December 2012 - Libraries

Re: Maximum and Minimum monoids
by wren ng thornton 01 Jan '13

01 Jan '13

On 12/29/12 8:29 AM, Strake wrote:> and (Max a) and (Min a) are not properly monoids. They're monoids just fine. It's just that their identity element need not be in the set being folded over. > We could define these: > > Supr, Infi :: * -> * Please please, if we're going to abbreviate mathematical terms then we ought to stick to the standard mathematical abbreviations: sup, inf. Personally, I'm fine with Min and Max as they are, because in particular they capture the fact that we're dealing with total orders here. That is, other than for empty sets, they do in fact return the maximum/minimum. Whereas Sup and Inf bring to mind the fact that what we're dealing with is a complete lattice, which need not be a total order. While Sup and Inf make perfectly good monoids/semigroups, I'd prefer if they properly allow all complete lattices rather than being unnecessarily restricted to Ord. -- Live well, ~wren

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Maximum and Minimum monoids
by Gabriel Gonzalez 01 Jan '13

01 Jan '13

I don't know if this has been brought up before or not, but would it be possible to add the Maximum and Minimum monoids to Data.Monoid? The following implementations extend the traditional semigroups using Maybe. ****** newtype Maximum a = Maximum { getMaximum :: Maybe a } instance (Ord a) => Monoid (Maximum a) where mempty = Maximum Nothing mappend (Maximum Nothing) m2 = m2 mappend m1 (Maximum Nothing) = m1 mappend (Maximum (Just a1)) (Maximum (Just a2)) = Maximum (Just (max a1 a2)) newtype Minimum a = Minimum { getMinimum :: Maybe a } instance (Ord a) => Monoid (Minimum a) where mempty = Minimum Nothing mappend (Minimum Nothing) m2 = m2 mappend m1 (Minimum Nothing) = m1 mappend (Minimum (Just a1)) (Minimum (Just a2)) = Minimum (Just (min a1 a2)) ****** These also give the correct behavior when folding empty structures by returning Nothing. The reason I'm asking is that my `pipes` library uses `WriterT` to implement folds and having the above monoids lets me implement minimum and maximum folds elegantly. I can always provide these monoids myself, but I feel like they belong in Data.Monoid.

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Proposal: add conversion functions to Data.Fixed.
by Jeff Shaw 01 Jan '13

01 Jan '13

Hello, In my work I've run into an annoyance: Data.Fixed.Fixed does not have a Binary instance in the binary package. This is a result of the representation of Fixed being hidden, which is understandable. The problem is that when I want to create a Binary instance for Fixed, I need to use unsafeCoerce, which is nasty, and there's no built-in way to safely deal with problems involving the desired resolution being different when deseralizing. Today I played around with the source to Fixed, and came up with this proposed solution: - | Get a Fixed value's Integer representation and resolution. fromFixed :: (HasResolution a) => Fixed a -> (Integer,Integer) fromFixed f@(MkFixed i) = (i, resolution f) -- | Given an Integer representation and resolution of a Fixed value, -- create a new Fixed value with an arbitrary resolution. toFixed :: (HasResolution a) => (Integer,Integer) -> Fixed a toFixed (i,r0) = withResolution (\r1 -> MkFixed $ (i * r1) `div` r0) These functions lead to this useful derivation: convertFixed = toFixed . fromFixed, which leads to the possibility of having many possibly useful utility functions, such as toUni, fromUni, toDeci, fromDeci, etc. From fromFixed and toFixed, a safe instance for Binary (Fixed a) can be built (I haven't tested this, but it looks right): instance (HasResolution a) => Binary (Fixed a) where put = put . fromFixed get = get >>= return . toFixed Here is my patch: https://docs.google.com/open?id=0B8oSBVnQGD_wNGEtT1JRSnpPUms. Please respond with any comments, concerns, or ideas. Discussion period: 3 weeks. Thanks, Jeff

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Proposal: Add foldMapWithKey to Data.Map and Data.IntMap
by Edward Kmett 30 Dec '12

30 Dec '12

I'd like to add foldMapWithKey to Data.Map and Data.IntMap. Right now you can foldrWithKey and foldlWithKey on Data.Map and Data.IntMap, but you cannot foldMapWithKey. You can fake this by using fold . mapWithKey but this requires converting the entire structure. You can also implement it with traverseWithKey and the use of the Const Monoid in a manner similar to foldMapDefault. When you have a monoid that can take advantage of the balanced nature of the tree, it'd be nice to be able to not lose the original near balanced parenthesization of the source tree. We can use this in the lens library for more efficient zippers on these containers, but in general having access to a 'balanced' foldMap is useful for any such container, and it seems odd not to be able to get at the keys in this orientation when you can go so every other way. A patch that implements the proposed operation is available here: https://github.com/haskell/containers/pull/24 Discussion Period: 2 weeks -Edward

