Let’s start with the same number dividing example, which returns 0 when the error happens. Maybe instead we can indicate that the result was not successful without any explicit numerical value?

And we can specify types of wrapped values in a function return annotation, for example Result[float, ZeroDivisionError] returns either Success[float] or Failure[ZeroDivisionError].

Now you can easily spot them! The rule is: if you see a Result it means that this function can throw an exception. And you even know its type in advance.

we also wrap them in Failure to solve the second problem: spotting potential exceptions is hard

exceptions are not exceptional, they represent expectable problems

You can use container values, that wraps actual success or error value into a thin wrapper with utility methods to work with this value. That’s exactly why we have created @dry-python/returns project. So you can make your functions return something meaningful, typed, and safe.

Write special-case classes. For example, you will have User base class with multiple error-subclasses like UserNotFound(User) and MissingUser(User). It might be used for some specific situations, like AnonymousUser in django, but it is not possible to wrap all your possible errors in special-case classes. It will require too much work from a developer. And over-complicate your domain model.

Exceptions are not exceptional

Exceptions are just like notorious goto statements that torn the fabric of our programs.

Not all cases can be covered and easily restored. And sometimes when we will reuse this function for different use-cases we will find out that it requires different restore logic.

But why do we return 0? Why not 1? Why not None? And while None in most cases is as bad (or even worse) than the exceptions, turns out we should heavily rely on business logic and use-cases of this function.

And checked exceptions won’t be supported in the nearest future.

Almost everything in python can fail with different types of exceptions: division, function calls, int, str, generators, iterables in for loops, attribute access, key access, even raise something() itself may fail. I am not even covering IO operations here. And checked exceptions won’t be supported in the nearest future.

You still need to have a solid experience to spot these potential problems in a perfectly readable and typed code.

So, despite your code is type safe it is not safe to be used.

print will never be actually executed. Because 1 / 0 is an impossible operation and ZeroDivisionError will be raised.

So, the sad conclusion is: all problems must be resolved individually depending on a specific usage context. There’s no silver bullet to resolve all ZeroDivisionErrors once and for all. And again, I am not even covering complex IO flows with retry policies and expotential timeouts.

Our mission is to allow people to make money via educational efforts and to dedicate the rest of their time to creating great open source products.

Examples

despite initially appearing to be an appropriate and effective response to a problem, has more bad consequences than good ones

there are two key elements to an anti-pattern that distinguish it from a bad habit, bad practice, or bad idea

But so far everything brought up has just been about the relative advantages of checked exceptions, and that issue is closed. We won't do it.

I'm not a fan of listing exceptions functions can throw, especially here in Python, where it's easier to ask forgiveness than permission.

certainly I wouldn't want it to start telling me that I'm not catching these!

I'm not a fan of checking exceptions either

In my past life with Java, I've had mixed feelings about exception checking. It's saved me from some mistakes more than it's been annoying. Maybe the checking of exceptions could be controlled by some notion of "unchecked exceptions"?

This PEP provides a standardized means to leverage existing tooling to package and distribute type information with minimal work and an ordering for type checkers to resolve modules and collect this information for type checking.

Brings functional programming to Python land

Provides a bunch of primitives to write declarative business logic

Enforces better architecture

Make your functions return something meaningful, typed, and safe!

include Dry::Monads::Do.for(:call)

Railway Oriented Programming is a way to gracefully handle errors in your application

This is how the same example would look like using raw monads:

Do notation provides an alternative to bind, which also flattens the code.

Better control over flow of our application: more ways to add branching

bind applies unwrapped Success value to the block, which should return a Result object. No-op on Failure

It allows us to reuse steps

The Result object that we pass around keeps accumulating data and becomes enormous, so we have to use **rest in our function signatures

The DSL has a weaker control over the program’s flow — we can’t have conditions unless we add a special step

However, you don’t need to have an extensive knowledge of monads to use ROP in your code.

Railway Oriented Programming comes from functional programming, so it is tightly related to the usual FP concepts like monads, composition, and many others.

I want to emphasize that Result is just an alternative name for the Either monad.

Back to what railway oriented programming really is. Below is a visual representation of what this looks like:

One thing I really like about this style is the elegance and readability it provides to your codebase.

