[Camllist] Why must types be always defined at the top level?
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Date:   (:) 
From:  Brian Hurt <bhurt@s...> 
Subject:  Re: [Camllist] Why must types be always defined at the top level? 
On Thu, 24 Jun 2004, Xavier Leroy wrote: > There is a general need to have polymorphic operations that are 1 not > defined on all instantiations of their types, and 2 can be defined > differently on different instantiations. Haskell type classes are an > example of a *general* mechanism that addresses this need; GCaml's > "extentional polymorphism" is another. Ocaml modules are another way to implement this. Wether it's a "good" way or not is open to debate. > As to whether equality should be defined on floats, there are pros and > cons. My standpoint is that it's eventually better to stick to > established standards (that is, IEEE float arithmetic) rather than try > to reinvent a wheel likely to be even squarer than these standards. > Prof. Kahan found it worthwhile to fully define equality over floats; > I'll abide by his wisdom. There are legitimate reasons to want floating point equality. It's generally not what you want, but I had an example of needing it in Ocaml just the other day. Basically, I was writting a root finding algorithm by range subdivision. Given a function f and a range [a,b] where sign(f(a)) != sign(f(b)), find the root in that range, the x such that a <= x <= b, and f(x) = 0 (or as close as can come). The function I ended up with was this: let root_find f a b = let rec loop a b fa_is_neg = let c = (a +. b) /. 2. in if (c = a)  (c = b) then (* note the floating point equality *) (* We've bottomed out pick a or b depending upon if f(a) or f(b) is closer to 0. *) if (abs_float (f a)) > (abs_float (f b)) then a else b else let fc_is_neg = ((f c) < 0.) in if (fa_is_neg == fc_is_neg) then loop c b fc_is_neg else loop a c fa_is_neg in loop a b ((f a) < 0.) ;; Basically, I run the algorithm out until the distance between a and b is 1ulp, then stop.  "Usenet is like a herd of performing elephants with diarrhea  massive, difficult to redirect, aweinspiring, entertaining, and a source of mindboggling amounts of excrement when you least expect it."  Gene Spafford Brian  To unsubscribe, mail camllistrequest@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/camlbugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners