Module: Rumonade::Monad

Includes:
Enumerable
Included in:
Either::LeftProjection, Either::RightProjection, Option
Defined in:
lib/rumonade/monad.rb

Overview

Mix-in for common monad functionality dependent on implementation of monadic methods unit and bind

Notes:

  • Classes should include this module AFTER defining the monadic methods unit and bind

  • When Monad is mixed into a class, if the class already contains methods in METHODS_TO_REPLACE, they will be renamed to add the suffix _without_monad, and replaced with the method defined here which has the suffix _with_monad

Constant Summary collapse

DEFAULT_METHODS_TO_REPLACE_WITH_MONAD =

Methods to replace when mixed in – unless class defines METHODS_TO_REPLACE_WITH_MONAD

[:map, :flat_map, :flatten]

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.included(base) ⇒ Object

When mixed into a class, this callback is executed



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# File 'lib/rumonade/monad.rb', line 15

def self.included(base)
  methods_to_replace = base::METHODS_TO_REPLACE_WITH_MONAD rescue DEFAULT_METHODS_TO_REPLACE_WITH_MONAD

  base.class_eval do
    # optimization: replace flat_map with an alias for bind, as they are identical
    alias_method :flat_map_with_monad, :bind

    methods_to_replace.each do |method_name|
      alias_method "#{method_name}_without_monad".to_sym, method_name if public_instance_methods.include? method_name
      alias_method method_name, "#{method_name}_with_monad".to_sym
    end
  end
end

Instance Method Details

#can_flatten_in_monad? ⇒ Boolean

Returns true if #flatten_with_monad can call recursively on contained values members (eg. elements of an array).

NOTE: Is overridden in Hash.

Returns:

  • (Boolean) 


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# File 'lib/rumonade/monad.rb', line 99

def can_flatten_in_monad?
  true
end

#each(lam = nil, &blk) ⇒ Object

Applies the given procedure to each element in this monad



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# File 'lib/rumonade/monad.rb', line 32

def each(lam = nil, &blk)
  bind { |v| (lam || blk).call(v) }; nil
end

#flat_map_with_monad(lam = nil, &blk) ⇒ Object

Returns the results of applying the given function to each element in this monad

NOTE: normally aliased as flat_map when Monad is mixed into a class



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# File 'lib/rumonade/monad.rb', line 46

def flat_map_with_monad(lam = nil, &blk)
  bind(lam || blk)
end

#flatten_with_monad(depth = nil) ⇒ Object

Returns a monad whose elements are the ultimate (non-monadic) values contained in all nested monads

NOTE: normally aliased as flatten when Monad is mixed into a class

Examples:

[Some(Some(1)), Some(Some(None))], [None]].flatten_with_monad
#=> [1]


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# File 'lib/rumonade/monad.rb', line 58

def flatten_with_monad(depth=nil)
  if depth.is_a? Integer
    depth.times.inject(self) { |e, _| e.shallow_flatten }
  else
    bind do |x|
      if x.is_a?(Monad) && x.can_flatten_in_monad?
        x.flatten_with_monad
      else
        self.class.unit(x)
      end
    end
  end
end

#map_with_monad(lam = nil, &blk) ⇒ Object

Returns a monad whose elements are the results of applying the given function to each element in this monad

NOTE: normally aliased as map when Monad is mixed into a class



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# File 'lib/rumonade/monad.rb', line 39

def map_with_monad(lam = nil, &blk)
  bind { |v| self.class.unit((lam || blk).call(v)) }
end

#select(lam = nil, &blk) ⇒ Object Also known as: find_all

Returns a monad whose elements are all those elements of this monad for which the given predicate returned true



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# File 'lib/rumonade/monad.rb', line 73

def select(lam = nil, &blk)
  bind { |x| (lam || blk).call(x) ? self.class.unit(x) : self.class.empty }
end

#shallow_flatten ⇒ Object

Returns a monad whose elements are the values contained in the first level of nested monads

This method is equivalent to the Scala flatten call (single-level flattening), whereas #flatten is in keeping with the native Ruby flatten calls (multiple-level flattening).

Examples:

[Some(Some(1)), Some(Some(None)), [None]].shallow_flatten
#=> [Some(1), Some(None), None]
[Some(1), Some(None), None].shallow_flatten
#=> [1, None]
[1, None].shallow_flatten
#=> [1]


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# File 'lib/rumonade/monad.rb', line 91

def shallow_flatten
  bind { |x| x.is_a?(Monad) ? x : self.class.unit(x) }
end