Mutation visitor

mutation.rb

require "syntax_tree"

# Create a new visitor that is going to visit the parsed tree.
visitor = SyntaxTree::Visitor::MutationVisitor.new

# Define a mutation on the visitor that is going to search for the given pattern
# to find nodes that match it and then call the given block to mutate the node
# in place in the tree.
visitor.mutate("If[predicate: Assign | OpAssign]") do |node|
  # Get the existing If's predicate node.
  predicate = node.predicate

  # Create a new predicate node that wraps the existing predicate node
  # in parentheses.
  predicate =
    SyntaxTree::Paren.new(
      lparen: SyntaxTree::LParen.default,
      contents: predicate,
      location: predicate.location
    )

  # Return a copy of this node with the new predicate.
  node.copy(predicate: predicate)
end

# Parse the source code into a tree.
source = "if a = 1; end"
program = SyntaxTree.parse(source)

# Visit the tree with the mutation visitor and obtain the new tree.
mutated = program.accept(visitor)

# Format the mutated tree back into source code.
SyntaxTree::Formatter.format(source, mutated)
# => "if (a = 1)\nend\n"