This style of error handling uses monadic behavior — a substitute way of handling errors.

let's be honest, print-and-play is A LOT of work (printing, cutting, laminating, sleeving, etc) and it is not everyone's cup of tea.

The game seems perfectly balanced, what is so rare for an assymétric game

The author will offer presentation/game sessions on Tabletopia, in French and English. In addition, the game will be freely available for players on Tabletopia as soon as the written rules are available.

This is the perfect game when you are not wanting something with stakes or stress.

Difficult enough to prove a worthy challenge, with an over-complexity that might have benefitted from a little self-restraint.

It seems like such a beautiful little visual novel and while I wasn’t expecting a masterpiece of localisation based on its low price, I was expecting to be able to read it. But that just cannot be done. Developers from Japan, China, Taiwan, Indonesia, and every other emerging game development centre through Asia-Pacific, listen to me carefully: You can have the most beautiful aesthetics and a heartwarming concept for your game. If the localisation isn’t going to be good, though, do not bother with an English release, because it is going to get reviews like this one. Make “invest in proper translation” your big resolution for 2021. I do not want to play any other games like Lily in the Hollow - Resurrection ever again.

The cars handling can be best explained as "its like steering a drunk sailor on a boat."

This tedium would be unacceptable in an action game, but Windbound is a survival game. In survival games, death is supposed to mean something. Loss of progress represents the stakes; repetition is the barrier of entry.

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.

this implies direct work with nil-able values which may end up with errors.

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".

Writing code in this style is tedious and error-prone.

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.

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.

The gem was inspired by the Kleisli gem.

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 ifs and elses.

built using nullary type constructors

Also, in non-functional programming, a function without arguments can be meaningful and not necessarily constant (due to side effects).

The latter are important examples which usually also exist in "purely" functional programming languages.

Often, such functions have in fact some hidden input which might be global variables, including the whole state of the system (time, free memory, …).

Though rarer in computer science, one can use category theory directly, which defines a monad as a functor with two additional natural transformations. So to begin, a structure requires a higher-order function (or "functional") named map to qualify as a functor:

For historical reasons, this map is instead called fmap in Haskell.

can transform monadic values m a applying f to the unwrapped value a

In fact, the Product comonad is just the dual of the Writer monad and effectively the same as the Reader monad (both discussed below)

procedure to wrap values of any basic type within the monad (yielding a monadic value)

A combinator, typically called bind (as in binding a variable) and represented with an infix operator >>=, that unwraps a monadic variable, then inserts it into a monadic function/expression, resulting in a new monadic value:(mx >>= f) : (M T, T → M U) → M U

A type constructor M that builds up a monadic type M T

A type converter, often called unit or return, that embeds an object x in the monad:.mw-parser-output .block-indent{padding-left:3em;padding-right:0;overflow:hidden}unit(x) : T → M T

allows monads to simplify a wide range of problems

another to compose functions that output monadic values (called monadic functions)

Monads achieve this by providing their own data type (a particular type for each type of monad), which represents a specific form of computation

In functional programming, a monad is an abstraction that allows structuring programs generically

Supporting languages may use monads to abstract away boilerplate code needed by the program logic.

Whatever language or default programming paradigm a developer uses, following the monad pattern brings many of the benefits of purely functional programming.

By reifying a specific kind of computation, a monad not only encapsulates the tedious details of that computational pattern, but it does so in a declarative way, improving the code's clarity.

category of functors (from values to computations)

To emphasize how Just acts on the underlying value by wrapping it, it can be redefined as a function too, called eta for now

With >>= available, add can now be redefined as something much more compact:

these two functions >>= and eta were designed to simplify add, but they clearly do not depend on the specifics of add in any way, just the Maybe type

While other monads will embody different logical processes, and some may have extra properties, all of them will have three similar components (directly or indirectly) that follow the basic outline of this example.

Maybe T can be understood as a "wrapping" type, wrapping the type T into a new type with built-in exception handling

This is done to avoid confusion by differentiating between cases where a variable carries a defined value and those where it does not.

Undefined values or operations are one particular problem that robust software should prepare for and handle gracefully.

Since monads make semantics explicit for a kind of computation, they can also be used to implement convenient language features.

Research beginning in the late 1980s and early 1990s established that monads could bring seemingly disparate computer-science problems under a unified, functional model.

handling potential undefined values (with the Maybe monad)

The exception can be avoided by using ? operator on the nullable value instead:

The @ ? annotation can be used to denote a nullable value.

