Many applications leverage monad transformer stacks to control IO effects, handle error conditions, pass configurations, and more. As the requirements of these applications grow, the complexity of these transformer stacks can grow proportionally. If a specific monad transformer stack is used in type signatures, then small changes to the stack can result in large changes to the code. This talk will demonstrate tools from the mtl and the lens library that allow each function's type to describe the required parts of a transformer stack, without depending on the concrete stack itself.

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