Date: 2021-06-08
Accepted
- Given we are structuring Hydra node as a reactive core we need a way to ensure a strict separation of pure and impure (or effectful) code
- Various patterns have been proposed in Haskell to model effects:
- Tagless final encoding also known as MTL-style although using typeclasses to implement is not necessary, whereby Effect(s) are expressed as typeclass(es) which are propagated as constraints
- Free monads, or any variant thereof like Eff, freer, extensible-effects, whereby effect(s) are expressed as ADTs which are interpreted in the context of an Effect stack
- Handle pattern also known as record-of-functions whereby effects are grouped together in a datatype with a single record constructor
- All options considered have both advocates and detractors and there is no widely accepted pattern
- We did a couple experiments to explore various options
- Effectful components of the Hydra node will be defined using the Handle pattern
- For example, the on-chain component is defined as:
newtype OnChain m = OnChain
{ postTx :: MonadThrow m => OnChainTx -> m ()
}
- Instantiation to concrete IO is pushed at the outermost layer, eg. in the
Mainor tests
- These tradeoffs also appear in other functional languages like F#