class Prism::Pattern

A pattern is an object that wraps a Ruby pattern matching expression. The expression would normally be passed to an ‘in` clause within a `case` expression or a rightward assignment expression. For example, in the following snippet:

case node
in ConstantPathNode[ConstantReadNode[name: :Prism], ConstantReadNode[name: :Pattern]]
end

the pattern is the ConstantPathNode[...] expression.

The pattern gets compiled into an object that responds to call by running the compile method. This method itself will run back through Prism to parse the expression into a tree, then walk the tree to generate the necessary callable objects. For example, if you wanted to compile the expression above into a callable, you would:

callable = Prism::Pattern.new("ConstantPathNode[ConstantReadNode[name: :Prism], ConstantReadNode[name: :Pattern]]").compile
callable.call(node)

The callable object returned by compile is guaranteed to respond to call with a single argument, which is the node to match against. It also is guaranteed to respond to ===, which means it itself can be used in a ‘case` expression, as in:

case node
when callable
end

If the query given to the initializer cannot be compiled into a valid matcher (either because of a syntax error or because it is using syntax we do not yet support) then a Prism::Pattern::CompilationError will be raised.

# File lib/prism/pattern.rb, line 63
def initialize(query)
  @query = query
  @compiled = nil
end

# File lib/prism/pattern.rb, line 70
def compile
  result = Prism.parse("case nil\nin #{query}\nend")

  case_match_node = result.value.statements.body.last
  raise CompilationError, case_match_node.inspect unless case_match_node.is_a?(CaseMatchNode)

  in_node = case_match_node.conditions.last
  raise CompilationError, in_node.inspect unless in_node.is_a?(InNode)

  compile_node(in_node.pattern)
end

# File lib/prism/pattern.rb, line 86
def scan(root)
  return to_enum(:scan, root) unless block_given?

  @compiled ||= compile
  queue = [root]

  while (node = queue.shift)
    yield node if @compiled.call(node) # steep:ignore
    queue.concat(node.compact_child_nodes)
  end
end