# 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 [2.1.1 (2020-06-04)]

```
[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 [2.1.1 (2020-06-04)]

No exception is raised.

## 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 [2.1.1 (2020-06-04)]

`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.

```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 [2.1.1 (2020-06-04)]

```
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 [2.1.1 (2020-06-04)]

```
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.

```ruby
defined?(Foo)
```

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

```
nil
```

#### mruby [2.1.1 (2020-06-04)]

`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 [2.1.1 (2020-06-04)]

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 [2.1.1 (2020-06-04)]

` '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 [2.1.1 (2020-06-04)]

```
$ ./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. Maybe in the
future, the simpler behavior will be adopted to CRuby as well.

#### 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 [mruby 2.1.1]

```
$ ./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)
```

## 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`.

## `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 `falsy`. mruby outputs `truthy`.