Last Modified
2016-11-15 11:31:48 -0600


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.


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

1/2 gives 0.5

Since mruby does not have Bignum, bigger integers are represented by Float numbers. To enhance interoperability between Fixnum and Float, mruby provides Float#upto and other iterating methods for the Float class. As a side effect, 1/2 gives 0.5 not 0.

Array passed to puts

Passing an Array to puts results in different output.

puts [1,2,3]

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


mruby [1.2.0 (2015-11-17)]

[1, 2, 3]

Kernel.raise in rescue clause

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

  1 / 0

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

ZeroDivisionError is raised.

mruby [1.2.0 (2015-11-17)]

No exception is raised.

Check of infinite recursion

mruby does not check infinite recursion across C extensions.

def test; eval 'test'; end; test

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

SystemStackError is raised.

mruby [1.2.0 (2015-11-17)]

Segmentation fault.

Fiber execution can’t cross C function boundary

mruby’s Fiber is implemented in a similar way 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)
    @feld = str

p "foobar"

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


mruby [1.2.0 (2015-11-17)]

ArgumentError is raised.

Method visibility

For simplicity reasons no method visibility (public/private/protected) is supported.

class VisibleTest

  def public_method; end

  def private_method; end


p, false)
p, true)

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


mruby [1.2.0 (2015-11-17)]



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


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


mruby [1.2.0 (2015-11-17)]

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)]


mruby [1.2.0 (2015-11-17)]

Syntax error

Operator modification

An operator can’t be overwritten by the user.

class String
  def +

'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 [1.2.0 (2015-11-17)]


Behavior of the operator wasn’t changed.

Kernel.binding missing

Kernel.binding is not implemented as it is not in the ISO standard.

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