Abstract

Type safety of imperative programs is an area fraught with difficulty and requiring great care. The SML solution to the problem, originally involving imperative type variables, has been recently simplified to the syntactic-value restriction. In Haskell, the problem is addressed in a rather different way using explicit monadic state. We present an operational semantics for state in Haskell and the first full proof of type safety. We demonstrate that the semantic notion of value provided by the explicit monadic types is able to avoid any problems with generalization. 1 Introduction When Launchbury and Peyton Jones introduced encapsulated monadic state [11, 12], it came equipped with a denotational semantics and a model-theoretic proof that different state threads did not interact with each other. The encapsulation operator runST had a type which statically guaranteed freedom of interaction, and the guarantee relied on a parametricity proof. What the paper failed to provide was any form...

Citations