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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.
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