dry-monads is a set of common monads for Ruby. Monads provide an elegant way of handling errors, exceptions and chaining functions so that the code is much more understandable and has all the error handling, without all the
elses. The gem was inspired by the Kleisli gem.
What is a monad, anyway? Simply, a monoid in the category of endofunctors. The term comes from category theory and some believe monads are tough to understand or explain. It’s hard to say why people think so because you certainly don’t need to know category theory for using them, just like you don’t need it for, say, using functions.
Moreover, the best way to develop intuition about monads is looking at examples rather than learning theories.
How to use it?
Let’s say you have code like this
user = User.find_by(id: params[:id]) if user address = user.address end if address city = address.city end if city state = city.state end if state state_name = state.name end user_state = state_name || "No state"
Writing code in this style is tedious and error-prone. There were created several “cutting-corners” means to work around this issue. The first is ActiveSupport’s
.try which is a plain global monkey patch on
Object. Another solution is using the Safe Navigation Operator
&. introduced in Ruby 2.3 which is a bit better because this is a language feature rather than an opinionated runtime environment pollution. However, some people think these solutions are hacks and the problem reveals a missing abstraction. What kind of abstraction?
When all objects from the chain of objects are there we could have this instead:
state_name = User.find_by(id: params[:id]).address.city.state.name user_state = state_name || "No state"
By using the
Maybe monad you can preserve the structure of this code at a cost of introducing a notion of
state_name = Maybe(User.find_by(id: params[:id])).maybe(&:address).maybe(&:city).maybe(&:state).maybe(&:name) user_state = state_name.value_or("No state")
Maybe(...) wraps the first value and returns a monadic value which either can be a
Some(address) but leaves
None intact. To get the final value you can use
value_or which is a safe way to unwrap a
nil-able value. In other words, once you’ve used
Maybe you cannot hit
nil with a missing method. This is remarkable because even
&. doesn’t save you from omitting
|| "No state" at the end of the computation. Basically, that’s what they call “Type Safety”.
A more expanded example is based on composing different monadic values. Suppose, we have a user and address, both can be
nil, and we want to associate the address with the user:
user = User.find_by(id: params[:user_id]) address = Address.find_by(id: params[:address_id]) if user && address user.update(address_id: address.id) end
Again, this implies direct work with
nil-able values which may end up with errors. A monad-way would be using another method,
maybe_user = Maybe(User.find_by(id: params[:user_id])) maybe_user.bind do |user| maybe_address = Maybe(Address.find_by(id: params[:address_id])) maybe_address.bind do |address| user.update(address_id: address.id) end end
One can say this code is opaque compared to the previous example but keep in mind that in real code it often happens to call methods returning
Maybe values. In this case, it might look like this:
find_user(params[:user_id]).bind do |user| find_address(params[:address_id]).bind do |address| Some(user.update(address_id: address.id)) end end
Finally, since 1.0, dry-monads has support for
do notation which simplifies this code even more, making it almost regular yet
user = yield find_user(params[:user_id]) address = yield find_address(params[:address_id]) Some(user.update(address_id: address.id))
Another widely spread monad is
Result (also known as
Either) that serves a similar purpose. A notable downside of
Maybe is plain
None which carries no information about where this value was produced.
Result solves exactly this problem by having two constructors for
def find_user(user_id) user = User.find_by(id: user_id) if user Success(user) else Failure(:user_not_found) end end def find_address(address_id) address = Address.find_by(id: address_id) if address Success(address) else Failure(:address_not_found) end end
You can compose
find_user(params[:user_id]).bind do |user| find_address(params[:address_id]).bind |address| Success(user.update(address_id: address.id)) end end
The inner block can be simplified with
find_user(params[:user_id]).bind do |user| find_address(params[:address_id]).fmap |address| user.update(address_id: address.id) end end
Or, again, the same code with
user = yield find_user(params[:user_id]) address = yield find_address(params[:address_id]) Success(user.update(address_id: address.id))
The result of this piece of code can be one of
Failure(:address_not_found). This style of programming called “Railway Oriented Programming” and can check out dry-transaction and watch a nice video on the subject. Also, see dry-matcher for an example of how to use monads for controlling the flow of code with a result.
A word of warning
do came around here was a warning about over-using monads, turned out with
do notation code does not differ much from regular Ruby code. Just don’t wrap everything with
Maybe, come up with conventions.
If you’re interested in functional programming in general, consider learning other languages such as Haskell, Scala, OCaml, this will make you a better programmer no matter what programming language you use on a daily basis. And if not earlier then maybe after that dry-monads will become another instrument in your Ruby toolbox :)