Patme

This gem is my experiment on elixir-style pattern matching in ruby. In it's current state it is no more than a proof of concept and I suggest not to use it in production.

Info

Patme module stores your instance methods internally and removes them from your class. Then, when method is called, it tries to pattern-match given arguments with existing method implementations. If implementation was found, it executes it, otherwise you'll get a NoMethodError.

Currently this gem supports 3 types of arguments: specific, arbitrary and optional. In method definition def foo(agr1, arg2=1, _arg3=false) arg1 is an arbitrary argument, arg2 is specific and arg3 is optional.

Patme supports class-based pattern matching. If method's specific argument is a Class, then it will be matched based on given argument's class using is_a?

Usage

Recursion example

require 'patme'

class Factorial
  include Patme::PatternMatching

  def calculate(n=0)
    1
  end

  def calculate(n)
    n * calculate(n-1)
  end
end

factorial_calculator = Factorial.new

factorial_calculator.calculate(0) # => 1
factorial_calculator.calculate(5) # => 120
factorial_calculator.calculate(-1) # => endless recursion, don't do so ^_^

Class-based pattern matching

require 'patme'

SeriousError = Class.new(StandardError)
RegularError = Class.new(StandardError)
UnexpectedError = Class.new(StandardError)

class ErrorHandler
  include Patme::PatternMatching

  def initialize(error)
    @error = error
  end

  def handle
    do_handle(@error)
  end

  # Will match if error.is_a?(SeriousError)
  def do_handle(error=SeriousError)
    puts "Alarm! SeriousError! Details: #{error.message}"
  end

  # Will match if error.is_a?(RegularError)
  def do_handle(error=RegularError)
    puts "Don't worry, just a RegularError: #{error.message}"
  end

  # Will match for all other StandardErrors
  def do_handle(error=StandardError)
    puts "Unexpected error: #{error.class}. Details: #{error.message}"
  end
end

begin
  raise SeriousError, 'boom!'
rescue => error
  ErrorHandler.new(error).handle
end
# => Alarm! SeriousError! Details: boom!

begin
  raise RegularError, 'we need to fix this'
rescue => error
  ErrorHandler.new(error).handle
end
# => Don't worry, just a RegularError: we need to fix this

begin
  raise UnexpectedError, 'something went completely wrong'
rescue => error
  ErrorHandler.new(error).handle
end
# => Unexpected error: UnexpectedError. Details: something went completely wrong

Full-featured example

require 'patme'

class MyClass
  include Patme::PatternMatching

  # This method will match only when arg1  ='test'
  def foo(arg1='test')
    "ran foo('test') with #{arg1}"
  end

  # Will match only when arg1  ='other'
  def foo(arg1='other')
    "ran foo('other') with #{arg1}"
  end

  # You can also use other basic types
  def foo(arg1={a: 1, b: 2})
    "ran foo({a: 1, b: 2}) with #{arg1}"
  end

  # Will match when arg2 = 'test' no matter what arg1 is
  def foo(arg1, arg2='test')
    "ran foo(any, 'test') with [#{arg1}, #{arg2}]"
  end

  # Will match with any arg1 and both with and without arg2
  # if arg2 is not supplied, method will receive arg2 = 'default'
  def foo(arg1, _arg2="default")
    "ran foo(any, optional) with [#{arg1}, #{_arg2}]"
  end


  # Will match with any one argument.
  def bar(arg1)
    "ran bar(any) with #{arg1}"
  end

  # Won't ever match because previous definition of bar will be pattern-matched
  # before this one
  def bar(arg1='never')
    "ran bar('never') with #{arg1}"
  end


  def baz(arg1='test')
    "ran baz('test') with #{arg1}"
  end
end

my_obj = MyClass.new

my_obj.foo('test') # => "ran foo('test') with test"
my_obj.foo('other') # => "ran foo('other') with other"
my_obj.foo({a: 1, b: 2}) # => "ran foo({a: 1, b: 2}) with {:a => 1, :b => 2}"
my_obj.foo(1, 'test') # => "ran foo(any, 'test') with [1, test]"
my_obj.foo(1) # => "ran foo(any, optional) with [1, default]"
my_obj.foo(1, 'some') # => "ran foo(any, optional) with [1, some]"

my_obj.bar(1) # => "ran bar(any) with 1"
my_obj.bar('never') # => "ran bar(any) with never"

my_obj.baz('test') # => "ran baz('test') with test"
my_obj.baz(1) # => NoMethodError

Limitations

Pattern-matching in it's current implementation is slow and the more implementations of a method you have the slower it would be:

require 'spec_helper'
require 'benchmark'

class BenchmarkDummy
  def no_pm(arg1, arg2)
    ['no_pm', arg1, arg2]
  end

  include Patme::PatternMatching

  def pm(arg1='test', arg2='lol')
    ['specific', arg1, arg2]
  end

  def pm(arg1, arg2)
    ['arbitrary', arg1, arg2]
  end
end

n = 1_000_000
foo = BenchmarkDummy.new

Benchmark.bm do |x|
  x.report('no_pm       '){ n.times{foo.no_pm('test', 'lol')} }
  x.report('pm_specific '){ n.times{foo.pm('test', 'lol')} }
  x.report('pm_arbitrary'){ n.times{foo.pm('some', 'arg')} }
end

#                   user     system      total        real
# no_pm         0.200000   0.000000   0.200000 (  0.205747)
# pm_specific   3.570000   0.000000   3.570000 (  3.574598)
# pm_arbitrary  5.210000   0.000000   5.210000 (  5.210410)

Only pattern matching on instance methods is supported.

Including Patme::PatternMatching into your class makes all of the methods below the include statement be pattern-matched (I think I'll fix this issue later).

Todos

1. Add support for class methods

2. Add something to tell Patme not to pattern-match on every method

   # Possible example
   class Hello
     include Patme::PatternMatching
   
     # Comment after method header will tell Patme not to touch this method
     def tell(name) # ::pm_off
       puts "Hello, #{name}"
     end
   
     # Or maybe it'll be better to use an annotation
     pm_start
     def yell(name="Boss", message)
       raise CommonSenceError, "don't yell on your boss"
     end
   
     def yell(name, message)
       puts message.upcase
     end
     pm_stop
   end

3. Add guards

   # Possible example
   class Factorial
     include Patme::PatternMatch
   
     def calculate(n=0)
       1
     end
   
     def calculate(n) # ::when( n < 0 )
       raise ArgumentError, "Can't calculate factorial of a negative number: #{n}"
     end
   
     def calculate(n) # ::when( n > 0 )
       n * self.of(n-1)
     end
   end

4. Add support for keyword arguments (key: - arbitrary, key: value - specific, _key: value - optional)

5. Your suggestions?

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/vizvamitra/patme.

License

The gem is available as open source under the terms of the MIT License.