u/gelisamin r/haskell • Recommends
On “Is there any reason why mtl-style is focused on having a Monad constraint in its classes?”
Probably for historical reasons.
Applicatives compose nicely, as witnessed by the Applicative instance for `Compose`. Monads don't compose so nicely, which is why we need Monad transformers. And the API for Monad transformers requires a lot of calls to `lift`, which some people find annoying. So, the Monad Transformer Library (mtl) was created, using type classes to determine how many calls to `lift` are needed and to add them automatically using type inference and instance resolution. Since the intent is to use an mtl-constrained type instead of a concrete Monad transformer stack containing the same pieces, it made sense for the mtl type to provide the same API as the concrete Monad transf…