can-i-haz: Generic implementation of the Has and CoHas patterns

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Dependencies base (>=4.7 && <5), mtl [details]
License BSD-3-Clause
Copyright 2019 Georg Rudoy
Author Georg Rudoy
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Uploaded by 0xd34df00d at 2019-12-15T23:59:13Z
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Last success reported on 2019-12-16 [all 1 reports]


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Generic implementation of the Has-pattern (mostly useful with MonadReader and MonadState) and its dual CoHas (mostly useful with MonadError).


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?

  1. It could be MonadReader AppEnv m for both, introducing unnecessary coupling.
  2. 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.


Perhaps the best source is the Haddock docs.


Thanks lyxia @ #haskell for the type families-based derivation of the GHas instance.