The unabated debate about exactly how much category theory one needs to know to understand that strange beast of IO prompts a thought if monads, like related continuations, are the things that are destined to be rediscovered time and time again. An old (1994) paper on category theory monads and functional programming included an interesting historical side-note. It turns out that the RealWorld-passing trick underlying the implementation of the IO monad, the trick that made it possible to embed truly side-effecting operations into pure Haskell -- the trick is 45 years old. It has been first published in February 1965. That 1965 paper also anticipated State and Writer monad, call/cc, streams and delayed evaluations, relation of streams with co-routines, and even stream fusion. First, here is the historical aside, cited from Jonathan M. D. Hill and Keith Clarke An Introduction to Category Theory, Category Theory Monads, and Their Relationship to Functional Programming. 1994 http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.6497 [blockquote] 3 An historical aside Monads are typically equated with single-threadedness, and are therefore used as a technique for incorporating imperative features into a purely functional language. Category theory monads have little to do with single-threadedness; it is the sequencing imposed by composition that ensures single-threadedness. In a Wadler-ised monad this is a consequence of bundling the Kleisli star and flipped compose into the bind operator. There is nothing new in this connection. Peter Landin in his Algol 60 used functional composition to model semi-colon. Semi-colon can be thought of as a state transforming operator that threads the state of the machine throughout a program. The work of Peyton-Jones and Wadler has turned full circle back to Landin's earlier work as their use of Moggi's sequencing monad enables real side-effects to be incorporated into monad operations such as print. This is similar to Landin's implementation of his sharing machine where the assignandhold function can side-effect the store of the sharing machine because of the sequencing imposed by functional composition. Landin defined that `Imperatives are treated as null-list producing functions' [In Landin's paper, () is the syntactic representation of the empty list and not the unit.]. The assignandhold imperative is subtly different in that it enables Algol's compound statements to be handled. The function takes a store location and a value as its argument, and performs the assignment to the store of the sharing machine, returning the value assigned as a result of the function. Because Landin assumed applicative order reduction, the K combinator was used to return (), and the imperative was evaluated as a side effect by the unused argument of the K-combinator. Statements are formed by wrapping such an imperative in a lambda expression that takes () as an argument. Two consecutive Algol-60 assignments would be encoded in the lambda calculus as: Algol 60 | Lambda Calculus x:= 2; | ( (\() -> K () (assignandhold x 2)) . x:= -3; | (\() -> K () (assignandhold x (-3))) ) () By using a lambda with () as its parameter, () can be thought of as the `state of the world' that is threaded throughout a program by functional composition. [/blockquote] Peter Landin's paper is remarkable indeed: P. J. Landin. A correspondence between ALGOL 60 and Church's lambda notation. Communications of the ACM, 8(2):89-101, February 1965. (Part 2 in CACM Vol 8(2) 1965, pages 158-165.) http://portal.acm.org/citation.cfm?id=363749 First the reader will notice the `where' notation. Peter Landin even anticipated the debate on `let' vs `where', saying ``The only consideration in choosing between `let' and `where' will be the relative convenience of writing an auxiliary definition before or after the expression it qualifies.'' The ()-passing trick is described in full on p100, with remarkable clarity: ``Statements. Each statement is rendered as a 0-list- transformer, i.e. a none-adic function producing the nullist for its result. It achieves by side-effects a transformation of the current state of evaluation. ... Compound statements are considered as functional products (which we indicate informally by infixed dots).'' A remark ``However, input/output devices can be modeled as named lists, with special, rather restricted functions associated. ... Writing is modeled by a procedure that, operates on a list, and appends a new final segment derived from other variables. (Alternatively, a purely functional approach can be contrived by including the transformed list among the results.)'' anticipated the State and Writer monads as well. Another remark, in the section on Streams, ``This correspondence [laws of head/tail/cons] serves two related purposes. It enables us to perform operations on lists (such as generating them, mapping them, concatenating them) without using an `extensive,' item-by-item representation of the intermediately resulting lists; and it enables us to postpone the evaluation of the expressions specifying the items of a list until they arc actually needed. The second of these is what interests us here.'' and footnote 6, ``It appears that in stream-transformers we have a functional analogue of what Conway [12] calls "co-routines." show that Peter Landin understood streams, on-demand evaluation and even stream fusion.