# 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.
```ruby
begin
1 / 0
rescue
raise
end
```
#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
`ZeroDivisionError` is raised.
#### mruby [3.1.0 (2022-05-12)]
`RuntimeError` is raised instead of `ZeroDivisionError`. To re-raise the exception, you have to do:
```ruby
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.
```ruby
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 [3.1.0 (2022-05-12)]
`ArgumentError` is raised.
## `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 [3.1.0 (2022-05-12)]
`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 [3.1.0 (2022-05-12)]
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 [3.1.0 (2022-05-12)]
`'ab'`
Behavior of the operator wasn't changed.
## `Kernel#binding` is not supported until [3.0.0 (2021-03-05)]
`Kernel#binding` method is not supported.
#### Ruby [ruby 2.5.1p57 (2018-03-29 revision 63029)]
```shell
$ ruby -e 'puts Proc.new {}.binding'
#<Binding:0x00000e9deabb9950>
```
## `nil?` redefinition in conditional expressions
Redefinition of `nil?` is ignored in conditional expressions.
```ruby
a = "a"
def a.nil?
true
end
puts(a.nil? ? "truthy" : "falsy")
```
Ruby outputs `truthy`. mruby outputs `falsy`.
## Argument Destructuring
```ruby
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:
```ruby
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`.
Keyword argument expansion has similar restrictions. The following example, gives `[1, 1]` for CRuby, mruby raises `NoMethodError` for `b`.
```ruby
def g(a: 1, b: a)
p [a,b]
end
g(a:1)
```
## No Double Dispatch in Module Loading
To make implementation simpler, mruby does not use double dispatching in module loading (`include`/`prepend`/`extend`).
Those method internally called corresponding actual load methods (`append_features`/`prepend_features`/`extend_object`).
But they are rarely overloaded, consumes more memory, and make loading little bit slower. As a Ruby implementation for the smaller device,
we decided mruby simpler.
```ruby
module M
def self.append_features(mod)
p :append
end
end
class C
include M
end
```
#### Ruby [ruby 3.5.0dev (2025-04-21 85bab61565))]
Prints `:append`.
#### mruby [3.5.0 (2025-04-28)]
Nothing printed (since `include` does not call `append_features` internally).
