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.

Attributes

query [R]

The query that this pattern was initialized with.

Public Class Methods

new (query)

Source

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

Create a new pattern with the given query. The query should be a string containing a Ruby pattern matching expression.

Public Instance Methods

compile ()

Source

# 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

Compile the query into a callable object that can be used to match against nodes.

scan (root) { |node| ... }

Source

# 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

Scan the given node and all of its children for nodes that match the pattern. If a block is given, it will be called with each node that matches the pattern. If no block is given, an enumerator will be returned that will yield each node that matches the pattern.

Private Instance Methods

combine_and (left, right)

Source

# File lib/prism/pattern.rb, line 102
def combine_and(left, right)
  ->(other) { left.call(other) && right.call(other) }
end

Shortcut for combining two procs into one that returns true if both return true.

combine_or (left, right)

Source

# File lib/prism/pattern.rb, line 108
def combine_or(left, right)
  ->(other) { left.call(other) || right.call(other) }
end

Shortcut for combining two procs into one that returns true if either returns true.

compile_alternation_pattern_node (node)

Source

# File lib/prism/pattern.rb, line 143
def compile_alternation_pattern_node(node)
  combine_or(compile_node(node.left), compile_node(node.right))
end

in foo | bar

compile_array_pattern_node (node)

Source

# File lib/prism/pattern.rb, line 118
def compile_array_pattern_node(node)
  compile_error(node) if !node.rest.nil? || node.posts.any?

  constant = node.constant
  compiled_constant = compile_node(constant) if constant

  preprocessed = node.requireds.map { |required| compile_node(required) }

  compiled_requireds = ->(other) do
    deconstructed = other.deconstruct

    deconstructed.length == preprocessed.length &&
      preprocessed
        .zip(deconstructed)
        .all? { |(matcher, value)| matcher.call(value) }
  end

  if compiled_constant
    combine_and(compiled_constant, compiled_requireds)
  else
    compiled_requireds
  end
end

in [foo, bar, baz]

compile_constant_name (node, name)

Source

# File lib/prism/pattern.rb, line 168
def compile_constant_name(node, name)
  if Prism.const_defined?(name, false)
    clazz = Prism.const_get(name)

    ->(other) { clazz === other }
  elsif Object.const_defined?(name, false)
    clazz = Object.const_get(name)

    ->(other) { clazz === other }
  else
    compile_error(node)
  end
end

Compile a name associated with a constant.

compile_constant_path_node (node)

Source

# File lib/prism/pattern.rb, line 148
def compile_constant_path_node(node)
  parent = node.parent

  if parent.is_a?(ConstantReadNode) && parent.slice == "Prism"
    name = node.name
    raise CompilationError, node.inspect if name.nil?

    compile_constant_name(node, name)
  else
    compile_error(node)
  end
end

in Prism::ConstantReadNode

compile_constant_read_node (node)

Source

# File lib/prism/pattern.rb, line 163
def compile_constant_read_node(node)
  compile_constant_name(node, node.name)
end

in ConstantReadNode in String

compile_error (node)

Source

# File lib/prism/pattern.rb, line 113
def compile_error(node)
  raise CompilationError, node.inspect
end

Raise an error because the given node is not supported.

compile_hash_pattern_node (node)

Source

# File lib/prism/pattern.rb, line 184
def compile_hash_pattern_node(node)
  compile_error(node) if node.rest
  compiled_constant = compile_node(node.constant) if node.constant

  preprocessed =
    node.elements.to_h do |element|
      key = element.key
      if key.is_a?(SymbolNode)
        [key.unescaped.to_sym, compile_node(element.value)]
      else
        raise CompilationError, element.inspect
      end
    end

  compiled_keywords = ->(other) do
    deconstructed = other.deconstruct_keys(preprocessed.keys)

    preprocessed.all? do |keyword, matcher|
      deconstructed.key?(keyword) && matcher.call(deconstructed[keyword])
    end
  end

  if compiled_constant
    combine_and(compiled_constant, compiled_keywords)
  else
    compiled_keywords
  end
end

in InstanceVariableReadNode[name: Symbol] in { name: Symbol }

compile_nil_node (node)

Source

# File lib/prism/pattern.rb, line 214
def compile_nil_node(node)
  ->(attribute) { attribute.nil? }
end

in nil

compile_node (node)

Source

# File lib/prism/pattern.rb, line 243
def compile_node(node)
  case node
  when AlternationPatternNode
    compile_alternation_pattern_node(node)
  when ArrayPatternNode
    compile_array_pattern_node(node)
  when ConstantPathNode
    compile_constant_path_node(node)
  when ConstantReadNode
    compile_constant_read_node(node)
  when HashPatternNode
    compile_hash_pattern_node(node)
  when NilNode
    compile_nil_node(node)
  when RegularExpressionNode
    compile_regular_expression_node(node)
  when StringNode
    compile_string_node(node)
  when SymbolNode
    compile_symbol_node(node)
  else
    compile_error(node)
  end
end

Compile any kind of node. Dispatch out to the individual compilation methods based on the type of node.

compile_regular_expression_node (node)

Source

# File lib/prism/pattern.rb, line 219
def compile_regular_expression_node(node)
  regexp = Regexp.new(node.unescaped, node.closing[1..])

  ->(attribute) { regexp === attribute }
end

in /foo/

compile_string_node (node)

Source

# File lib/prism/pattern.rb, line 227
def compile_string_node(node)
  string = node.unescaped

  ->(attribute) { string === attribute }
end

in “” in “foo”

compile_symbol_node (node)

Source

# File lib/prism/pattern.rb, line 235
def compile_symbol_node(node)
  symbol = node.unescaped.to_sym

  ->(attribute) { symbol === attribute }
end

in :+ in :foo