Class: RuboCop::NodePattern

Inherits:

Object

Object
RuboCop::NodePattern

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 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 ...)
'{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 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...)
                    # (#== 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 Classes: Compiler

Constant Summary collapse

Invalid =

Class.new(StandardError)

Instance Method Summary collapse

#initialize(str) ⇒ NodePattern constructor

A new instance of NodePattern.

Constructor Details

#initialize(str) ⇒ `NodePattern`

Returns a new instance of NodePattern.

# File 'lib/rubocop/node_pattern.rb', line 488

def initialize(str)
  compiler = Compiler.new(str)
  src = "def match(node0#{compiler.emit_trailing_params});" \
        "#{compiler.emit_method_code}end"
  instance_eval(src)
end