Class: RuboCop::NodePattern

  • Object
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Defined in:


This class performs a pattern-matching operation on an AST node.

Initialize a new NodePattern with, then pass an AST node to NodePattern#match. Alternatively, use one of the class macros in NodePattern::Macros to define your own pattern-matching method.

If the match fails, nil will be returned. If the match succeeds, the return value depends on whether a block was provided to #match, and whether the pattern contained any "captures" (values which are extracted from a matching AST.)

  • With block: #match yields the captures (if any) and passes the return value of the block through.
  • With no block, but one capture: the capture is returned.
  • With no block, but multiple captures: captures are returned as an array.
  • With no captures: #match returns true.

Pattern string format examples

':sym' # matches a literal symbol '1' # matches a literal integer 'nil' # matches a literal nil 'send' # matches (send ...) '(send)' # matches (send) '(send ...)' # matches (send ...) 'class' # matches (send ...) or (class ...) '(class)' # matches (send) or (class) '(send const)' # matches (send (const ...)) '(send _ :new)' # matches (send :new) '(send $_ :new)' # as above, but whatever matches the $_ is captured '(send $_ $_)' # you can use as many captures as you want '(send !const ...)' # ! negates the next part of the pattern '$(send const ...)' # arbitrary matching can be performed on a capture '(send _recv _msg)' # wildcards can be named (for readability) '(send ... :new)' # you can specifically match against the last child # (this only works for the very last) '(send $...)' # capture all the children as an array '(send $... int)' # capture all children but the last as an array '(send _x :+ _x)' # unification is performed on named wildcards # (like Prolog variables...) '(int odd?)' # words which end with a ? are predicate methods, # are are called on the target to see if it matches # any Ruby method which the matched object supports # can be used '(int [!1 !2])' # [] contains multiple patterns, ALL of which must # match in that position # ({} is pattern union, [] is intersection) '(send %1 _)' # % stands for a parameter which must be supplied to # #match at matching time # it will be compared to the corresponding value in # the AST using #== # a bare '%' is the same as '%1' '^^send' # each ^ ascends one level in the AST # so this matches against the grandparent node

You can nest arbitrarily deep:

# matches node parsed from 'Const =' or 'Const =':
'(casgn nil const (send (const nil {:Class :Module}) :new)))'
# matches a node parsed from an 'if', with a '==' comparison,
# and no 'else' branch:
'(if (send _ :== _) _ nil)'

Note that patterns like 'send' are implemented by calling #send_type? on the node being matched, 'const' by #const_type?, 'int' by #int_type?, and so on. Therefore, if you add methods which are named like #prefix_type? to the AST node class, then 'prefix' will become usable as a pattern.

Also node that if you need a "guard clause" to protect against possible nils in a certain place in the AST, you can do it like this: [!nil <pattern>]

Defined Under Namespace

Modules: Macros Classes: Compiler

Constant Summary collapse

Invalid =

Instance Method Summary collapse

Constructor Details

#initialize(str) ⇒ NodePattern

Returns a new instance of NodePattern.

# File 'lib/rubocop/node_pattern.rb', line 383

def initialize(str)
  compiler =
  src = 'def match(node0' << compiler.emit_trailing_param_list << ');' <<
        compiler.emit_method_code << 'end'