Why use an effect system?

[atravers]

@jeukshi

Java is great at having good ideas and making them look bad.

No, checked exceptions are a bad idea:

• As mentioned earlier, from The C++ Programming Language, 4th edition (2011):

  @mixphix

  Exception specifications provide runtime enforcement of which exceptions a function is allowed to throw. They were added at the energetic initiative of people from Sun Microsystems. Exception specifications turned out to be worse than useless for improving code readability, reliability, and performance.

• There’s also these observations from 2003:

  • Let’s start with versioning, because the issues are pretty easy to see there. Let’s say I create a method foo that declares it throws exceptions A, B, and C. In version two of foo, I want to add a bunch of features, and now foo might throw exception D. It is a breaking change for me to add D to the throws clause of that method, because existing caller of that method will almost certainly not handle that exception.

    Adding a new exception to a throws clause in a new version breaks client code. […]

  • The scalability issue is somewhat related to the versionability issue. In the small, checked exceptions are very enticing. With a little example, you can show that you’ve actually checked that you caught the FileNotFoundException, and isn’t that great? Well, that’s fine when you’re just calling one API. The trouble begins when you start building big systems where you’re talking to four or five different subsystems. Each subsystem throws four to ten exceptions. Now, each time you walk up the ladder of aggregation, you have this exponential hierarchy below you of exceptions you have to deal with. You end up having to declare 40 exceptions that you might throw. And once you aggregate that with another subsystem you’ve got 80 exceptions in your throws clause. It just balloons out of control.

    In the large, checked exceptions become such an irritation that people completely circumvent the feature. […]

So how many more programming languages will have to avoid them before people are convinced that checked exceptions are a bad idea?

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@jackdk

class Monad m => MonadWhatever m where
  liftWhatever :: Free (Coyoneda WhateverAPICall) a -> m a 

New Haskellers are having enough problems with the monadic interface:

Still, today, over 25 years after the introduction of the concept of monads to the world of functional programming, beginning functional programmers struggle to grasp the concept of monads. This struggle is exemplified by the numerous blog posts about the effort of trying to learn about monads. From our own experience we notice that even at university level, bachelor level students often struggle to comprehend monads and consistently score poorly on monad-related exam questions.

Visual Support for Learning Monads (2017)

…do they really need more problems by “giving” them extra artefacts from one of the most abstract branches of mathematics?