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This gem provides Option, Either, and Try monads implemented an idiomatic way. It is highly inspired by scala's implementation.


Add this line to your application's Gemfile:

gem 'fear'

And then execute:

$ bundle

Or install it yourself as:

$ gem install fear


Option (Documentation)

Represents optional (nullable) values. Instances of Option are either an instance of Some or the object None.

The most idiomatic way to use an Option instance is to treat it as a collection and use map, flat_map, select, or each:

include Fear::Option::Mixin

def normalize_name(name)
    .select { |n| n.length != 0 }

normalize_name('robert paulson ') #=> 'ROBERT PAULSON'
normalize_name(nil) #=> 'NONAME' 

This allows for sophisticated chaining of Option values without having to check for the existence of a value.

Try (Documentation)

The Try represents a computation that may either result in an exception, or return a successfully computed value. Instances of Try, are either an instance of Success or Failure.

For example, Try can be used to perform division on a user-defined input, without the need to do explicit exception-handling in all of the places that an exception might occur.

include Fear::Try::Mixin

dividend = Try { Integer(params[:dividend]) }
divisor = Try { Integer(params[:divisor]) }

result = dividend.flat_map { |x| divisor.map { |y| x / y } }

if result.success?
  puts "Result of #{dividend.get} / #{divisor.get} is: #{result.get}"
  puts "You must've divided by zero or entered something wrong. Try again"
  puts "Info from the exception: #{result.exception.message}"

Either (Documentation)

Represents a value of one of two possible types (a disjoint union.) An instance of Either is either an instance of Left or Right.

A common use of Either is as an alternative to Option for dealing with possible missing values. In this usage, None is replaced with a Left which can contain useful information. Right takes the place of Some. Convention dictates that Left is used for failure and Right is used for success.

For example, you could use Either<String, Fixnum> to select whether a received input is a String or an Fixnum.

include Fear::Either::Mixin

input = Readline.readline('Type Either a string or an Int: ', true)
result = begin
rescue ArgumentError

    -> (x) { "You passed me the Int: #{x}, which I will increment. #{x} + 1 = #{x+1}" },
    -> (x) { "You passed me the String: #{x}" }

For composition

Provides syntactic sugar for composition of multiple monadic operations. It supports two such operations - flat_map and map. Any class providing them is supported by For.

include Fear::For::Mixin

def divide(dividend, divisor)
  For(x: dividend, y: divisor) do 
    x / y

dividend = Try { Integer(params[:dividend]) } #=> Try(4)
divisor = Try { Integer(params[:divisor]) } #=> Try(2)

divide(dividend, divisor) #=> Try(2)

It would be translated to

Success(4).flat_map do |x|
  Success(2).map do |y|
    x / y

If one of operands is Failure, the result is Failure

dividend = Try { 42 }
divisor = Try { Integer('ehuton') }

divide(dividend, divisor) #=> Failure(<ArgumentError: invalid value for Integer(): "ehuton">)

For works with arrays as well

For(a: [1, 2], b: [2, 3], c: [3, 4]) do 
  a * b * c
end #=> [6, 8, 9, 12, 12, 16, 18, 24]

would be translated to:

[1, 2].flat_map do |a|
  [2, 3].flat_map do |b|
    [3, 4].map do |c|
      a * b * c

Pattern Matching

Option, Either, and Try contains enhanced version of #=== method. It performs matching not only on container itself, but on enclosed value as well. I'm writing all the options in a one case statement in sake of simplicity.

case Some(42)
when Some(42)                #=> matches
when Some(41)                #=> does not match
when Some(Fixnum)            #=> matches
when Some(String)            #=> does not match
when Some((40..43))          #=> matches
when Some(-> (x) { x > 40 }) #=> matches


To simplify testing, you may use fear-rspec gem. It provides a bunch of rspec matchers.


  1. Fork it ( https://github.com/bolshakov/fear/fork )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request