Effable

A data structure for emission plans

-- λ> import Effable
-- λ> :info Effable

type Effable :: (* -> *) -> * -> *
newtype Effable m b

instance Semigroup   (Effable m b)
instance Monoid      (Effable m b)

instance Functor     (Effable m)
instance Applicative (Effable m)
instance Monad       (Effable m)

instance Alternative (Effable m)
instance MonadPlus   (Effable m)

instance IsString b => IsString (Effable m b)

Links:

• Hackage page
• API documentation
• github.com/carlwr/effable

An Effable m b is

• a pure plan for the later emission of bs

• a representation of an ordered sequence of bs, each annotated with an emission wrapper m () -> m ()

• emitted to its eventual result m () with run (interpretation/elimination)

• fairly opaque

  • its constructor is not exported

  • observing it is only supported through run/runWith

• niche: see Caveats section

Why?

Compared to just working in the monadic m b context, Effable brings one particular distinguishing feature:

An Effable can undergo decoration with emission wrappers (wrap, wrapInside) after which it is still a pure Effable.

This means that even after having been modified with wrappers, or any other supported transformation...

• ...it is still a Functor, Applicative and Monad in b

• ...it can be transformed further and combined into more complex structures with <>

Emission wrappers can be applied to granular constituents of an Effable as the user code is building it. The Effable machinery will track the wrappers behind-the-scenes through all supported transformations so that the eventual emission respects them.

Caveats and usage scope

• effectful predicates and emission wrappers typically run more than once when a plan is emitted

  • these actions should yield the same value across evaluations, otherwise the inclusion of branches will be inconsistent

  • therefore, Effable is only suitable when these actions are read-like (return the same value over repeated evaluations and are free of externally observable side-effects)

• the nesting of combinators and use of <*> and >>= will grow the internal Effable representation, and the number of times actions are run at emission, combinatorially

Intuition

Effables are kept pure through all supported transformations by representing all possible outcomes of actions that affect structure. Running the actions is deferred to emission time. This comes with the cost of the internal representation carrying a complexity proportional to all possible outcomes.

Metaphorically, Effable is the many-worlds interpretation of actions meaning actions can be represented without them interacting with the actual world (= purity) and its representation is not collapsed to an outcome until it is observed (= run).

"Effable"?

Effable as in sayable or utterable.

Or, Eff-able as in able to be effected or effectuated - something with the potential of becoming effects.