It reminds us that rather than asking an object for data and acting on that data, we should instead tell an object what to do.

It’s definitely better than littering our code with endless if-statements.

As you can see, we end up with a lot of boilerplate if-statements. The code is more verbose. And it’s difficult to follow the main logic.

In JavaScript, we have a built-in language feature for dealing with exceptions. We wrap problematic code in a try…catch statement. This lets us write the ‘happy path’ in the try section, and then deal with any exceptions in the catch section. And this is not a bad thing. It allows us to focus on the task at hand, without having to think about every possible error that might occur.

And they are not the only way to handle errors.

In this article, we’ll take a look at using the ‘Either monad’ as an alternative to try...catch.

The .chain() method allows us to switch over to the left track if an error occurs. Note that the switches only go one way.

This stuff is intoxicating once you get into it.

Don’t worry if you get confused at first. Everyone does. I’ve listed some other references at the end that may help. But don’t give up.

And a word of warning. If you haven’t come across things like monads before, they might seem really… different. Working with tools like these takes a mind shift. And that can be hard work to start with.

polymorphic method inside shape class, its possible to discriminate them. without if’s.

Since an inverse is the contrapositive of the converse, inverse and converse are logically equivalent to each other.

In logic and mathematics, statements p {\displaystyle p} and q {\displaystyle q} are said to be logically equivalent if they are provable from each other under a set of axioms,[1] or have the same truth value in every model.

So every program starts in the IO monad. From here you can get any input you need, call into relatively "pure" code with the inputs, and then output the result in some way. The reverse does not work. You cannot call into IO code from pure code like you can call into a Maybe function from pure code.

The IO monad wraps computations in the following context: "This computation can read information from or write information to the terminal, file system, operating system, and/or network". If you want to get user input, print a message to the user, read information from a file, or make a network call, you'll need to do so within the IO Monad. These are "side effects". We cannot perform them from "pure" Haskell code.

A Monad wraps a value or a computation with a particular context. A monad must define both a means of wrapping normal values in the context, and a way of combining computations within the context.

So if you don't yet understand functors or applicative functors, check out part 1 and part 2 of this series!

Each of the programming language generations aims to provide a higher level of abstraction of the internal computer hardware details, making the language more programmer-friendly, powerful, and versatile.

It turns out that, given a set of constraints defining a particular problem, deriving an efficient algorithm to solve it is a very difficult problem in itself. This crucial step cannot yet be automated and still requires the insight of a human programmer.

Purely functional programming may also be defined by forbidding state changes and mutable data.

purely functional programming usually designates a programming paradigm—a style of building the structure and elements of computer programs—that treats all computation as the evaluation of mathematical functions.

Purely functional data structures are persistent. Persistency is required for functional programming; without it, the same computation could return different results.

Most C++ template idioms will carry over to D without alteration, but D adds some additional functionality

it is inconvenient to write specific implementations for each datatype contained, especially if the code for each datatype is virtually identical. For example, in C++, this duplication of code can be circumvented by defining a class template

Comparison to imperative languages

In mathematics, a structure is a set endowed with some additional features on the set (e.g. an operation, relation, metric, or topology)

This is a useful approach to error handling, but please don’t take it to extremes! See my post on “Against Railway-Oriented Programming”.

Modelling the flow of a program where chunks of code are executed in a certain order, with a successful “happy path” and an “error-out” path is called a Railway. It popped up in functional languages

Intuitively, you understand the flow just by looking at the BPMN diagram. And, heck, we haven’t even discussed BPMN or any terminology, yet!

If anything here did fail in “validate omniauth”, all other steps in the chain would be skipped as the flow would follow the path leading to the failure terminus.

Things could go wrong in two places here. First, the validation could fail if Github sends us data we don’t understand. Second, we might not know the user signing in, meaning the “find user” logic has to error-out

Whatever data from the outside is needed in the activity has to be passed explicitely into the activity’s call method.

In other words: the controllers usually contain only routing and rendering code and dispatch instantly to a particular operation/activity class.

At this stage, routing, controllers, etc is irrelevant. Just imagine a Rails controller action, a Sinatra router or a Hanami action as follows.

