Modular type class machinery for monad transformer stacks.
The layers package provides the type class machinery needed to make monads built out of stacks of monad transformers easy to use. It introduces the concept of monad layers, which are a generalisation of monad transformers. The type class machinery provided by and the design patterns suggested by layers allow for much more modularity than is possible with the existing type class machinery and design patterns. With layers it is possible to use arbitrary monad interfaces (monad interfaces are what we call the sort of type classes that you see in the mtl and similar packages) with arbtirary monad transformers (by monad transformers here, we are specifically to monad constructors, such as the ones defined in transformers), without ever having to explicitly define how to lift specific interfaces through specific transformers.
layers improves upon and/or replaces, in part or in whole, the following list of packages: transformers, mtl, mmtl, transformers-base, monad-control, lifted-base, monad-peel, MonadCatchIO-transformers, MonadCatchIO-mtl, exception-transformers, monad-abort-fd and probably more too. There have been many attempts to either improve upoin or work around the deficiencies of the existing type class machinery for monad transformer stacks, but we believe layers is the most complete of any of these so far.
A comprehensive overview of the motivation behind layers and an explanation of the design decisions taken is given in Documentation.Layers.Overview. It is highly recommended that you read this if you are considering using this package. The core type classes of the package are defined in Control.Monad.Layer (this can be thought of as equivalent to or a replacement of Control.Monad.Trans.Class from transformers). The rest of the modules in this package export monad interfaces, in the Control.Monad.Interface hierarchy, including replacements for all of the monad interfaces of the mtl package.