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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.
+
+
+## ```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.
+
+```ruby
+puts [1,2,3]
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+```
+1
+2
+3
+```
+
+#### mruby [1.3.0 (2017-7-4)]
+
+```
+[1, 2, 3]
+```
+
+## ```Kernel.raise``` in rescue clause
+
+```Kernel.raise``` without arguments does not raise the current exception within
+a rescue clause.
+
+```ruby
+begin
+  1 / 0
+rescue
+  raise
+end
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+```ZeroDivisionError``` is raised.
+
+#### mruby [1.3.0 (2017-7-4)]
+
+No exception is raised.
+
+## Check of infinite recursion
+
+mruby does not check infinite recursion across C extensions.
+
+```ruby
+def test; eval 'test'; end; test
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+```SystemStackError``` is raised.
+
+#### mruby [1.3.0 (2017-7-4)]
+
+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.
+
+```ruby
+class Liste < Array
+  def initialize(str = nil)
+    @feld = str
+  end
+end
+
+p Liste.new "foobar"
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+``` [] ```
+
+#### mruby [1.3.0 (2017-7-4)]
+
+```ArgumentError``` is raised.
+
+## Method visibility
+
+For simplicity reasons no method visibility (public/private/protected) is
+supported.
+
+```ruby
+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 [1.3.0 (2017-7-4)]
+
+```
+true
+true
+```
+
+## defined?
+
+The ```defined?``` keyword is considered too complex to be fully
+implemented. It is recommended to use ```const_defined?``` and
+other reflection methods instead.
+
+```ruby
+defined?(Foo)
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+```
+nil
+```
+
+#### mruby [1.3.0 (2017-7-4)]
+
+```NameError``` is raised.
+
+## ```alias``` on global variables
+
+Aliasing a global variable works in CRuby but is not part
+of the ISO standard.
+
+```ruby
+alias $a $__a__
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+``` nil ```
+
+#### mruby [1.3.0 (2017-7-4)]
+
+Syntax error
+
+## Operator modification
+
+An operator can't be overwritten by the user.
+
+```ruby
+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 [1.3.0 (2017-7-4)]
+
+``` 'ab' ```
+Behavior of the operator wasn't changed.
+
+## ```Kernel.binding``` missing
+
+```Kernel.binding``` is not implemented as it is not in the
+ISO standard.