# 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 ``` #### CRuby `ZeroDivisionError` is raised. #### mruby `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" ``` #### CRuby `[]` #### mruby `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) ``` #### CRuby ``` nil ``` #### mruby `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__ ``` #### CRuby `nil` #### mruby Syntax error ## Operator modification Operators on some of the primitive classes cannot be overridden, as they are optimized in the VM. ```ruby class String def + end end 'a' + 'b' ``` #### CRuby `ArgumentError` is raised. The re-defined `+` operator does not accept any arguments. #### mruby `'ab'` Behavior of the operator wasn't changed. ## `Kernel#binding` is not supported without mruby-binding gem `Kernel#binding` method requires the `mruby-binding` gem (included in the `metaprog` gembox). Without this gem, `binding` is not available. ## `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 ``` #### CRuby Prints `:append`. #### mruby Nothing printed (since `include` does not call `append_features` internally). ## No `#hash` call for small hashes For performance reasons, mruby avoids calling the `#hash` method on keys when a hash table is small. This means that custom `#hash` methods on key objects may not be executed. ## Pattern Matching Pattern matching is only partially supported in mruby. Currently, only the rightward assignment operator (`=>`) with simple variable binding is implemented. ```ruby expr => var # Supported: assigns expr to var ``` #### CRuby Full pattern matching with `case/in` syntax and various pattern types: ```ruby case [1, 2, 3] in [a, b, c] puts "#{a}, #{b}, #{c}" # => "1, 2, 3" end case {name: "Alice", age: 30} in {name:, age:} puts "#{name} is #{age}" # => "Alice is 30" end ``` #### mruby Only rightward assignment with simple variable binding: ```ruby [1, 2, 3] => x puts x # => [1, 2, 3] ``` The following are **not supported**: - `case/in` syntax - Array patterns: `in [a, b, c]` - Hash patterns: `in {name:, age:}` - Guard clauses: `in pattern if condition` - Pin operator: `in ^variable` - Find patterns: `in [*, x, *]` - Alternative patterns: `in pattern1 | pattern2` - Boolean pattern check: `value in pattern` Note: mruby does provide `Array#deconstruct` and `Hash#deconstruct_keys` methods for future pattern matching compatibility. ## No Refinements Module refinements (`refine`, `using`) are not supported in mruby. ## No `Encoding` Class mruby does not have an `Encoding` class. Strings are treated as byte sequences by default. UTF-8 aware string operations can be enabled with the `MRB_UTF8_STRING` compile flag. ## Integer Precision Varies by Boxing Mode Integer size depends on the value boxing configuration: | Configuration | Integer range | | ----------------------------- | ---------------- | | Word boxing, 64-bit (default) | roughly +/- 2^62 | | Word boxing, 32-bit (default) | roughly +/- 2^30 | | NaN boxing (64-bit only) | -2^31 to 2^31-1 | Code relying on 64-bit integer precision may behave differently across configurations. The `mruby-bigint` gem provides arbitrary-precision integers when included. ## No `ObjectSpace.each_object` by Default `ObjectSpace` is only available via the `mruby-objectspace` gem (included in the `stdlib` gembox). Even with the gem, `ObjectSpace.each_object` has limited functionality compared to CRuby.