Monads and Effects

A tension in language design has been between simple semantics on the one hand, and rich possibilities for side-effects, exception handling and so on on the other. The introduction of monads has made a large step towards reconciling these alternatives. First proposed by Moggi as a way of structuring semantic descriptions, they were adopted by Wadler to structure Haskell programs. Monads have been used to solve long-standing problems such as adding pointers and assignment, inter-language working, and exception handling to Haskell, without compromising its purely functional semantics. The course introduces monads, effects, and exemplifies their applications in programming (Haskell) and in compilation (MLj). The course presents typed metalanguages for monads and related categorical notions, and then describes how they can be further refined by introducing effects.