Category theorists invented monads in the 1960's to concisely express certain aspects of universal algebra. Functional programmers invented list comprehensions in the 1970's to concisely express certain programs involving lists. This paper shows how list comprehensions may be generalised to an arbitrary monad, and how the resulting programming feature can concisely express in a pure functional language some programs that manipulate state, handle exceptions, parse text, or invoke continuations. A new solution to the old problem of destructive array update is also presented. No knowledge of category theory is assumed.

In combinator parsing, the text of parsers resembles BNF notation. We present the basic method, and a number of extensions. We address the special problems presented by white-- space, and parsers with separate lexical and syntactic phases. In particular, a combining form for handling the "offside rule" is given. Other extensions to the basic method include an "into" combining form with many useful applications, and a simple means by which combinator parsers can produce more informative error messages. 1 Introduction Broadly speaking, a parser may be defined as a program which analyses text to determine its logical structure. For example, the parsing phase in a compiler takes a program text, and produces a parse tree which expounds the structure of the program. Many programs can be improved by having their input parsed. The form of input which is acceptable is usually defined by a context--free grammar, using BNF notation. Parsers themselves may be built by hand, but are most often ge...