📝 the practical takeaway.

Author: robrix

semantic-core/src/Control/Monad/Module.hs

@@ -10,6 +10,8 @@ import Control.Effect.Carrier
 
 -- | Modules over monads allow lifting of a monad’s product (i.e. 'Control.Monad.join') into another structure composed with the monad. A right-module @f m@ over a monad @m@ therefore allows one to extend @m@’s '>>=' operation to values of @f m@ using the '>>=*' operator.
 --
+--   In practical terms, this means that we can describe syntax which cannot itself bind or be substituted for variables, but which can be substituted inside when containing a substitutable expression monad. For example, we might not want to allow variables in a declaration context, but might still want to be able to substitute for e.g. globally-bound variables inside declarations; a 'RightModule' instance expresses this relationship nicely.
+--
 --   Note that we are calling this a right-module following Maciej Piróg, Nicolas Wu, & Jeremy Gibbons in _Modules Over Monads and their Algebras_; confusingly, other sources refer to this as a left-module.
 --
 --   Laws: