Hi! I recently got engaged in a thread on comp.lang.functional about ML and Lisp. I posted some simple but efficient OCaml code that is difficult to translate into Lisp: let rec ( +: ) f g = match f, g with | `Q n, `Q m -> `Q (n +/ m) | `Q (Int 0), e | e, `Q (Int 0) -> e | f, `Add(g, h) -> f +: g +: h | f, g -> `Add(f, g) let rec ( *: ) f g = match f, g with | `Q n, `Q m -> `Q (n */ m) | `Q (Int 0), e | e, `Q (Int 0) -> `Q (Int 0) | `Q (Int 1), e | e, `Q (Int 1) -> e | f, `Mul(g, h) -> f *: g *: h | f, g -> `Mul(f, g) let rec simplify = function | `Q _ | `Var _ as e -> e | `Add(f, g) -> simplify f +: simplify g | `Mul(f, g) -> simplify f *: simplify g;; This code does some simple rearrangements of symbolic expressions to simplify them, e.g. 2+1*x+0 -> 2+x. It works with arbitrary-precision rational arithmetic. Does Ruby have pattern matching? If so, what does the above look like in Ruby? If not, how else can you express this elegantly in Ruby? Lisp doesn't have pattern matching but Pascal Constanza wrote quite an elegant solution in Lisp using dynamic method dispatch: (defstruct add x y) (defstruct mul x y) (defgeneric simplify-add (x y) (:method ((x number) (y number)) (+ x y)) (:method ((x (eql 0)) y) y) (:method (x (y (eql 0))) x) (:method (x (y add)) (simplify-add (simplify-add x (add-x y)) (add-y y))) (:method (x y) (make-add :x x :y y))) (defgeneric simplify-mul (x y) (:method ((x number) (y number)) (* x y)) (:method ((x (eql 0)) y) 0) (:method (x (y (eql 0))) 0) (:method ((x (eql 1)) y) y) (:method (x (y (eql 1))) x) (:method (x (y mul)) (simplify-mul (simplify-mul x (mul-x y)) (mul-y y))) (:method (x y) (make-mul :x x :y y))) (defgeneric simplify (exp) (:method (exp) exp) (:method ((exp add)) (simplify-add (simplify (add-x exp)) (simplify (add-y exp)))) (:method ((exp mul)) (simplify-mul (simplify (mul-x exp)) (simplify (mul-y exp))))) This has the advantage that Lisp optimises the above so that it is only 10x slower than the OCaml. Unlike the OCaml, it can be extended and automatically reoptimised at run-time.

on 2007-02-17 21:01

on 2007-02-18 11:11

On 17.02.2007 20:47, Jon Harrop wrote: > let rec ( *: ) f g = match f, g with > > This code does some simple rearrangements of symbolic expressions to > simplify them, e.g. 2+1*x+0 -> 2+x. It works with arbitrary-precision > rational arithmetic. > > Does Ruby have pattern matching? If so, what does the above look like in > Ruby? If not, how else can you express this elegantly in Ruby? Not that I am aware of. Even a quick search on RAA doesn't reveal anything: http://raa.ruby-lang.org/search.rhtml?search=pattern I guess the major issue here between Ruby and Lisp is that you do not have access to Ruby expressions natively the same way as you have in Lisp. There are no macros that integrate nicely with the language as Lisp macros do. The Ruby solution would likely involve creating a parser for the expressions you want to simplify, then simplifying the syntax tree and outputting it again. For getting at the Ruby parse tree there are indeed libraries, so that parse should not be too hard. In any case I guess the solution will only be half as elegant as the other two you presented. :-) Side note: the feature that comes closest to pattern matching is the assignment grouping of expressions: irb(main):014:0> a,(b,(c,d),e)=1,[2,[3,4],5] => [1, [2, [3, 4], 5]] irb(main):015:0> a => 1 irb(main):016:0> b => 2 irb(main):017:0> c => 3 irb(main):018:0> d => 4 irb(main):019:0> e => 5 > (:method (x (y add)) > (simplify-mul (simplify-mul x (mul-x y)) (mul-y y))) > slower than the OCaml. Unlike the OCaml, it can be extended and > automatically reoptimised at run-time. I have to admit that I am only familiar with very basic Lisp. But it does look elegant. :-) Kind regards robert

on 2007-02-18 12:28

>> | f, g -> `Add(f, g) >> | `Add(f, g) -> simplify f +: simplify g >> | `Mul(f, g) -> simplify f *: simplify g;; >> >> This code does some simple rearrangements of symbolic expressions to >> simplify them, e.g. 2+1*x+0 -> 2+x. It works with arbitrary-precision >> rational arithmetic. >> >> Does Ruby have pattern matching? If so, what does the above look like in >> Ruby? If not, how else can you express this elegantly in Ruby? I've showed kinda pattern matching for Ruby here: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/... I'm now working on more full-fledged library (place for it, but no files still at http://rubyforge.org/projects/pattern-match). V.

on 2007-02-21 03:07

On 2/17/07, Jon Harrop <jon@ffconsultancy.com> wrote: > | f, `Add(g, h) -> f +: g +: h > | `Q _ | `Var _ as e -> e > | `Add(f, g) -> simplify f +: simplify g > | `Mul(f, g) -> simplify f *: simplify g;; > > This code does some simple rearrangements of symbolic expressions to > simplify them, e.g. 2+1*x+0 -> 2+x. It works with arbitrary-precision > rational arithmetic. > > Does Ruby have pattern matching? If so, what does the above look like in > Ruby? If not, how else can you express this elegantly in Ruby? > check out this article on artima by Topher Cyl: http://www.artima.com/rubycs/articles/patterns_sex... Phil

on 2007-02-21 03:36

Phil Tomson wrote: > check out this article on artima by Topher Cyl: > http://www.artima.com/rubycs/articles/patterns_sex... Great article. Thanks. :-)