This paper summarizes my experience in implementing a compiler for a functional language. The language is ML1 [Milner 84] and the compiler was first implemented in 1980 as a personal project when I was a postgraduate student at the University of Edinburgh2. In this paper, “the” ML compiler refers to my VAX implementation.

At the time, I was familiar with programming language semantics but knew very little about compiler technology; interpreters had been my main programming concern. Major influences in the design of this compiler have been [Steele 77] [Steele 78] and the implementation folklore for statically and dynamically scoped dialects of Lisp [Allen 78]. As a result, the internal structure of the compiler is fairly unorthodox, if compared for example with [Aho 78].

Anyway, a compiler for a language like ML has to be different. ML is interactive, statically scoped, strongly typed, polymorphic, and has first class higher-order functions, type inference and dynamic allocation. These features preclude many well-known implementation styles, particularly the ones used for Lisp (because of static scoping), the Algol family (because of functional values) and C (because of nested scoping and strong typing). The interaction of these features is what gives ML its “character”, and makes compilation challenging.