- Haskell-Cafe - Haskell.org

Zippers for gui elements?
by Gü?nther Schmidt 18 Apr '09

18 Apr '09

Hi all, I need to control some gui elements, ie. stuff like when dis button is clicked make dat one inactive. Some time ago, wish I'd remember where, I read about using Zippers for that sort of thing. Does anybody know where I might have read that? Günther

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Re: [Haskell-cafe] General function to count list elements?
by nowgate＠yahoo.com 18 Apr '09

18 Apr '09

Hi John, Yes, I see now what the hullabaloo was about. I've only been hacking Haskell for a few weeks, so I'm still seeing the "forest" rather than the "trees". But I'm coming along. Thanks. Michael --- On Sat, 4/18/09, John Dorsey <haskell(a)colquitt.org> wrote: From: John Dorsey <haskell(a)colquitt.org> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 2:36 PM Michael, > I had a count function that worked fine for an enum type, but thought > why not go polymorphic with it. So I changed it and got the error I > originally inquired about. For variety, I'll go a slightly different direction. If you generalize count to use any predicate, instead of always equality... gcount :: (a -> a -> Bool) -> a -> [a] -> Int gcount pred x0 xs = length (filter (pred x0) xs) count = gcount (==) This will work with any type that you can write a predicate for with the type (a -> a -> Bool). I can even use this with functions, if I'm careful. ghci> gcount (\f g -> True) (*2) [id,(const 1),(*3)] 3 ghci> gcount (\f g -> f 1 == g 1) (^2) [id,(const 1),(*3)] 2 By the way, do you see why everyone's bothing you about comparing functions? The type you gave count, which didn't have an Eq constraint, was an assertion that you could compare two values of *any* type. If there's a type that's not comparable, then count's type was wrong. Functions are the canonical example of an incomparable type. When you're bored some time, read a bit about the Curry-Howard correspondence. It's interesting, even if (like me) you don't grok all of its implications. Regards, John

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Re: [Haskell-cafe] General function to count list elements?
by michael rice 18 Apr '09

