Monadic Encapsulation of Effects: a Revised Approach

Launchbury and Peyton-Jones came up with an ingenious idea for embedding regions of imperative programming in a pure functional language like Haskell. The key idea was based on a simple modification of Hindley-Milner’s type system. Our first contribution is to propose a more natural encapsulation construct exploiting higher-order kinds, which achieves the same encapsulation effect, but avoids the bogus type parameter of the original proposal. The second contribution is a stronger type safety result, namely encapsulation of strict state in higherorder lambda-calculus. We formalise the intended implementation as a very simple big-step operational semantics on untyped terms, which captures interesting implementation details not captured by the reduction semantics proposed previously.