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Limitations and Differences

The philosophy of mruby is to be a lightweight implementation of the Ruby ISO standard. These two objectives are partially contradicting. Ruby is an expressive language with complex implementation details which are difficult to implement in a lightweight manner. To cope with this, limitations to the "Ruby Compatibility" are defined.

This document is collecting these limitations.

Integrity

This document does not contain a complete list of limitations. Please help to improve it by submitting your findings.

Kernel.raise in rescue clause

Kernel.raise without arguments does not raise the current exception within a rescue clause.

begin
  1 / 0
rescue
  raise
end

Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]

ZeroDivisionError is raised.

mruby [3.0.0 (2021-03-05)]

RuntimeError is raised instead of ZeroDivisionError. To re-raise the exception, you have to do:

begin
  1 / 0
rescue => e
  raise e
end

Fiber execution can't cross C function boundary

mruby's Fiber is implemented similarly to Lua's co-routine. This results in the consequence that you can't switch context within C functions. Only exception is mrb_fiber_yield at return.

Array does not support instance variables

To reduce memory consumption Array does not support instance variables.

class Liste < Array
  def initialize(str = nil)
    @field = str
  end
end

p Liste.new "foobar"

Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]

[]

mruby [3.0.0 (2021-03-05)]

ArgumentError is raised.

Method visibility

For simplicity reasons no method visibility (public/private/protected) is supported. Those methods are defined, but they are dummy methods.

class VisibleTest

  def public_method; end

  private
  def private_method; end

end

p VisibleTest.new.respond_to?(:private_method, false)
p VisibleTest.new.respond_to?(:private_method, true)

Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]

false
true

mruby [3.0.0 (2021-03-05)]

true
true

Visibility Declaration

The declaration form of following visibility methods are not implemented.

  • public
  • private
  • protected
  • module_function

Especially, module_function method is not dummy, but no declaration form.

module TestModule
  module_function
  def test_func
    p 'test_func called'
  end

  test_func
end

p 'ok'

Ruby [ruby 2.5.5p157 (2019-03-15 revision 67260)]

ok

mruby [3.0.0 (2021-03-05)]

test.rb:8: undefined method 'test_func' (NoMethodError)

defined?

The defined? keyword is considered too complex to be fully implemented. It is recommended to use const_defined? and other reflection methods instead.

defined?(Foo)

Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]

nil

mruby [3.0.0 (2021-03-05)]

NameError is raised.

alias on global variables

Aliasing a global variable works in CRuby but is not part of the ISO standard.

alias $a $__a__

Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]

nil

mruby [3.0.0 (2021-03-05)]

Syntax error

Operator modification

An operator can't be overwritten by the user.

class String
  def +
  end
end

'a' + 'b'

Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]

ArgumentError is raised. The re-defined + operator does not accept any arguments.

mruby [3.0.0 (2021-03-05)]

'ab' Behavior of the operator wasn't changed.

Kernel#binding is not supported

Kernel#binding method is not supported.

Ruby [ruby 2.5.1p57 (2018-03-29 revision 63029)]

$ ruby -e 'puts Proc.new {}.binding'
#<Binding:0x00000e9deabb9950>

mruby [3.0.0 (2021-03-05)]

$ ./bin/mruby -e 'puts Proc.new {}.binding'
trace (most recent call last):
        [0] -e:1
-e:1: undefined method 'binding' (NoMethodError)

Keyword arguments

mruby keyword arguments behave slightly different from CRuby 2.5 to make the behavior simpler and less confusing.

Ruby [ruby 2.5.1p57 (2018-03-29 revision 63029)]

$ ruby -e 'def m(*r,**k) p [r,k] end; m("a"=>1,:b=>2)'
[[{"a"=>1}], {:b=>2}]

mruby [3.0.0 (2021-03-05)]

$ ./bin/mruby -e 'def m(*r,**k) p [r,k] end; m("a"=>1,:b=>2)'
trace (most recent call last):
	[0] -e:1
-e:1: keyword argument hash with non symbol keys (ArgumentError)

nil? redefinition in conditional expressions

Redefinition of nil? is ignored in conditional expressions.

a = "a"
def a.nil?
  true
end
puts(a.nil? ? "truthy" : "falsy")

Ruby outputs falsy. mruby outputs truthy.

Argument Destructuring

def m(a,(b,c),d); p [a,b,c,d]; end
m(1,[2,3],4)  # => [1,2,3,4]

Destructured arguments (b and c in above example) cannot be accessed from the default expression of optional arguments and keyword arguments, since actual assignment is done after the evaluation of those default expressions. Thus:

def f(a,(b,c),d=b)
  p [a,b,c,d]
end
f(1,[2,3])

CRuby gives [1,2,3,nil]. mruby raises NoMethodError for b.