can-i-haz
Generic
implementation of the Has-pattern (mostly useful with MonadReader
and MonadState
)
and its dual CoHas
(mostly useful with MonadError
).
Motivation
Assume there are two types representing the MonadReader
environments
for different parts of an app:
data DbConfig = DbConfig { .. }
data WebConfig = WebConfig { .. }
as well as a single type containing both of those:
data AppEnv = AppEnv
{ dbConfig :: DbConfig
, webConfig :: WebConfig
}
What should be the MonadReader
constraint of the DB module and web module respectively?
- It could be
MonadReader AppEnv m
for both, introducing unnecessary coupling.
- Or it could be
MonadReader DbConfig m
for the DB module
and MonadReader WebConfig m
for the web module respectively,
but combining them becomes a pain.
Or, it could be MonadReader r m, Has DbConfig r
for the DB module,
where Has
class allows projecting DbConfig
out of some r
,
and similarly for the web module!
This approach keeps both modules decoupled, while allowing using them in the same monad stack.
The only downside is that now one has to define the Has
class
and write tediuos instances for the AppEnv
type
(and potentially other types in case of tests).
The solution
This library saves you from this unnecessary boilerplate!
The only thing you have to do is to append the deriving
-clause:
data AppEnv = AppEnv
{ dbConfig :: DbConfig
, webConfig :: WebConfig
} deriving (Generic, Has DbConfig, Has WebConfig)
and use ask extract
instead of ask
(but this is something you’d have to do anyway).
Reversing the arrows: CoHas
There is a dual (but arguably less frequent) problem of combining different parts of an application
living in different MonadError
environments.
The duality is due to us now wanting to inject values of a type into a “wider” sum type
(as opposed to projecting values out of some product type).
The CoHas
class serves exactly this purpose.
Documentation
Perhaps the best source is the Haddock docs.
Acknowledgements
Thanks lyxia @ #haskell for the type families-based derivation of the GHas
instance.