class Concurrent::Maybe

A `Maybe` encapsulates an optional value. A `Maybe` either contains a value of (represented as `Just`), or it is empty (represented as `Nothing`). Using `Maybe` is a good way to deal with errors or exceptional cases without resorting to drastic measures such as exceptions.

`Maybe` is a replacement for the use of `nil` with better type checking.

For compatibility with {Concurrent::Concern::Obligation} the predicate and accessor methods are aliased as `fulfilled?`, `rejected?`, `value`, and `reason`.

## Motivation

A common pattern in languages with pattern matching, such as Erlang and Haskell, is to return either a value or an error from a function Consider this Erlang code:

“`erlang case file:consult(“data.dat”) of

{ok, Terms} -> do_something_useful(Terms);
{error, Reason} -> lager:error(Reason)

end. “`

In this example the standard library function `file:consult` returns a [tuple](erlang.org/doc/reference_manual/data_types.html#id69044) with two elements: an [atom](erlang.org/doc/reference_manual/data_types.html#id64134) (similar to a ruby symbol) and a variable containing ancillary data. On success it returns the atom `ok` and the data from the file. On failure it returns `error` and a string with an explanation of the problem. With this pattern there is no ambiguity regarding success or failure. If the file is empty the return value cannot be misinterpreted as an error. And when an error occurs the return value provides useful information.

In Ruby we tend to return `nil` when an error occurs or else we raise an exception. Both of these idioms are problematic. Returning `nil` is ambiguous because `nil` may also be a valid value. It also lacks information pertaining to the nature of the error. Raising an exception is both expensive and usurps the normal flow of control. All of these problems can be solved with the use of a `Maybe`.

A `Maybe` is unambiguous with regard to whether or not it contains a value. When `Just` it contains a value, when `Nothing` it does not. When `Just` the value it contains may be `nil`, which is perfectly valid. When `Nothing` the reason for the lack of a value is contained as well. The previous Erlang example can be duplicated in Ruby in a principled way by having functions return `Maybe` objects:

“`ruby result = MyFileUtils.consult(“data.dat”) # returns a Maybe if result.just?

do_something_useful(result.value)      # or result.just

else

logger.error(result.reason)            # or result.nothing

end “`

@example Returning a Maybe from a Function

module MyFileUtils
  def self.consult(path)
    file = File.open(path, 'r')
    Concurrent::Maybe.just(file.read)
  rescue => ex
    return Concurrent::Maybe.nothing(ex)
  ensure
    file.close if file
  end
end

maybe = MyFileUtils.consult('bogus.file')
maybe.just?    #=> false
maybe.nothing? #=> true
maybe.reason   #=> #<Errno::ENOENT: No such file or directory @ rb_sysopen - bogus.file>

maybe = MyFileUtils.consult('README.md')
maybe.just?    #=> true
maybe.nothing? #=> false
maybe.value    #=> "# Concurrent Ruby\n[![Gem Version..."

@example Using Maybe with a Block

result = Concurrent::Maybe.from do
  Client.find(10) # Client is an ActiveRecord model
end

# -- if the record was found
result.just? #=> true
result.value #=> #<Client id: 10, first_name: "Ryan">

# -- if the record was not found
result.just?  #=> false
result.reason #=> ActiveRecord::RecordNotFound

@example Using Maybe with the Null Object Pattern

# In a Rails controller...
result = ClientService.new(10).find    # returns a Maybe
render json: result.or(NullClient.new)

@see hackage.haskell.org/package/base-4.2.0.1/docs/Data-Maybe.html Haskell Data.Maybe @see github.com/purescript/purescript-maybe/blob/master/docs/Data.Maybe.md PureScript Data.Maybe

Constants

NONE

Indicates that the given attribute has not been set. When `Just` the {#nothing} getter will return `NONE`. When `Nothing` the {#just} getter will return `NONE`.

