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Fast is a "Find AST" tool to help you search in the code abstract syntax tree.

🍿Watch my talk at Ruby Kaigi: Grepping Ruby code like a boss

Ruby allow us to do the same thing in a few ways then it's hard to check how the code is written.

Using the AST will be easier than try to cover the multiple ways we can write the same code.

You can define a string like %|| or '' or "" but they will have the same AST representation.

AST representation

Each detail of the ruby syntax have a equivalent identifier and some content. The content can be another expression or a final value.

Fast uses parser gem behind the scenes to parse the code into nodes.

First get familiar with parser gem and understand how ruby code is represented.

When you install parser gem, you will have access to ruby-parse and you can use it with -e to parse an expression directly from the command line.


ruby-parse -e 1

It will print the following output:

(int 1)

And trying a number with decimals:

ruby-parse -e 1.1
(float 1)

Building a regex that will match decimals and integer looks like something easy and with fast you use a node pattern that reminds the syntax of regular expressions.

Syntax for find in AST

The current version cover the following elements:

  • () to represent a node search
  • {} is for any matches like union conditions with or operator
  • [] is for all matches like intersect conditions with and operator
  • $ is for capture current expression
  • _ is something not nil
  • nil matches exactly nil
  • ... is a node with children
  • ^ is to get the parent node of an expression
  • ? is for maybe
  • \1 to use the first previous captured element
  • "" surround the value with double quotes to match literal strings

Jump to Syntax.


Use Fast.ast to convert simple code to AST objects. You can use it as ruby-parse but directly from the console.

Fast.ast("1")       # => s(:int, 1)
Fast.ast("method")  # => s(:send, nil, :method)
Fast.ast("a.b")     # => s(:send, s(:send, nil, :a), :b)
Fast.ast("1 + 1")   # => s(:send, s(:int, 1), :+, s(:int, 1))
Fast.ast("a = 2")   # => s(:lvasgn, :a, s(:int, 2))
Fast.ast("b += 2")  # => s(:op_asgn, s(:lvasgn, :b), :+, s(:int, 2))

It uses astrolable gem behind the scenes:

=> Astrolabe::Node
=> :int
=> [1]

See also ast_from_file.


Fast.match? is the most granular function that tries to compare a node with an expression. It returns true or false and some node captures case it find something.

Let's start with a simple integer in Ruby:


The AST can be represented with the following expression:

(int 1)

The ast representation holds node type and children.

Let's build a method s to represent Parser::AST::Node with a #type and #children.

def s(type, *children), children)

A local variable assignment:

value = 42

Can be represented with:

ast = s(:lvasgn, :value, s(:int, 42))

Now, lets find local variable named value with an value 42:

Fast.match?('(lvasgn value (int 42))', ast) # true

Lets abstract a bit and allow some integer value using _ as a shortcut:

Fast.match?('(lvasgn value (int _))', ast) # true

Lets abstract more and allow float or integer:

Fast.match?('(lvasgn value ({float int} _))', ast) # true

Or combine multiple assertions using [] to join conditions:

Fast.match?('(lvasgn value ([!str !hash !array] _))', ast) # true

Matches all local variables not string and not hash and not array.

We can match "a node with children" using ...:

Fast.match?('(lvasgn value ...)', ast) # true

You can use $ to capture a node:

Fast.match?('(lvasgn value $...)', ast) # => [s(:int), 42]

Or match whatever local variable assignment combining both _ and ...:

Fast.match?('(lvasgn _ ...)', ast) # true

You can also use captures in any levels you want:

Fast.match?('(lvasgn $_ $...)', ast) # [:value, s(:int), 42]

Keep in mind that _ means something not nil and ... means a node with children.

Then, if do you get a method declared:

def my_method
It will be represented with the following structure:

ast =
  s(:def, :my_method,
    s(:send, nil, :call_other_method))

Keep an eye on the node (args).

Then you know you can't use ... but you can match with (_) to match with such case.