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Retiring as maintainer of dataenc
by Magnus Therning 29 Dec '12

29 Dec '12

Hi all, A few years ago I found a need to base64 encode a bit of text inside vim and realised that there was no convenient way of doing it. This resulted in dataenc[1] and a pair of tools built on it, omnicodec[2]. The main goals were to make dataenc correct, support many encodings, and easy to maintain. This resulted in very simple (some would say naïve) code, with fairly good test coverage. As some have noticed, this simplicity in algorithm results in rather bad performance on large data. Which is why I've been planning for a replacement for a while now. This replacement is now here, sandi[3]. The goals of sandi are similar to dataenc, correct, many encodings, fairly simple, but also reasonably fast. This means that I personally think dataenc has reached the end of its life and I won't spend any more time on maintaining it. If anyone is interested in taking over maintenance then please announce it on the list. /M [1]: http://hackage.haskell.org/package/dataenc [2]: http://hackage.haskell.org/package/omnicodec [3]: http://hackage.haskell.org/package/sandi -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

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ANN: sandi 0.1.1 - a data encoding/decoding library
by Magnus Therning 29 Dec '12

29 Dec '12

The other day I released version 0.1.1 of sandi, a data encoding/decoding library supporting the following encodings: base16, base32, base32hex, base64, base64url, base85, quoted-printable, uuencode, xxencode, yEnc I believe it's reasonably fast, but I still have to look more closely at criterion to get some hard numbers confirming that :) More details can be found at http://hackage.haskell.org/package/sandi /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Perl is another example of filling a tiny, short-term need, and then being a real problem in the longer term. -- Alan Kay

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Re: Maximum and Minimum monoids
by Gabriel Gonzalez 29 Dec '12

29 Dec '12

I'm still not convinced by the Bounded instance for Maybe, which seems like a hack. I would prefer to just use the Monoid instance for "Maybe (Max a)" and just fmap "getMaximum" over the result rather than pretend that Maybe is Bounded when it's not. Moreover, the entire Bounded constraint seems wrong in principle. There is no intuitive reason why some type should be bounded to be able to take a maximum of a set of elements. The Ord constraint should be sufficient to define a maximum. I also don't think it is a good idea to have direct Monoid instances for Max and Min, because they behave weirdly on empty lists. Do you really think that "minimum [] :: Int" should be defined? I still believe that the correct answer for "minimum []" should be Nothing or a similar type with a distinguished infinity like those that Roman proposed.``

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Re: Maximum and Minimum monoids
by Gabriel Gonzalez 28 Dec '12

28 Dec '12

I am definitely in favor of adding "semigroups" to the platform if drops the containers dependency. Optionally, we could then fix the Monoid instance for Maybe so that it has a proper semigroup context and we wouldn't have to use the Option type. I would be happy with that solution. However, if we do that then rename the types to "Max" and "Min" instead of "Maximum and "Minimum", since "Maybe (Max a)" is long enough as it is.

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Re: Maximum and Minimum monoids
by John Lato 27 Dec '12

27 Dec '12

+1 for something like this. I've written these at least once, and encountered them on a few more occasions. I would prefer specific types as Roman suggests, I think. I prefer to match on Plus/MinusInfinity rather than going via Maybe. John L. > From: Roman Cheplyaka <roma(a)ro-che.info> > Subject: Re: Maximum and Minimum monoids To: Gabriel Gonzalez <gabriel439(a)gmail.com> > Cc: libraries(a)haskell.org > Message-ID: <20121227202510.GA24842@sniper> > Content-Type: text/plain; charset=us-ascii > > Wouldn't it be better to have a real algebraic type instead of wrapping > Maybe? > > Something like > > data Maximum a = MinusInfinity | Maximum a > data Minimum a = PlusInfinity | Minimum a > > Maximum x is more concise than Maximum (Just x), and MinusInfinity is > more descriptive than Maximum Nothing. getMaximum/getMinimum functions > can still return Maybes. > > Anyway, I'm +1 to having something along these lines. > > Roman >

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What is the best usage of haskell in terms of build-in capabilities or external libraries?
by Peng Yu 19 Dec '12

19 Dec '12

Hi, When a language is selected for a project, a practical matter is what the language provides. For example, if I need to use regex a lot, I probably want to use perl rather than other languages, because perl is the best in terms of regex and have much influence on other languages that have support on regex. If I need to do a lot of statistics, I'd better use R, because there are already a lot of statical packages implemented in R. And if there is a statistical method, it is most likely to implemented in R first. If I want to implement matrix numerical algorithm, I'd better use Fortran. After reading some wiki pages of haskell, it is not clear to what area haskell is really good for. One strong point seems to be a testbed for trying new languages. But this seems to be the case for other functional languages as well. Is there an application area in which haskell is much stronger than other different languages? -- Regards, Peng

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