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Free them all

Armando Santos bolt12

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With scoped effects, handlers must be a part of the program

It is seductive to imagine that effect handlers in an algebraic effect system are not part of the program itself but metalanguage-level folds over the program tree. And in traditional free-like formulations, this is in fact the case. The Eff monad represents the program tree, which has only two cases:

data Eff effs a where
  Pure :: a -> Eff effs a
  Op :: Op effs a -> (a -> Eff effs b) -> Eff effs b

data Op effs a where