AI generated content

[flip111]

A bit of an open topic...

I can imagine for some use cases it can be nice to supplement the website with content generated from an AI. I don't mean to integrate a chatbot or other API to AI. But to generate it once based on specific package (version) and then serve it statically.

Example prompts would be:

• Show a short example that demonstrate the working of this package
• Can you explain how this package compares to other haskell packages of the same type?

I'm not sure how good the quality would be, but worth a try i suppose. Though i would always put a clear mark with that it is AI generated content.

[tchoutri]

It's very hard to do right and ultimately not worth the effort. See pkgxdev/pantry#5358

[flip111]

We don't know which methods they used to generate the content. Also they didn't seem to have a disclaimer that it was AI generated. I'm not sure if it will be worth the effort. Here an example by ChatGPT 3.5

---

This is a part of the readme of the fused-effects package in haskell

Overview

fused-effects is an effect system for Haskell that values expressivity, efficiency, and rigor. It provides an encoding of algebraic, higher-order effects, includes a library of the most common effects, and generates efficient code by fusing effect handlers through computations. It is suitable for use in hobbyist, research, and industrial contexts.

Readers already familiar with effect systems may wish to start with the usage instead. For those interested, this talk at Strange Loop outlines the history of and motivation behind effect systems and fused-effects itself.
Algebraic effects

In fused-effects and other systems with algebraic (or, sometimes, extensible) effects, effectful programs are split into two parts: the specification (or syntax) of the actions to be performed, and the interpretation (or semantics) given to them.

In fused-effects, effect types provide syntax and carrier types provide semantics. Effect types are datatypes with one constructor for each action, invoked using the send builtin. Carriers are monads, with an Algebra instance specifying how an effect’s constructors should be interpreted. Carriers can handle more than one effect, and multiple carriers can be defined for the same effect, corresponding to different interpreters for the effect’s syntax.
Higher-order effects

Unlike some other effect systems, fused-effects offers higher-order (or scoped) effects in addition to first-order algebraic effects. In a strictly first-order algebraic effect system, operations like local or catchError, which specify some action limited to a given scope, must be implemented as interpreters, hard-coding their meaning in precisely the manner algebraic effects were designed to avoid. By specifying effects as higher-order functors, this limitation is removed, meaning that these operations admit a variety of interpretations. This means, for example, that you can introspect and redefine both the local and ask operations provided by the Reader effect, rather than solely ask (as is the case with certain formulations of algebraic effects).

As Nicolas Wu et al. showed in Effect Handlers in Scope, this has implications for the expressiveness of effect systems. It also has the benefit of making effect handling more consistent, since scoped operations are just syntax which can be interpreted like any other, and are thus simpler to reason about.
Fusion

In order to maximize efficiency, fused-effects applies fusion laws, avoiding the construction of intermediate representations of effectful computations between effect handlers. In fact, this is applied as far as the initial construction as well: there is no representation of the computation as a free monad parameterized by some syntax type. As such, fused-effects avoids the overhead associated with constructing and evaluating any underlying free or freer monad.

Instead, computations are performed in a carrier type for the syntax, typically a monad wrapping further monads, via an instance of the Carrier class. This carrier is specific to the effect handler selected, but since it isn’t described until the handler is applied, the separation between specification and interpretation is maintained. Computations are written against an abstract effectful signature, and only specialized to some concrete carrier when their effects are interpreted.

Since the interpretation of effects is written as a typeclass instance which ghc is eager to inline, performance is excellent: approximately on par with mtl.

Finally, since the fusion of carrier algebras occurs as a result of the selection of the carriers, it doesn’t depend on complex RULES pragmas, making it easy to reason about and tune.
Usage
Package organization

The fused-effects package is organized into two module hierarchies:

    those under Control.Effect, which provide effects and functions that invoke these effects’ capabilities.
    those under Control.Carrier, which provide carrier types capable of executing the effects described by a given effect type.

An additional module, Control.Algebra, provides the Algebra interface that carrier types implement to provide an interpretation of a given effect. You shouldn’t need to import it unless you’re defining your own effects.
Invoking effects

Each module under the Control.Effect hierarchy provides a set of functions that invoke effects, each mapping to a constructor of the underlying effect type. These functions are similar to, but more powerful than, those provided by mtl. In this example, we invoke the get and put functions provided by Control.Effect.State, first extracting the state and then updating it with a new value:

action1 :: Has (State String) sig m => m ()
action1 = get >>= \ s -> put ("hello, " ++ s)

The Has constraint requires a given effect (here State) to be present in a signature (sig), and relates that signature to be present in a carrier type (m). We generally, but not always, program against an abstract carrier type, usually called m, as carrier types always implement the Monad typeclass.

To add effects to a given computation, add more Has constraints to the signature/carrier pair sig and m. For example, to add a Reader effect managing an Int, we would write:

action2 :: (Has (State String) sig m, Has (Reader Int) sig m) => m ()
action2 = do
  i <- ask
  put (replicate i '!')

Running effects

Effects are run with effect handlers, specified as functions (generally starting with run…) unpacking some specific monad with a Carrier instance. For example, we can run a State computation using runState, imported from the Control.Carrier.State.Strict carrier module:

example1 :: Algebra sig m => [a] -> m (Int, ())
example1 list = runState 0 $ do
  i <- get
  put (i + length list)

runState returns a tuple of both the computed value (the ()) and the final state (the Int), visible in the result of the returned computation. The get function is resolved with a visible type application, due to the fact that effects can contain more than one state type (in contrast with mtl’s MonadState, which limits the user to a single state type).