18 Apr '09

I wasn't comparing functions, you were. I was just trying to answer your questions. I had a count function that worked fine for an enum type, but thought why not go polymorphic with it. So I changed it and got the error I originally inquired about. Thanks. Michael --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 1:18 PM 2009/4/18 michael rice <nowgate(a)yahoo.com>: >> To compare two functions in C, I would compare their machine addresses. >> > > Why would you need that at all? > > How would *you* do it? Do what? As for comparing functions, I would not do it at all, because comparing functions makes no sense: one way of comparison (extensional equality) is not implementable, whereas the other (object identity) breaks referential transparency and is impossible to reason about and generally doesn't have a single reason to work correctly at all. Object identity has no reason to exist in a language without mutable objects, like Haskell. That's probably the key. As for implementing the count function, I would implement it in the way that people already suggested to you: by putting a constraint onto its type, namely a constraint that makes it polymorphic *over types whose values can be compared for equality*, t.i. over members of the Eq class. This is perfectly correct and does not cause any mathematical unsoundness. > > Michael > > ================ > > --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: > > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > Subject: Re: [Haskell-cafe] General function to count list elements? > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Saturday, April 18, 2009, 12:39 PM > > 2009/4/18 michael rice <nowgate(a)yahoo.com>: >> I know functions can be compared in Scheme >> >> Welcome to DrScheme, version 4.1 [3m]. >> Language: Swindle; memory limit: 128 megabytes. >>> (equal? equal? equal?) >> #t >>> >> > > That's not the functions being compared, but the memory addresses of > the code implementing them. If your goal is comparing functions to > answer a question "Are these two values indistinguishable?", equal? > doesn't help you, because it may answer 'false' even if these two > values are indistinguishable from a mathematical point of view. > >> >> but apparently not in Haskell >> >> [michael@localhost ~]$ ghci >> 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. >> Prelude> (==) (==) (==) >> >> <interactive>:1:0: >> No instance for (Eq (a -> a -> Bool)) >> arising from a use of `==' at <interactive>:1:0-13 >> Possible fix: add an instance declaration for (Eq (a -> a -> Bool)) >> In the expression: (==) (==) (==) >> In the definition of `it': it = (==) (==) (==) >> Prelude> >> >> though I'm new at Haskell and may not be posing the question properly. >> >> I would think a language with 1st-class support for functions would >> certainly include comparing them. >> > > Again, this is first-class support for memory addresses of code > representing functions. > >> To compare two functions in C, I would compare their machine addresses. >> > > Why would you need that at all? > >> Michael >> >> >> --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: >> >> From: Eugene Kirpichov <ekirpichov(a)gmail.com> >> Subject: Re: [Haskell-cafe] General function to count list elements? >> To: "michael rice" <nowgate(a)yahoo.com> >> Cc: haskell-cafe(a)haskell.org >> Date: Saturday, April 18, 2009, 11:39 AM >> >> Could you then provide an example of two functions that *are* equal, >> or, even better, a definition of equality for arbitrary functions? >> Since Haskell may be compiled into C, this must be a definition that >> is implementable in C. >> >> 2009/4/18 michael rice <nowgate(a)yahoo.com>: >>> Though I haven't tried it out, it's trying to use my function to count >>> functions. >>> >>> The first argument is the identity function. >>> >>> The second argument is a list of a different form of the identity >>> function. >>> >>> Though the two identity functions, given the same input, would produce >>> the >>> same output, I doubt they would be equal. >>> >>> So my guess at an answer would be zero. >>> >>> Michael >>> >>> --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: >>> >>> From: Eugene Kirpichov <ekirpichov(a)gmail.com> >>> Subject: Re: [Haskell-cafe] General function to count list elements? >>> To: "michael rice" <nowgate(a)yahoo.com> >>> Cc: haskell-cafe(a)haskell.org >>> Date: Saturday, April 18, 2009, 11:03 AM >>> >>> What should >>> >>> count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) >>> >>> return? >>> >>> 2009/4/18 michael rice <nowgate(a)yahoo.com>: >>>> 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 >>>> >>>> ============= >>>> >>>> [michael@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(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>>> >>> >>> >>> >>> -- >>> Eugene Kirpichov >>> Web IR developer, market.yandex.ru >>> >>> >> >> >> >> -- >> Eugene Kirpichov >> Web IR developer, market.yandex.ru >> >> > > > > -- > Eugene Kirpichov > Web IR developer, market.yandex.ru > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Re: [Haskell-cafe] General function to count list elements?
by michael rice 18 Apr '09

18 Apr '09

> To compare two functions in C, I would compare their machine addresses. > Why would you need that at all? How would *you* do it? Michael ================ --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 12:39 PM 2009/4/18 michael rice <nowgate(a)yahoo.com>: > I know functions can be compared in Scheme > > Welcome to DrScheme, version 4.1 [3m]. > Language: Swindle; memory limit: 128 megabytes. >> (equal? equal? equal?) > #t >> > That's not the functions being compared, but the memory addresses of the code implementing them. If your goal is comparing functions to answer a question "Are these two values indistinguishable?", equal? doesn't help you, because it may answer 'false' even if these two values are indistinguishable from a mathematical point of view. > > but apparently not in Haskell > > [michael@localhost ~]$ ghci > 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. > Prelude> (==) (==) (==) > > <interactive>:1:0: > No instance for (Eq (a -> a -> Bool)) > arising from a use of `==' at <interactive>:1:0-13 > Possible fix: add an instance declaration for (Eq (a -> a -> Bool)) > In the expression: (==) (==) (==) > In the definition of `it': it = (==) (==) (==) > Prelude> > > though I'm new at Haskell and may not be posing the question properly. > > I would think a language with 1st-class support for functions would > certainly include comparing them. > Again, this is first-class support for memory addresses of code representing functions. > To compare two functions in C, I would compare their machine addresses. > Why would you need that at all? > Michael > > > --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: > > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > Subject: Re: [Haskell-cafe] General function to count list elements? > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Saturday, April 18, 2009, 11:39 AM > > Could you then provide an example of two functions that *are* equal, > or, even better, a definition of equality for arbitrary functions? > Since Haskell may be compiled into C, this must be a definition that > is implementable in C. > > 2009/4/18 michael rice <nowgate(a)yahoo.com>: >> Though I haven't tried it out, it's trying to use my function to count >> functions. >> >> The first argument is the identity function. >> >> The second argument is a list of a different form of the identity >> function. >> >> Though the two identity functions, given the same input, would produce the >> same output, I doubt they would be equal. >> >> So my guess at an answer would be zero. >> >> Michael >> >> --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: >> >> From: Eugene Kirpichov <ekirpichov(a)gmail.com> >> Subject: Re: [Haskell-cafe] General function to count list elements? >> To: "michael rice" <nowgate(a)yahoo.com> >> Cc: haskell-cafe(a)haskell.org >> Date: Saturday, April 18, 2009, 11:03 AM >> >> What should >> >> count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) >> >> return? >> >> 2009/4/18 michael rice <nowgate(a)yahoo.com>: >>> 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 >>> >>> ============= >>> >>> [michael@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(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> >> >> >> >> -- >> Eugene Kirpichov >> Web IR developer, market.yandex.ru >> >> > > > > -- > Eugene Kirpichov > Web IR developer, market.yandex.ru > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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General function to count list elements?
by michael rice 18 Apr '09

18 Apr '09

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 ============= [michael@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

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Computing a sorted list of products lazily
by Jason Dagit 18 Apr '09

18 Apr '09

Hello, A colleague of mine recently asked if I knew of a lazy way to solve the following problem: Given two sets of sorted floating point numbers, can we lazily generate a sorted list of the products from their Cartesian product? The algorithm should return the same result as: sortProduct a b = sort [ x * y | x <- a, y <- b ] But, my friend is not satisfied with the strictness of sort. In particular, he doesn't want to have to hold the whole list in memory to sort it. He would like to compute the results one product at a time and in the correct order. Is there an existing algorithm for this problem? We searched but we don't seem to know the right search terms. Additionally, my friend is not writing this in Haskell, but a Haskell solution is fine because we should be able to transform it into a Java solution by hand. We think we know an algorithm that will produce the right result but it's a bit messy and we doubt we're the first to tackle this problem. Any help would be greatly appreciated. Thanks in advance! Jason

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Synchronising cabal package version with self-reported version
by Denis Bueno 18 Apr '09

18 Apr '09

Hi all, In a command-line app of mine I want to have a --version flag, like many GNU apps do to report their version. The only way I know to provide the version number is to hardcode it in the source code somewhere. That means I have the version number in two places: the .cabal file and the source code. I just know that one day they'll get unsynchronised, and --version will be wrong. Is there any way to put the version number in _one_ place, and have both the .cabal and source refer to it? Or even any easier way to synchronise both version numbers? Thanks, Denis

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Debugging a RecSel Error
by Gü?nther Schmidt 18 Apr '09

18 Apr '09

Hi all, I'm trying to find the spot in my source code that triggered a RecSel Exception ("No match in record selector

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Parsec question
by Michael P Mossey 18 Apr '09

18 Apr '09

I want to write a parser that can read a file with this format: the file has sections which are demarcated by keywords. Keywords always begin with two forward slashes and consist of letters, digits, and underscore. The text can be anything, including special characters. For instance: //keyword some text and more text //another_keyword and) some { more text //ya_keyword $$ -- text I'm not sure how to write a parser that considers anything but a double slash to be a valid part of the text. Thanks, Mike

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Announce: funsat-0.6
by Denis Bueno 17 Apr '09

17 Apr '09

Hello haskell-cafe, funsat is a modern, DPLL-style SAT solver written in Haskell. Funsat solves formulas in conjunctive normal form and produces a total variable assignment for satisfiable problems. Funsat is intended to be reasonably efficient for practical problems and convenient to use as a constraint-solving backend in other software. Version 0.6 is available from Hackage: * http://hackage.haskell.org/cgi-bin/hackage-scripts/package/funsat New in 0.6: * A representation for logical circuits (and, or, not, onlyif, iff, if-then-else) supporting efficient conversion to CNF (for solving) has been added. * Now uses the BSD3 license. Please report any bugs to the github page linked from hackage. Denis

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