Let's test a few other examples. You can go deeply with the arrays. Let's suppose we have a hardcore call to a.b.c.d and the following AST represents it:

ast =
        s(:send, nil, :a),

You can search using sub-arrays with pure values, or shortcuts or procs:

Fast.match?([:send, [:send, '...'], :d], ast) # => true
Fast.match?([:send, [:send, '...'], :c], ast) # => false
Fast.match?([:send, [:send, [:send, '...'], :c], :d], ast) # => true

Shortcuts like ... and _ are just literals for procs. Then you can use procs directly too:

Fast.match?([:send, [ -> (node) { node.type == :send }, [:send, '...'], :c], :d], ast) # => true

And also work with expressions:

Fast.match?('(send (send (send (send nil $_) $_) $_) $_)', ast) # => [:a, :b, :c, :d]

If something does not work you can debug with a block:

Fast.debug { Fast.match?([:int, 1], s(:int, 1)) }

It will output each comparison to stdout:

int == (int 1) # => true
1 == 1 # => true

Search allows you to go deeply in the AST, collecting nodes that matches with the expression. It also returns captures if they exist.'(int _)', Fast.ast('a = 1')) # => s(:int, 1)

If you use captures, it returns the node and the captures respectively:'(int $_)', Fast.ast('a = 1')) # => [s(:int, 1), 1]

You can also bind external parameters in the search using extra arguments:'(int %1)', Fast.ast('a = 1'), 1) # => [s(:int, 1)]


To pick just the captures and ignore the nodes, use Fast.capture:

Fast.capture('(int $_)', Fast.ast('a = 1')) # => 1


And if I want to refactor a code and use delegate <attribute>, to: <object>, try with replace:

Fast.replace '(def $_ ... (send (send nil $_) \1))', ast do |node, captures|
  attribute, object = captures
  replace(node.location.expression, "delegate :#{attribute}, to: :#{object}")


Now let's imagine we have real files like sample.rb with the following code:

def good_bye
  message = ["good", "bye"]
  puts message.join(' ')

And we decide to remove the message variable and put it inline with the puts.

Basically, we need to find the local variable assignment, store the value in memory. Remove the assignment expression and use the value where the variable is being called.

assignment = nil
Fast.replace_file('({ lvasgn lvar } message )','sample.rb')  do |node, _|
  if node.type == :lvasgn
    assignment = node.children.last
  elsif node.type == :lvar
    replace(node.location.expression, assignment.location.expression.source)

It will return an output of the new source code with the changes but not save the file. You can use ()[#rewrite_file] if you're confident about the changes.


Fast.capture_file can be used to combine capture and file system.

Fast.capture_file("$(casgn)", "lib/fast/version.rb") # => s(:casgn, nil, :VERSION, s(:str, "0.1.3"))
Fast.capture_file("(casgn nil _ (str $_))", "lib/fast/version.rb") # => "0.1.3"


Fast.capture_all can be used to combine capture_file from multiple sources:

Fast.capture_all("(casgn nil $_)") # => { "./lib/fast/version.rb"=>:VERSION, "./lib/fast.rb"=>[:LITERAL, :TOKENIZER], ...}

The second parameter can also be passed with to filter specific folders:

Fast.capture_all("(casgn nil $_)", "lib/fast") # => {"lib/fast/shortcut.rb"=>:LOOKUP_FAST_FILES_DIRECTORIES, "lib/fast/version.rb"=>:VERSION}


Fast.rewrite_file works exactly as the replace but it will override the file from the input.


This method parses the code and load into a AST representation.



You can use search_file and pass the path for search for expressions inside files.

Fast.search_file(expression, 'file.rb')

It's simple combination of Fast.ast_from_file with


You'll be probably looking for multiple ruby files, then this method fetches all internal .rb files

Fast.ruby_files_from(['lib']) # => ["lib/fast.rb"]


Combines the search_file with ruby_files_from multiple locations and returns tuples with files and results.

Fast.search_all("(def ast_from_file)")
=> {"./lib/fast.rb"=>[s(:def, :ast_from_file,
    s(:arg, :file)),

You can also override the second param and pass target files or folders:

Fast.search_all("(def _)", '../other-folder')

Use Fast with SQL

Did you like the Ruby AST?

Check out the SQL Support.

You can do things like:

Fast.parse_sql('select 1').replace('ival', '2') # => "select 2"
Learn how to create a SQL Formatter