extensible-effects is based on the work
[Extensible Effects: An Alternative to Monad Transformers](http://okmij.org/ftp/Haskell/extensible/).
Please read the [paper](http://okmij.org/ftp/Haskell/extensible/exteff.pdf) for details.

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## Advantages


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

## Disadvantages

### For GHC version 7.8 and upwards
  * The extensibility comes at the cost of some ambiguity. Note, however, that
    the extensibility can be traded back, but that detracts from some of the
    advantages. For details see section 4.1 in the
    [paper](http://okmij.org/ftp/Haskell/extensible/exteff.pdf). This issue
    manifests itself in a few ways:
    * 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](https://github.com/suhailshergill/extensible-effects/issues/31).
  * Requires a `Typeable` instance on the return type. This is no longer a
    limitation on GHC versions 7.8 and above.
    * fixed by
      [#38](https://github.com/suhailshergill/extensible-effects/issues/38).

### For GHC versions prior to 7.8
  * Neither `Eff` nor `(:>)` has a `Typeable` instance, and can thus often not
    be used as a return type (e.g. `State` type) for other `Eff`s. 
    * fixed by
      [#38](https://github.com/suhailshergill/extensible-effects/issues/38).