They help streamlining the control flow, and take away control code while providing you with an incredibly cool developer experience.

Activities are a necessary abstraction on top of Ruby.

provide interfaces so you don’t have to think about them

You’re allowed to blame us for a terrible developer experience in Trailblazer 2.0. It’s been quite painful to find out which step caused an exception. However, don’t look back in anger! We’ve spent a lot of time on working out a beautiful way for both tracing and debugging Trailblazer activities in 2.1.

An activity is a high-level concept to structure code flow

Trailblazer is an architectural pattern that comes with Ruby libraries to implement that pattern.

Whether this is the life-cycle of a <user> entity or just a sign-up function, it has to be defined and coded somewhere.

Trailblazer strikes when it comes to organizing business code.

on one of the most popular websites of F #

This is the most popular article “railway oriented programming” on one of the most popular websites of F #.

No contribution is too small Even if you find a single-character typo, we're happy to take the change! Although the codebase can feel daunting for beginners, we and other contributors are happy to help you along.

An operation has two invocation styles. This is the only difference to an Activity.

Operations are often confused as god objects that do “everything”. However, operations are nothing but orchestrators.

Here comes a sample screenshot.

The activity gem is an extraction from Trailblazer 2.0, where we only had operations. Operations expose a linear flow which goes into one direction, only. While this was a massive improvement over messily nested code, we soon decided it’s cool being able to model non-linear flows. This is why activities are the major concept since Trailblazer 2.1.

Feel free to pick and choose what you need for your applications.

I started Trailblazer GmbH 4 years ago with my relocation from Australia back to Europe. One of our consulting clients is the central police department of a German state that has kept me busy for more than three years now.

In addition to the organically formed core team

Endpoint is the missing link between your routing (Rails, Hanami, …) and the “operation” to be called. It provides standard behavior for all cases 404, 401, 403, etc and lets you hook in your own logic like Devise or Tyrant authentication, again, using TRB activity mechanics.

TRAILBLAZER-WORKFLOW is another dream ‘o mine come true. It allows creating long-term processes (or state machines) based on BPMN diagrams that can be modeled using our editor.

Not only does it help to structure myself,

There are two definitions of ‘Enterprise’ 1 - Enterprise as a business. In fact, in French, ‘enterprise’ literally means ‘business’ 2- Enterprise as a large business. This is the most common use of the term in business, differentiating between small, medium, and large businesses. In this context, there is no official rule, however it is generally accepted for enterprise to mean companies with over 1,000 employees and/or $1B in revenue

This would be the taxonomy:

Our company reinvests our funds straight back into developing all Trailblazer gems, better documentation and support, plus building up a nice team of fantastic people to help you with Trailblazer.

would love to hear from you if you’re excited about our gems, high-level abstractions and improving our documentation

We’re actively looking to grow this team and would love to hear from you

And: we will never stop innovating.

which entails computer programming (process of writing and maintaining the source code), but also encompasses a planned and structured process from the conception of the desired software to its final manifestation

Software architecture is about making fundamental structural choices that are costly to change once implemented.

Software architecture refers to the fundamental structures of a software system

The adapter is where authentication, policy checks, and eventually your domain logic happen. All termini of the protocol’s activity are standardized end events - that’s how protocol and adapter communicate.

An endpoint links your routing with your business code. The idea is that your controllers are pure HTTP routers, calling the respective endpoint for each action. From there, the endpoint takes over, handles authentication, policies, executing the domain code, interpreting the result, and providing hooks to render a response.

The endpoint gem is the missing link between your routing framework and your business code. It acts like a mix of before_filters and “responder” that handles authentication and authorization, invoking your actual logic, and rendering a response.

Instead of dealing with a mix of before_filters, Rack-middlewares, controller code and callbacks, an endpoint is just another activity and allows to be customized with the well-established Trailblazer mechanics.

providing best practices for both HTML-rendering controllers and APIs.

While routing and redirecting/rendering still happens in Rails, all remaining steps are handled in the endpoint.

their main intent is to reduce fuzzy controller code

Tyrant "Freedom choked in dread we live, since tyrant was enthroned."

The trailblazer-workflow gem is currently a paid feature.

They do not maintain a to-do list (mentally or physically).

I admit, I was a bit cynical -- I mean, how much can what I scribble on the back of a used envelope really change my life?