# 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