Haskell is a purely functional programming language: by default, Haskell functions do not cause side effects such as system I/O, nondeterminism, or exception handling. As such, Haskell programs are generally expressed in terms of monad transformers, which provide the facility to compose different side effects into a single interface powerful enough to express the programmer's needs. The monad transformer library, mtl, is mature and powerful, but complicates and in some cases constrains the construction and generalization of user-specified monads. A class of libraries known as 'effect systems' have emerged in an attempt to solve the problems associated with monad transformers. Effect systems provide a compositional approach to program construction, generally using a single monadic type specialzed with a programmer-provided list of capabilities. Effect systems are more powerful and flexible than monad transformers, but their adoption in industry has been minimal, due both to their incompatiblity with mtl and their historically poor performance. The new fused-effects library changes the status quo: it provides an extensible and flexible vocabulary for program construction, yielding a more expressive interface than mtl without sacrificing any performance characteristics. I'll describe the history of effect systems, outline the tradeoffs associated with programming with effects, and demonstrate the use of fused-effects in practice. Patrick Thomson GitHub, Inc. @importantshock Patrick is a senior engineer on GitHub's Semantic Code team, building systems to understand and analyze the corpus of code on GitHub. He enjoys peaceful countryside walks and loud rap shows.

While no level of type safety is sufficient to ensure all programs' correctness, the fact that the Semantic Code team spends the majority of its time working on features rather than debugging production crashes is truly remarkable—and this can largely be attributed to our choice of language. ​ but in Haskell: its brevity, power, and focus on correctness lets researchers focus on the nature of the problem rather than the onerous task of fitting advanced research into conventional languages. Writing in Haskell allows us to build on top of the work of others rather than getting stuck in a cycle of reading, porting, and bug-fixing. ​ a reputation for being difficult to learn. Some of that is well deserved, but half of it has more to do with how many of us first learned imperative programming and the switch to a functional paradigm takes some patience.