What's the Precedence of Ruby'S Method Call

What's the precedence of method calls with and without parentheses?

Prelude

This aims to test all possible scenarios.

Note that when saying "operator X has higher precedence than method invocation" what is meant is in arguments. Aka:

invocation foo X bar

as opposed to (call on object)

X invocation

As far as the second case is concerned, method calls always have higher precedence.

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Short answer

It doesn't fit:

• It causes SyntaxError in some cases
• It has higher precedence than rescue, but lower than assignment

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Summary

• not can't be used after method invocation regardless of brackets
• Using brackets (()) with method invocations sometimes causes a SyntaxError. These cases are: and, or, if, unless, until, while and rescue
• In cases when brackets don't cause an error, they don't change the precedence in any way
• All operators, except for and, or, postfix if, unless, until, while, rescue have higher precedence than method invocation

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Lets try it:

class Noone < BasicObject
  undef_method :!

  def initialize(order)
    @order = order
  end

  def method_missing(name, *args)
    @order << name
    self
  end
end

---

First unary:

# + and - will become binary
unary_operators = %i(! ~ not defined?)

puts 'No brackets'
unary_operators.each do |operator|
  puts operator

  order = []
  foo = Noone.new order
  bar = Noone.new order
  begin
    eval("foo.meta #{operator} bar")
  rescue SyntaxError => e
    puts e
  end
  p order
  puts '-----------'
end

puts 'Brackets'
unary_operators.each do |operator|
  puts operator

  order = []
  foo = Noone.new order
  bar = Noone.new order
  begin
    eval("foo.meta(#{operator} bar)")
  rescue SyntaxError => e
    puts e
  end
  p order
  puts '-----------'
end

Points taken:

• not after a method invocation is a SyntaxError
• all unary operators have higher precedence than method invocation regardless of brackets

---

Now binary:

binary_operators = %i(
  **
  * / %
  + -
  << >>
  &
  | ^
  > >= < <=
  <=> == === =~
  .. ...
  or and
)

puts 'No brackets'
binary_operators.each do |operator|
  order = []
  foo = Noone.new order
  bar = Noone.new order
  baz = Noone.new order
  begin
    eval("foo.meta bar #{operator} baz")
  rescue SyntaxError => e
    puts e
  end
  p order
end

puts 'Brackets'
binary_operators.each do |operator|
  order = []
  foo = Noone.new order
  bar = Noone.new order
  baz = Noone.new order
  begin
    eval("foo.meta( bar #{operator} baz)")
  rescue SyntaxError => e
    puts e
  end
  p order
end

Points taken:

• brackets around method invocation with and or or is a SyntaxError
• we have to test and and or further without brackets
• .. and ... call <=>. We have to test this further
• we couldn't test a few other binary operators this way, namely &&, ||, ==, !=, modifier rescue, if, unless, until, while
• other than the above mentioned, operators have higher precedence, regardless of brackets

---

def yes
  puts 'yes'
  true
end

def no
  puts 'no'
  false
end

def anything(arg)
  puts 'Anything'
  arg
end

anything yes and no
anything no or yes
anything yes && no
anything no || yes
anything(yes && no)
anything(no || yes)

anything yes == no
anything(yes == no)
anything yes != no
anything(yes != no)

Points taken:

• and and or have lower precedence without brackets
• &&, ||, == and != have higher precedence regardless of brackets

---

def five(*args)
  p args
  5
end

five 2..7
five(2..7)
five 2...7
five(2...7)

Points taken:

• .. and ... have higher precedence regardless of brackets

---

anything yes if no
anything(yes if no)
anything no unless yes
anything(no unless yes)

anything no until yes
anything(no until yes)
anything yes while no
anything(yes while no)

Points taken:

• brackets with if, unless, until, while cause a SyntaxError
• all of the above have lower precedence than method invocation without brackets

---

def error
  puts 'Error'
  raise
end

anything error rescue yes
anything(error rescue yes)

Points taken:

• brackets around rescue cause a SyntaxError
• rescue has lower precedence if no brackets are present

---

Ternary:

anything yes ? no : 42
anything(yes ? no : 42)

Points taken:

• ternary has higher precedence regardless of brackets

---

Assignment (left for last as it changes yes and no):

anything yes = no
anything(no = five(42))

Points taken:

• Assignment has higher precedence than invocation

Note that += and the like are just shortcuts for + and = so they exhibit the same behaviour.

Precedence of operators in ruby

Because of Operator Precedence, you should either do:

if object && (object.is? ball)

or

if object && object.is?(ball)

The reason is "&&" and "||" have higher precedence than "and" and "or". Thus if you don't use parenthesis for the latter "object.is? ball", "object.is?" will be grabbed first by "&&". However, if you use "and" without parenthesis, it works fine, since it has lowest priority in this case:

if object and object.is? ball

Ruby operator precedence

If x is integer, then x**2.to_s is x.**(2.to_s). Because Fixnum#** method exist. In your case ** is a method, not an operator.

In this case, no operator precedence and associativity comes into existence.

In the expression - x.**(2.to_s)

• x is Fixnum object.

• ** is a method called on x.

• 2.to_s is simply an expression, which will be evaluated first, and passed as an argument to the method **.

My answer is as per your example. But there are several edge cases in Ruby. Just to get the taste of those, give some time to read this thread why does a + f b not parse?.

Are there Ruby precedence issues with using Proc.call vs. Proc.[]?

The #call technique allows the operator precedence to potentially obscure intent:

p = Proc::new do |a1| Proc::new do |a2| "#{a1.inspect}:#{a2.inspect}" end end
p.call([1,2,3]).call [1]
=> => "[1, 2, 3]:[1]"
p.call [1,2,3][1]
=> #<Proc:0x7ffa08dc@(irb):1>
p.call([1,2,3])[1]
=> "[1, 2, 3]:1"
p[[1,2,3]][[1]]
=> "[1, 2, 3]:[1]"

The [] syntax makes the syntactic association of the arguments to the method more robust, but you'd achieve the same effect by putting parentheses around the arguments to Proc#call.

Ruby precedence of methods in objects extended with multiple modules

method_added is in Module.

I actually meant included, not extended, but both are also in Module.

The mechanism would revolve around doing something like this:

module Foo
  def self.included(base)
    base.extend(FooMethods)
  end

  module FooMethods
    def bar
      # Whatever
    end
  end
end

Inside Foo.included you can determine, based on arbitrary criteria, whether or not the methods in question should be added to base (the entity including the module).

In your case, you could check to see if a "higher priority" module was already included, or see if the module is the "higher priority" module. Based on that you'd decide whether or not to add the methods.

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