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.
Kernel.raise without arguments does not raise the current exception within
a rescue clause.
begin
1 / 0
rescue
raise
endZeroDivisionError is raised.
RuntimeError is raised instead of ZeroDivisionError. To re-raise the exception, you have to do:
begin
1 / 0
rescue => e
raise e
endmruby'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.
To reduce memory consumption Array does not support instance variables.
class Liste < Array
def initialize(str = nil)
@field = str
end
end
p Liste.new "foobar"[]
ArgumentError is raised.
For simplicity reasons no method visibility (public/private/protected) is supported. Those methods are defined, but they are dummy methods.
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)false
true
true
true
The declaration form of following visibility methods are not implemented.
publicprivateprotectedmodule_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'
ok
test.rb:8: undefined method 'test_func' (NoMethodError)
The defined? keyword is considered too complex to be fully
implemented. It is recommended to use const_defined? and
other reflection methods instead.
defined?(Foo)nil
NameError is raised.
Aliasing a global variable works in CRuby but is not part of the ISO standard.
alias $a $__a__nil
Syntax error
An operator can't be overwritten by the user.
class String
def +
end
end
'a' + 'b'ArgumentError is raised.
The re-defined + operator does not accept any arguments.
'ab'
Behavior of the operator wasn't changed.
Kernel#binding method is not supported.
$ ruby -e 'puts Proc.new {}.binding'
#<Binding:0x00000e9deabb9950>
$ ./bin/mruby -e 'puts Proc.new {}.binding'
trace (most recent call last):
[0] -e:1
-e:1: undefined method 'binding' (NoMethodError)
mruby keyword arguments behave slightly different from CRuby 2.5 to make the behavior simpler and less confusing.
$ ruby -e 'def m(*r,**k) p [r,k] end; m("a"=>1,:b=>2)'
[[{"a"=>1}], {:b=>2}]
$ ./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)
Redefinition of nil? is ignored in conditional expressions.
a = "a"
def a.nil?
true
end
puts(a.nil? ? "truthy" : "falsy")Ruby outputs falsy. mruby outputs truthy.
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:
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.