Since this function returns a value in some carrier m, effect handlers can be chained to run multiple effects. Here, we get the list to compute the length of from a Reader effect:

example2 :: Algebra sig m => m (Int, ())
example2 = runReader "hello" . runState 0 $ do
  list <- ask
  put (length (list :: String))

(Note that the type annotation on list is necessary to disambiguate the requested value, since otherwise all the typechecker knows is that it’s an arbitrary Foldable. For more information, see the comparison to mtl.)

When all effects have been handled, a computation’s final value can be extracted with run:

example3 :: (Int, ())
example3 = run . runReader "hello" . runState 0 $ do
  list <- ask
  put (length (list :: String))

run is itself actually an effect handler for the Lift Identity effect, whose only operation is to lift a result value into a computation.

Alternatively, arbitrary Monads can be embedded into effectful computations using the Lift effect. In this case, the underlying Monadic computation can be extracted using runM. Here, we use the MonadIO instance for the LiftC carrier to lift putStrLn into the middle of our computation:

example4 :: IO (Int, ())
example4 = runM . runReader "hello" . runState 0 $ do
  list <- ask
  liftIO (putStrLn list)
  put (length list)

(Note that we no longer need to give a type annotation for list, since putStrLn constrains the type for us.)
Required compiler extensions

When defining your own effects, you may need -XKindSignatures if GHC cannot correctly infer the type of your constructor; see the documentation on common errors for more information about this case.

When defining carriers, you’ll need -XTypeOperators to declare a Carrier instance over (:+:), -XFlexibleInstances to loosen the conditions on the instance, -XMultiParamTypeClasses since Carrier takes two parameters, and -XUndecidableInstances to satisfy the coverage condition for this instance.

The following invocation, taken from the teletype example, should suffice for most use or construction of effects and carriers:

{-# LANGUAGE FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, TypeOperators, UndecidableInstances #-}

Defining new effects

The process of defining new effects is outlined in docs/defining_effects.md, using the classic Teletype effect as an example.

And this is the documentation page for Control.Carrier.Reader

# A carrier for Reader effects.

## runReader :: r -> ReaderC r m a -> m a
Run a Reader effect with the passed environment value.

runReader a ask = pure a
runReader a (pure b) = pure b
runReader a (local f m) = runReader (f a) m

## newtype ReaderC r m a

### Constructors
* ReaderC (r -> m a)

### Instances
* MonadTrans (ReaderC r)
* MonadFail m => MonadFail (ReaderC r m)
* MonadFix m => MonadFix (ReaderC s m)
* MonadIO m => MonadIO (ReaderC r m)
* Alternative m => Alternative (ReaderC r m)
* Applicative m => Applicative (ReaderC r m)
* Functor m => Functor (ReaderC r m)
* Monad m => Monad (ReaderC r m)
* (Alternative m, Monad m) => MonadPlus (ReaderC r m)
* MonadUnliftIO m => MonadUnliftIO (ReaderC r m)
* Algebra sig m => Algebra (Reader r :+: sig) (ReaderC r m)

## Reader effect

module Control.Effect.Reader

Write a code example demonstrating the use of Control.Carrier.Reader. Write it in the style of a tutorial.

---

Certainly! Let's create a simple tutorial-style example demonstrating the use of Control.Carrier.Reader in Haskell.

Understanding Reader Effects

The Control.Carrier.Reader module provides a way to handle reader effects in Haskell. Reader effects are useful when you need to pass around some immutable configuration or environment throughout your computation. This tutorial will walk you through the basics of using reader effects with the ReaderC carrier.

Example Scenario

Imagine we are building a simple application where we need to calculate the area of a rectangle. However, the dimensions of the rectangle are stored in a configuration that we want to pass around using reader effects.

Step 1: Import Necessary Modules

{-# LANGUAGE FlexibleContexts #-}

import Control.Carrier.Reader
import Control.Monad.IO.Class (liftIO)

Step 2: Define the Configuration Type

First, let's define a simple data type to represent our configuration. In this case, our configuration will contain the length and width of the rectangle.

data RectConfig = RectConfig
    { length :: Double
    , width :: Double
    }

Step 3: Define a Computation

Next, let's define a computation that calculates the area of the rectangle using the dimensions from our configuration. We'll use the ask function provided by the Control.Effect.Reader module to retrieve the configuration.

calculateArea :: (Has (Reader RectConfig) sig m) => m Double
calculateArea = do
    config <- ask
    return $ length config * width config

Step 4: Run the Computation

Now, let's run our computation by providing a configuration using the runReader function. We'll pass in a sample configuration and print out the calculated area.

main :: IO ()
main = do
    let config = RectConfig { length = 5.0, width = 3.0 }
    area <- runReader config calculateArea
    liftIO $ putStrLn $ "The area of the rectangle is: " ++ show area

Conclusion

In this example, we've demonstrated how to use Control.Carrier.Reader to handle reader effects in Haskell. We defined a computation that relies on a configuration, and then we ran that computation by providing a configuration using the runReader function. Reader effects are a powerful tool for passing immutable configuration or environment throughout your computations, making your code more modular and easier to test.

Feel free to experiment further with reader effects in your own Haskell projects!