Class: RuboCop::NodePattern

Inherits:
Object
  • Object
show all
Defined in:
lib/rubocop/node_pattern.rb

Overview

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

Initialize a new ‘NodePattern` with `NodePattern.new(pattern_string)`, 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 block and 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 ...)
'(op-asgn)'         # node types with hyphenated names also work
'{send class}'      # matches (send ...) or (class ...)
'({send class})'    # matches (send) or (class)
'(send const)'      # matches (send (const ...))
'(send _ :new)'     # matches (send <anything> :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 match against the last children
'(array <str sym>)' # you can match children in any order. This
                    # would match `['x', :y]` as well as `[:y, 'x']
'(_ <str sym ...>)' # will match if arguments have at least a `str` and
                    # a `sym` node, but can have more.
'(array <$str $_>)' # captures are in the order of the pattern,
                    # irrespective of the actual order of the children
'(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...)
                    # (#== is used to see if values unify)
'(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
                    # if a truthy value is returned, the match succeeds
'(int [!1 !2])'     # [] contains multiple patterns, ALL of which must
                    # match in that position
                    # in other words, while {} is pattern union (logical
                    # OR), [] is intersection (logical AND)
'(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'
                    # the number of extra parameters passed to #match
                    # must equal the highest % value in the pattern
                    # for consistency, %0 is the 'root node' which is
                    # passed as the 1st argument to #match, where the
                    # matching process starts
'^^send'            # each ^ ascends one level in the AST
                    # so this matches against the grandparent node
'#method'           # we call this a 'funcall'; it calls a method in the
                    # context where a pattern-matching method is defined
                    # if that returns a truthy value, the match succeeds
'equal?(%1)'        # predicates can be given 1 or more extra args
'#method(%0, 1)'    # funcalls can also be given 1 or more extra args

You can nest arbitrarily deep:

# matches node parsed from 'Const = Class.new' or 'Const = Module.new':
'(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 note 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>]`

The compiler code is very simple; don’t be afraid to read through it!

Defined Under Namespace

Modules: Macros

Constant Summary collapse

Invalid = 
Class.new(StandardError)

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(str) ⇒ NodePattern

Returns a new instance of NodePattern.



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# File 'lib/rubocop/node_pattern.rb', line 686

def initialize(str)
  @pattern = str
  compiler = Compiler.new(str)
  src = "def match(node0#{compiler.emit_trailing_params});" \
        "#{compiler.emit_method_code}end"
  instance_eval(src, __FILE__, __LINE__ + 1)
end

Instance Attribute Details

#pattern ⇒ Object (readonly)

Returns the value of attribute pattern.



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# File 'lib/rubocop/node_pattern.rb', line 684

def pattern
  @pattern
end

Instance Method Details

#==(other) ⇒ Object Also known as: eql? 



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# File 'lib/rubocop/node_pattern.rb', line 709

def ==(other)
  other.is_a?(NodePattern) &&
    Compiler.tokens(other.pattern) == Compiler.tokens(pattern)
end

#marshal_dump ⇒ Object 



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# File 'lib/rubocop/node_pattern.rb', line 705

def marshal_dump
  pattern
end

#marshal_load(pattern) ⇒ Object 



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# File 'lib/rubocop/node_pattern.rb', line 701

def marshal_load(pattern)
  initialize pattern
end

#match(*args) ⇒ Object 



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# File 'lib/rubocop/node_pattern.rb', line 694

def match(*args)
  # If we're here, it's because the singleton method has not been defined,
  # either because we've been dup'ed or serialized through YAML
  initialize(pattern)
  match(*args)
end

#to_s ⇒ Object 



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# File 'lib/rubocop/node_pattern.rb', line 715

def to_s
  "#<#{self.class} #{pattern}>"
end