace (most recent call last):
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[0] -e:1
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-e:1: keyword argument hash with non symbol keys (ArgumentError)
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```
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## Argument Destructuring
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```ruby
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def m(a,(b,c),d); p [a,b,c,d]; end
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m(1,[2,3],4) # => [1,2,3,4]
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```
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Destructured arguments (`b` and `c` in above example) cannot be accessed
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from the default expression of optional arguments and keyword arguments,
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since actual assignment is done after the evaluation of those default
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expressions. Thus:
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```ruby
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def f(a,(b,c),d=b)
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p [a,b,c,d]
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end
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f(1,[2,3])
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```
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CRuby gives `[1,2,3,nil]`. mruby raises `NoMethodError` for `b`.
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