Class: Prism::Pattern

Inherits:
Object show all
Defined in:
lib/prism/pattern.rb

Overview

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.

Defined Under Namespace

Classes: CompilationError

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(query) ⇒ Pattern

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



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# File 'lib/prism/pattern.rb', line 63

def initialize(query)
  @query = query
  @compiled = nil
end

Instance Attribute Details

#query ⇒ Object (readonly)

The query that this pattern was initialized with.



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# File 'lib/prism/pattern.rb', line 59

def query
  @query
end

Instance Method Details

#compile ⇒ Object

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



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# File 'lib/prism/pattern.rb', line 70

def compile
  result = Prism.parse("case nil\nin #{query}\nend")
  compile_node(result.value.statements.body.last.conditions.last.pattern)
end

#scan(root) ⇒ Object

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.



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# File 'lib/prism/pattern.rb', line 79

def scan(root)
  return to_enum(__method__, root) unless block_given?

  @compiled ||= compile
  queue = [root]

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