mruby 2.0.0

We are proudly announcing the first stable release of mruby 2.0 series – mruby 2.0.0.

This release includes the improvement of Ruby2.x compatibility and memory consumption.

New Features

Core Language Features

  • Keyword Arguments Implemented (Ruby 2.0 Compatible)
  • Argument Deconstructing Implemented

Core Libraries


  • Kernel#then was added as Kernel#yield_self (Ruby 2.6 Compatible)


  • Array#to_h can be called with a block (Ruby 2.6 Compatible)
  • Array#union was added (Ruby 2.6 Compatible)


  • String#lines can be called with a block
  • The following methods are added String#tr, String#tr!, String#tr_s, String#tr_s!, String#squeeze, String#squeeze!, String#count, String#delete, String#delete!


  • String#unpack1 was added


Meta-programming features are separated as a mrbgem. You need to link mruby-metaprog to use meta-programming. (See Breaking Changes for more detail)


matsumotory/mruby-sleep was bundled as a core library. You can use Kernel#sleep and Kernel#usleep methods.


mruby compiler (mrbc)

  • --remove-lv option was added By specifying --remove-lv, mrbc suppress the generation of local variable information to be used for debugging in the compiled binary. It reduces the size of compiled mruby binary and memory consumption when loaded.

mruby interpreter (mruby)

  • -d option was added By specifying -d, mruby execute the script in the debugging mode ($DEBUG=true).
  • -r option was added By specifying -r library, mruby loads the specified library before the execution.

interactive mruby (mirb)

  • -d option was added (Behaves as mruby -d)
  • -r option was added (Behaves as mruby -r)

Reducing Memory Consumption

Several improvements were done in mruby 2.0.0 to reduce memory consumption:

  • Instance variables are stored in segmented lists
  • Hash implementation replaced from khash
  • --remove-lv option was added to mruby compiler(mrbc)
  • And smaller improvements including VM stack and call frames.

Breaking Changes

There are two major breaking changes from mruby 1.4.1

1. Bytecode Overhauled

VM instruction format has been completely changed. The fixed width instruction until mruby 1.4.1 was abandoned. We chose variable length bytecode instructions in mruby 2.0. You need to recompile everything for mruby 2.0.

  • Binary Format Version: 0005
  • VM Version: 0002

2. Meta-programming features separated as mrbgem

From the assumption that meta-programming features are not frequently used in embedded system development, we separated meta-programming features as a mebgem (mruby-metaprog). The following methods have moved to mruby-metaprog gem.

  • Kernel module global_variables, local_variables, singleton_class, instance_variables, instance_variables_defined?, instance_variable_get, instance_variable_set, methods, private_methods, public_methods, protected_methods, singleton_methods, define_singleton_methods, send

  • Module class class_variables, class_variables_defined?, class_variable_get, class_variable_set, remove_class_variable, included_modules, instance_methods, remove_method, method_removed, constants

  • Module class (Singleton Methods)
    constants, nesting

3. No implicit conversion by to_int and to_str.

The objects with methods to_int or to_str are no longer converted to Integers or Strings respectively. Those implicit conversions cause recursive VM calls and had been the source of many bugs.

We have done 378 commits to 156 files, 7,949 lines were added, 5,430 lines removed since mruby1.4.1. For more detail of the updates, see Commit Log.

Evaluated mrbgems list (NPO mruby forum)

See the list.