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 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:

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 with two elements: an atom (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:

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