Re: [Haskell-cafe] Re: General function to count list elements?
A solution with enums would severely suffer from the expression problem... One would need to extent the enums every time one needs to support a new function. Maybe could be solved with type classes, don't know. On Mon, Apr 20, 2009 at 3:57 PM, Achim Schneider bars...@web.de wrote: Lennart Augustsson lenn...@augustsson.net wrote: On Sun, Apr 19, 2009 at 10:43 PM, Peter Verswyvelen bugf...@gmail.com wrote: For example, suppose you have a predicate a - Bool, and a list of these predicates [a - Bool], but you want to remove all functions that are obviously equal in the C way from the list for optimization... Okay big hack, and one could do this already with reallyUnsafePtrEquality# I guess... And when the need gets big enough you pull out StablePtr and use that. :) Waaagh! Don't give him ideas, he's going to do it... Make yourself an enum, generate your list, nub it, then transform it to a list of functions. Always do everything with the least information sanely feasible, and a function is more information than a value, even if you can't get at it, anymore. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: General function to count list elements?
Lennart Augustsson lenn...@augustsson.net wrote: On Sun, Apr 19, 2009 at 10:43 PM, Peter Verswyvelen bugf...@gmail.com wrote: For example, suppose you have a predicate a - Bool, and a list of these predicates [a - Bool], but you want to remove all functions that are obviously equal in the C way from the list for optimization... Okay big hack, and one could do this already with reallyUnsafePtrEquality# I guess... And when the need gets big enough you pull out StablePtr and use that. :) Waaagh! Don't give him ideas, he's going to do it... Make yourself an enum, generate your list, nub it, then transform it to a list of functions. Always do everything with the least information sanely feasible, and a function is more information than a value, even if you can't get at it, anymore. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: General function to count list elements?
On Mon, Apr 20, 2009 at 7:57 AM, Achim Schneider bars...@web.de wrote: Lennart Augustsson lenn...@augustsson.net wrote: On Sun, Apr 19, 2009 at 10:43 PM, Peter Verswyvelen bugf...@gmail.com wrote: For example, suppose you have a predicate a - Bool, and a list of these predicates [a - Bool], but you want to remove all functions that are obviously equal in the C way from the list for optimization... Okay big hack, and one could do this already with reallyUnsafePtrEquality# I guess... And when the need gets big enough you pull out StablePtr and use that. :) Waaagh! Don't give him ideas, he's going to do it... Nah, I don't think so. I think, even after all this mess, we've been relatively clear that Eq is how you do it. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: General function to count list elements?
Peter Verswyvelen bugf...@gmail.com wrote:
Sometimes I do miss the pragmatic C solution:- two function pointers
that are equal surely represent the same functions (although in C
nothing is really sure ;)
In haskell, they would, but C doesn't give you the same guarantee:
int evil = 0;
int wicked( int i_dare_you ) {
return i_dare_you + (++evil);
}
Clearly, wicked and wicked are the same function, but they surely
aren't indistinguishable, at least if you happen to call them.
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[Haskell-cafe] Re: General function to count list elements?
The problem is that you must note in the type signature the fact that 'a' must be a member of typeclass Eq in order to be able to use == count :: (Eq a) = a - [a] - Int count x ys = length (filter (== x) ys) JP michael rice wrote: Is there a general function to count list elements. I'm trying this count :: a - [a] - Int count x ys = length (filter (== x) ys) with this error upon loading = [mich...@localhost ~]$ ghci count GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( count.hs, interpreted ) count.hs:2:29: Could not deduce (Eq a) from the context () arising from a use of `==' at count.hs:2:29-32 Possible fix: add (Eq a) to the context of the type signature for `count' In the first argument of `filter', namely `(== x)' In the first argument of `length', namely `(filter (== x) ys)' In the expression: length (filter (== x) ys) Failed, modules loaded: none. Prelude = Not sure what it's trying to tell me other than I need an (Eq a) somewhere. Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
