sutton036

Most software developers are now familiar with inheritance and virtual methods as common techniques for extensibility from the object oriented paradigm. When faced with functional programming for the first time, these developers often ask how to write extensible code in this alien paradigm. The functional paradigm actually only provides a single form of extensibility: higher-order functions. These allow you to factor out "inner" functions. For example, code that often appears with the same first and last code blocks: let f x = first x stuff1 x last x let g x = first x stuff2 x last x can be factored into a general higher order function that is reused from the specific cases: let hof stuff x = first x stuff x last x let f = hof stuff1 x let g = hof stuff2 x Applying this aggressively leads to design patterns such as parser combinators and is a very powerful and lightweight technique for making code extensible. However, it does not make data types extensible. Conseque...

For around 30 years, the ML family of languages have been characterised by several unusual features including a novel and powerful higher-order module system. This language feature was pioneered by Dave MacQueen in the early 1980s and found its way into the Standard ML and OCaml languages. Higher-order modules allow one module to be parameterized over another module in the form of functors that map modules to modules. Higher-order modules turn out to be especially useful for parameterizing concrete data structures over more primitive data structures. Perhaps the most compelling application of higher-order modules comes from graph theory, where they allow algorithms over graphs to be abstracted over the concrete data structures used to represent the graphs. This application was discussed in our most recent OCaml Journal article " The A* algorithm " that describes a generalization of Dijkstra's shortest path algorithm that is widely used for route finding in game AI. Th...