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Generate equivalent source for ASTs from parser.

The following constraints apply:

  • No support for macruby extensions
  • Only support for the modern AST format
  • Only support for Ruby >= 3.0

Notable Users:

(if you want your tool to be mentioned here please PR the addition with a TLDR of your use case).

Public API:

While unparser is in the 0.x versions its public API can change any moment. I recommend to use ~> 0.x.y style version constraints that should give the best mileage.


require 'parser/current'
require 'unparser'

ast = Unparser.parse('your(ruby(code))')

Unparser.unparse(ast) # => 'your(ruby(code))'

To preserve the comments from the source:

require 'parser/current'
require 'unparser'

ast, comments = Unparser.parse_with_comments('your(ruby(code)) # with comments')

Unparser.unparse(ast, comments) # => 'your(ruby(code)) # with comments'

Passing in manually constructed AST:

require 'parser/current'
require 'unparser'

module YourHelper
  def s(type, *children), children)

include YourHelper

node = s(:def,
    s(:arg, :x)
    s(:lvar, :x),
    s(:int, 3)

Unparser.unparse(node) # => "def foo(x)\n  x + 3\nend"

Note: DO NOT attempt to pass in nodes generated via AST::Sexp#s, these ones return API incompatible AST::Node instances, unparser needs Parser::AST::Node instances.

Equivalent vs identical:

require 'unparser'

node = Unparser.parse(<<~'RUBY')
  %w[foo bar]

generated = Unparser.unparse(node) # ["foo", "bar"], NOT %w[foo bar] !

code == generated                 # false, not identical code
Unparser.parse(generated) == node # true, but identical AST

Summary: unparser does not reproduce your source! It produces equivalent source.

Ruby Versions:

Unparsers primay reason for existance is mutant and its supported Ruby-Versions.

Basically: All non EOL MRI releases.

If you need to generate Ruby Syntax outside of this band feel free to contact me (email in gemspec).


Unparser currently successfully round trips almost all ruby code around. Using Ruby >= 2.6. If there is a non round trippable example that is NOT subjected to known Limitations. please report a bug.

On CI unparser is currently tested against rubyspec with minor excludes.


Source parsed with magic encoding headers other than UTF-8 and that have literal strings. where parts can be represented in UTF-8 will fail to get reproduced.

A fix is possible as with latest updates the parser gem carries the information.



# -*- encoding: binary -*-



(str "\x98v\xAB\xCDE2\xEF\x01\x01#Eg\x89\xAB\xCD\xEF")




(str "\x98v\xAB\xCDE2\xEF\u0001\u0001#Eg\x89\xAB\xCD\xEF")


@@ -1,2 +1,2 @@
-(str "\x98v\xAB\xCDE2\xEF\x01\x01#Eg\x89\xAB\xCD\xEF")
+(str "\x98v\xAB\xCDE2\xEF\u0001\u0001#Eg\x89\xAB\xCD\xEF")


Install the gem unparser via your prefered method.


Various people contributed to this repository. See Contributors.

Included Libraries

For dependency reduction reasons unparser ships vendored (and reduced) versions of:


  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with version (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.

Known Users

  • RailsRocket - A no-code app builder that creates Rails apps


See LICENSE file.