extensible-effects: An Alternative to Monad Transformers

extensible-effects is based on the work Extensible Effects: An Alternative to Monad Transformers. Please read the paper and the followup freer paper for details. Additional explanation behind the approach can be found on Oleg's website.

Advantages

• Effects can be added, removed, and interwoven without changes to code not dealing with those effects.

Limitations

Ambiguity-Flexibility tradeoff

The extensibility of Eff comes at the cost of some ambiguity. A useful pattern to mitigate the ambiguity is to specialize the call to the handler of effects using type application or type annotation. Examples of this pattern can be seen in Example/Test.hs.

Note, however, that the extensibility can also be traded back, but that detracts from some of the advantages. For details see section 4.1 in the paper.

Some examples where the cost of extensibility is apparent:

• Common functions can't be grouped using typeclasses, e.g. the ask and getState functions can't be grouped with some

  class Get t a where
    ask :: Member (t a) r => Eff r a
  

  ask is inherently ambiguous, since the type signature only provides a constraint on t, and nothing more. To specify fully, a parameter involving the type t would need to be added, which would defeat the point of having the grouping in the first place.

• Code requires greater number of type annotations. For details see #31.

Current implementation only supports GHC version 7.8 and above

This is not a fundamental limitation of the design or the approach, but there is an overhead with making the code compatible across a large number of GHC versions. If this is needed, patches are welcome :)