Dr. Dobb's Journal - May 2008 - (Page 23)

d05digg_p2ds 3/13/08 8:44 AM Page 23 Perforce has the effect of pushing a function onto the stack without executing it. In Joy parlance, this is called a “quotation,” but I think of it as an anonymous function. Anonymous functions are first class values: They can be constructed dynamically and treated like any other primitive value (you can dup them, swap them, pop them, and so on). You can execute anonymous functions using higher order functions such as apply, if, and while. Closures (functions bound to values in the local environment) can be constructed in Cat using the papply primitive instruction. This has the effect of binding the top value on the stack to the function, a process known as “partial application” (partial application is frequently mislabeled as “currying”). An example of partial application is that if you wrote 1 [<=] papply, it has the same effect as if you wrote [1 int) {1+} define swapd : ('a 'b 'c -> 'b 'a 'c) { [swap] dip } define dupd : ('a 'b -> 'a 'a 'b) { [dup] dip } define bury : ('a 'b 'c -> 'c 'a 'b) { swap swapd } define fib // compute fibonnacci { dup 1 <= [dup dec fib swap fib +] [] if } define fact // compute factorial { dup list) {{ desc: This is a simple implementation of a quick-sort algorithm. test: in: [5 4 2 1 3 2 4] list qsort out: [5 4 4 3 2 2 1] list }} { // Does list have 0 or 1 elements? [small] // Base case do nothing [] // Argument-relation // Split the list using the head as a pivot // storing the pivot under for later use [uncons under [lt] papply split] // Append the pivot to the first list // then concatenate the two lists. [[swap cons] dip cat] bin_rec } Scott Buchholz Sybase “Perforce is the cornerstone of Monster’s software production lines.” Mark Conway Monster Worldwide “The bottom line is that Perforce has proven itself to be a reliable platform for global development at Symantec.” Russell Jackson Symantec Example 2: A simple quick-sort algorithm. define binrec(input, cond, term, unfold, fold) { input cond [term] // termination transformation [input unfold // split input [cond term unfold fold binrec] // self call with same arguments app2 // call function twice: // once with second value removed // once with top value removed ] if } See for yourself Free evaluation at www.perforce.com Example 3: Example implementation of the bin_rec function. May 2008 l www.ddj.com l Dr. Dobb’s Journal 23 http://siteseer.ist.psu.edu/meijer91functional.html http://siteseer.ist.psu.edu/meijer91functional.html http://siteseer.ist.psu.edu/launchbury95warm.html http://siteseer.ist.psu.edu/launchbury95warm.html http://www.perforce.com http://www.perforce.com http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - May 2008

Dr. Dobb's Journal - May 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Software Development Goes to the Movies Cat: A Functional Stack-Based Little Language Mojax: Mobile Ajax Framework Kernel-Mode Databases Getting Better Search Results Effective Concurrency The Agile Edge

Dr. Dobb's Journal - May 2008

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