extensible-effects-2.1.0.0: README.md

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) and
the followup [freer paper](http://okmij.org/ftp/Haskell/extensible/more.pdf) for
details. Additional explanation behind the approach can be found on [Oleg's website](http://okmij.org/ftp/Haskell/extensible/).

[![Build Status](https://travis-ci.org/suhailshergill/extensible-effects.svg?branch=master)](https://travis-ci.org/suhailshergill/extensible-effects)
[![Join the chat at https://gitter.im/suhailshergill/extensible-effects](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/suhailshergill/extensible-effects?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
[![Stories in Ready](https://badge.waffle.io/suhailshergill/extensible-effects.png?label=ready&title=Ready)](http://waffle.io/suhailshergill/extensible-effects)
[![Stories in progress](https://badge.waffle.io/suhailshergill/extensible-effects.png?label=in%20progress&title=In%20progress)](http://waffle.io/suhailshergill/extensible-effects)

## Advantages

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

## Limitations

### 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 :)

## Disadvantages

### Ambiguity-Flexibility tradeoff
  * 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).