Attributes

just[R]

The value of a `Maybe` when `Just`. Will be `NONE` when `Nothing`.

nothing[R]

The reason for the `Maybe` when `Nothing`. Will be `NONE` when `Just`.

reason[R]

The reason for the `Maybe` when `Nothing`. Will be `NONE` when `Just`.

value[R]

The value of a `Maybe` when `Just`. Will be `NONE` when `Nothing`.

Public Class Methods

from(*args) { |*args| ... } click to toggle source

Create a new `Maybe` using the given block.

Runs the given block passing all function arguments to the block as block arguments. If the block runs to completion without raising an exception a new `Just` is created with the value set to the return value of the block. If the block raises an exception a new `Nothing` is created with the reason being set to the raised exception.

@param [Array<Object>] args Zero or more arguments to pass to the block. @yield The block from which to create a new `Maybe`. @yieldparam [Array<Object>] args Zero or more block arguments passed as

arguments to the function.

@return [Maybe] The newly created object.

@raise [ArgumentError] when no block given.

# File lib/concurrent-ruby/concurrent/maybe.rb, line 137
def self.from(*args)
  raise ArgumentError.new('no block given') unless block_given?
  begin
    value = yield(*args)
    return new(value, NONE)
  rescue => ex
    return new(NONE, ex)
  end
end
just(value) click to toggle source

Create a new `Just` with the given value.

@param [Object] value The value to set for the new `Maybe` object.

@return [Maybe] The newly created object.

# File lib/concurrent-ruby/concurrent/maybe.rb, line 152
def self.just(value)
  return new(value, NONE)
end
new(just, nothing) click to toggle source

Create a new `Maybe` with the given attributes.

@param [Object] just The value when `Just` else `NONE`. @param [Exception, Object] nothing The exception when `Nothing` else `NONE`.

@return [Maybe] The new `Maybe`.

@!visibility private

# File lib/concurrent-ruby/concurrent/maybe.rb, line 224
def initialize(just, nothing)
  @just = just
  @nothing = nothing
end
nothing(error = '') click to toggle source

Create a new `Nothing` with the given (optional) reason.

@param [Exception] error The reason to set for the new `Maybe` object.

When given a string a new `StandardError` will be created with the
argument as the message. When no argument is given a new
`StandardError` with an empty message will be created.

@return [Maybe] The newly created object.

# File lib/concurrent-ruby/concurrent/maybe.rb, line 164
def self.nothing(error = '')
  if error.is_a?(Exception)
    nothing = error
  else
    nothing = StandardError.new(error.to_s)
  end
  return new(NONE, nothing)
end

Public Instance Methods

<=>(other) click to toggle source

Comparison operator.

@return [Integer] 0 if self and other are both `Nothing`;

-1 if self is `Nothing` and other is `Just`;
1 if self is `Just` and other is nothing;
`self.just <=> other.just` if both self and other are `Just`.
# File lib/concurrent-ruby/concurrent/maybe.rb, line 199
def <=>(other)
  if nothing?
    other.nothing? ? 0 : -1
  else
    other.nothing? ? 1 : just <=> other.just
  end
end
fulfilled?()
Alias for: just?
just?() click to toggle source

Is this `Maybe` a `Just` (successfully fulfilled with a value)?

@return [Boolean] True if `Just` or false if `Nothing`.

# File lib/concurrent-ruby/concurrent/maybe.rb, line 176
def just?
  ! nothing?
end
Also aliased as: fulfilled?
nothing?() click to toggle source

Is this `Maybe` a `nothing` (rejected with an exception upon fulfillment)?

@return [Boolean] True if `Nothing` or false if `Just`.

# File lib/concurrent-ruby/concurrent/maybe.rb, line 184
def nothing?
  @nothing != NONE
end
Also aliased as: rejected?
or(other) click to toggle source

Return either the value of self or the given default value.

@return [Object] The value of self when `Just`; else the given default.

# File lib/concurrent-ruby/concurrent/maybe.rb, line 210
def or(other)
  just? ? just : other
end
rejected?()
Alias for: nothing?