Readme for extensible-effects-1.9.2.2
extensible-effects is based on the work Extensible Effects: An Alternative to Monad Transformers. Please read the paper for details.
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. This issue
manifests itself in a few ways:
-
Common functions can't be grouped using typeclasses, e.g. the
ask
andgetState
functions can't be grouped with someclass Get t a where ask :: Member (t a) r => Eff r a
ask
is inherently ambiguous, since the type signature only provides a constraint ont
, and nothing more. To specify fully, a parameter involving the typet
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.
-
- Requires a
Typeable
instance on the return type. This is no longer a limitation on GHC versions 7.8 and above.- fixed by #38.
For GHC versions prior to 7.8
- Neither
Eff
nor(:>)
has aTypeable
instance, and can thus often not be used as a return type (e.g.State
type) for otherEff
s.- fixed by #38.