Yukihiro "Matz" Matsumoto
dccd66f9ef
The Difference Since Ruby1.9, the keyword arguments were emulated by Ruby using the hash object at the bottom of the arguments. But we have gradually moved toward keyword arguments separated from normal (positinal) arguments. At the same time, we value compatibility, so that Ruby3.0 keyword arguments are somewhat compromise. Basically, keyword arguments are separated from positional arguments, except when the method does not take any formal keyword arguments, given keyword arguments (packed in the hash object) are considered as the last argument. And we also allow non symbol keys in the keyword arguments. In that case, those keys are just passed in the `**` hash (or raise `ArgumentError` for unknown keys). The Instruction Changes We have changed `OP_SEND` instruction. `OP_SEND` instruction used to take 3 operands, the register, the symbol, the number of (positional) arguments. The meaning of the third operand has been changed. It is now considered as `n|(nk<<4)`, where `n` is the number of positional arguments, and `nk` is the number of keyword arguments, both occupies 4 bits in the operand. The number `15` in both `n` and `nk` means variable sized arguments are packed in the object. Positional arguments will be packed in the array, and keyword arguments will be packed in the hash object. That means arguments more than 14 values are always packed in the object. Arguments information for other instructions (`OP_SENDB` and `OP_SUPER`) are also changed. It works as the third operand of `OP_SEND`. the difference between `OP_SEND` and `OP_SENDB` is just trivial. It assigns `nil` to the block hidden arguments (right after arguments). The instruction `OP_SENDV` and `OP_SENDVB` are removed. Those instructions are replaced by `OP_SEND` and `OP_SENDB` respectively with the `15` (variable sized) argument information. Calling Convention When calling a method, the stack elements shall be in the order of the receiver of the method, positional arguments, keyword arguments and the block argument. If the number of positional or keyword arugument (`n` or `nk`) is zero, corresponding arguments will be empty. So when `n=0` and `nk=0` the stack layout (from bottom to top) will be: +-----------------------+ | recv | block (or nil) | +-----------------------+ The last elements `block` should be explicitly filled before `OP_SEND` or assigned to `nil` by `OP_SENDB` internally. In other words, the following have exactly same behavior: OP_SENDB clears `block` implicitly: ``` OP_SENDB reg sym 0 ``` OP_SEND clears `block` implicitly: ``` OP_LOADNIL R2 OP_SEND R2 sym 0 ``` When calling a method with only positional arguments (n=0..14) without keyword arguments, the stack layout will be like following: +--------------------------------------------+ | recv | arg1 | ... | arg_n | block (or nil) | +--------------------------------------------+ When calling a method with arguments packed in the array (n=15) which means argument splat (*) is used in the actual arguments, or more than 14 arguments are passed the stack layout will be like following: +-------------------------------+ | recv | array | block (or nil) | +-------------------------------+ The number of the actual arguments is determined by the length of the argument array. When keyword arguments are given (nk>0), keyword arguments are passed between positional arguments and the block argument. For example, when we pass one positional argument `1` and one keyword argument `a: 2`, the stack layout will be like: +------------------------------------+ | recv | 1 | :a | 2 | block (or nil) | +------------------------------------+ Note that keyword arguments consume `2*nk` elements in the stack when `nk=0..14` (unpacked). When calling a method with keyword arguments packed in the hash object (nk=15) which means keyword argument splat (**) is used or more than 14 keyword arguments in the actual arguments, the stack layout will be like: +------------------------------+ | recv | hash | block (or nil) | +------------------------------+ Note for mruby/c When mruby/c authors try to support new keyword arguments, they need to handle the new meaning of the argument information operand. If they choose not to support keyword arguments in mruby/c, it just raise error when `nk` (taken by `(c>>4)&0xf`) is not zero. And combine `OP_SENDV` behavior with `OP_SEND` when `n` is `15`. If they want to support keyword arguments seriously, contact me at <matz@ruby.or.jp> or `@yukihiro_matz`. I can help you.
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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.
Kernel.raise in rescue clause
Kernel.raise without arguments does not raise the current exception within
a rescue clause.
begin
1 / 0
rescue
raise
end
Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
ZeroDivisionError is raised.
mruby [3.0.0 (2021-03-05)]
RuntimeError is raised instead of ZeroDivisionError. To re-raise the exception, you have to do:
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.
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 [3.0.0 (2021-03-05)]
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.
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 [3.0.0 (2021-03-05)]
true
true
Visibility Declaration
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'
Ruby [ruby 2.5.5p157 (2019-03-15 revision 67260)]
ok
mruby [3.0.0 (2021-03-05)]
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.
defined?(Foo)
Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
nil
mruby [3.0.0 (2021-03-05)]
NameError is raised.
alias on global variables
Aliasing a global variable works in CRuby but is not part of the ISO standard.
alias $a $__a__
Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
nil
mruby [3.0.0 (2021-03-05)]
Syntax error
Operator modification
An operator can't be overwritten by the user.
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 [3.0.0 (2021-03-05)]
'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 [3.0.0 (2021-03-05)]
$ ./bin/mruby -e 'puts Proc.new {}.binding'
trace (most recent call last):
[0] -e:1
-e:1: undefined method 'binding' (NoMethodError)
nil? redefinition in conditional expressions
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.
Argument Destructuring
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.