March 2010 - Haskell-Cafe

How do you rewrite your code?
by Sean Leather 05 Mar '10

05 Mar '10

There are numerous threads on the Haskell Café involving rewriting, refactoring, refining, and in general improving code (for some definition of improve). I am interested in seeing examples of how Haskell code can be rewritten to make it better. Some general examples are: - Eta-reduce - Make more pointfree - Introduce monadic operators or do-notation - e.g. for Maybe, lists, State - Eliminate monadic operators or do-notation - Generalize types - e.g. change map to fmap, (++) to mappend - Use instances of Functor, Applicative, Alternative, Category, Arrow, Monoid, Traversable, etc. - Use library functions from Data.List, Data.Map, Data.Set, etc. - Use some form of generic programming (e.g. SYB, Uniplate, EMGM, Alloy) - Use other libraries not included in the Platform My question is simple: *How do you rewrite your code to improve it?* You can answer this in any way you like, but I think the most useful answer is to show a reasonably small, concrete example of what your code looked like before and after. Also, please describe how you think the rewrite improves such code. - Is it better style? More useful? More efficient? - Are the types (before and after) the same? - Are the semantics the same? - How did you prove or test equivalence? (e.g. Can you use equational reasoning to confirm the rewrite is valid? Did you use QuickCheck?) Here is an example that I find myself doing occasionally. For all x, f: x >>= return . f --> fmap f x or f <$> x -- requires importing Control.Applicative I think the right-hand side (RHS) is more concise and simpler. The types here do change: the type constructor has a Monad constraint in the left-hand side and a Functor constraint in the RHS. Types that are Monad instances are generally also Functor instances, so this is often possible. I'm convinced the semantics are preserved, though I haven't proven it. What's an example of a rewrite that you've encountered? Thanks, Sean

13 14

QOTM: It's a series of 3 TLAs designed to make people who know what they are feel more in-the-know than those who don't.
by caseyh＠istar.ca 04 Mar '10

04 Mar '10

CBV, CBN, CBL

2 1

ANNOUNCE: wai-0.0.0 (Web Application Interface)
by Michael Snoyman 04 Mar '10

04 Mar '10

Hello all, I'd like to announce the first release of the Web Application Interface package[1]. The WAI is a common protocol between web server backends and web applications, allowing application writers to target a single interface and have their code run on multiple server types. There are two previous implementations of the same idea: the Network.Wai module as implemented in the Hyena package, and Hack. Some distinguishing characteristics of the wai package include: * Unlike Hack, this does not rely upon lazy I/O for passing request and response bodies within constrained memory. * Unlike Hyena, the request body is passed via a "Source" instead of Enumerator, so that it can easily be converted to a lazy bytestring for those who wish. * Unlike both, it attempts to achieve more type safety by have explicit types of request headers, response headers, HTTP version and status code. * It also removes any variables which are not universal to all web server backends. For example, scriptName has been removed, since it has no meaning for standalone servers. This package also contains separate modules for conversions to and from Sources and Enumerators. I am also simultaneously releasing two other packages: web-encodings 0.2.4[2] includes a new function, parseRequestBody, which directly parses a request body in a WAI request. It handles both URL-encoded and multipart data, and can store file contents in a file instead of memory. This should allow dealing with large file submits in a memory-efficient manner. You can also receive the file contents as a lazy bytestring. Finally, wai-extra 0.0.0 [3] is a collection of middleware and backends I use regularly in web application development. On the middleware side, there are five modules, including GZIP encoding and storing session data in an encrypted cookie (ClientSession). On the backend side, it includes CGI and SimpleServer. The latter is especially useful for testing web applications, though some people have reported using it in a production environment. All of the code is directly ported from previous packages I had written against Hack, so they are fairly well tested. As far as stability, I don't expect the interface to change too drastically in the future. I am happy to hear any suggestions people have for moving forward, but expect future versions to be mostly backwards compatible with this release. Also, future versions will respect the Package Versioning Policy[4]. Thanks to everyone who contributed their input. Michael [1] http://hackage.haskell.org/package/wai [2] http://hackage.haskell.org/package/web-encodings [3] http://hackage.haskell.org/package/wai-extra [4] http://www.haskell.org/haskellwiki/Package_versioning_policy

1 0

How do you set up windows enviroment
by Maciej Piechotka 04 Mar '10

04 Mar '10

How do you set up Windows environment? I'd like to develop few platform-specific code or rather port it to this platform. I tried to set up environment but I failed (darcs hanging on copy via ssh, command line is... well slightly better then in W98 etc.). Does anyone have a tips for setting Haskell environment on Windows 7. Regards

6 10

Prelude.undefined
by Tom Hawkins 04 Mar '10

04 Mar '10

How do I track down an reference to an undefined value? My program must not be using a library correctly because the program makes no direct use of 'undefined'. Running with +RTS -xc yields: <GHC.Err.CAF>Test: Prelude.undefined

8 12

ANNOUNCE: Parsec 3.1.0
by Derek Elkins 04 Mar '10

04 Mar '10

Parsec is a monadic combinator library that is well-documented, simple to use, and produces good error messages. Parsec is not inherently lazy/incremental and is not well-suited to handling large quantities of simply formatted data. Parsec 3 adds to Parsec the ability to use Parsec as a monad transformer and generalizes the input Parsec accepts. Parsec 3 includes a compatibility layer for Parsec 2 and should be a drop-in replacement for code using Parsec 2. Code using the features of Parsec 3 should use the modules in Text.Parsec. Due almost entirely to the work of Antoine Latter there is a new version of Parsec 3 available. He documented some of his thoughts on this in this series of blog posts: http://panicsonic.blogspot.com/2009/12/adventures-in-parsec.html The main features of this release are: - the performance should be much better and comparable to Parsec 2 - notFollowedBy's type and behavior have been generalized Changes: - the changes to the core of Parsec lead to some changes to when things get executed when it is used as a monad transformer "In the new version bind, return and mplus no longer run in the inner monad, so if the inner monad was side-effecting for these actions the behavior of existing code will change." - notFollowedBy p now behaves like notFollowedBy (try p) which changes the behavior slightly when p consumes input, though the behavior should be more natural now. - the set of names exported from Text.Parsec.Prim has changed somewhat

4 3

How to break strings and put to a tuple
by Pradeep Wickramanayake 04 Mar '10

04 Mar '10

Hi, sortList2 :: String -> String sortList2 (x:xs) | x == ',' = "" | otherwise = [x] ++ sortList2 xs im breaking a one specific string and putting them to each word. But I need to put them to a tuple. Can someone help me with the code Please.

2 1

Profiling help (Warning: Euler spoilers)
by Louis Wasserman 04 Mar '10

04 Mar '10

James, Which version of Control.Monad.State are you importing? If you're just importing vanilla Control.Monad.State, that's actually sending you to Control.Monad.State.Lazy. Using Control.Monad.State.Strict might solve your problems, srsly. Laziness can result in epically failing memory leaks. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

2 3

ANN: iteratee-parsec 0.0.2
by Maciej Piechotka 04 Mar '10

04 Mar '10

Iteratee-parsec is a library which allows to have a parsec (3) parser in IterateeG monad. It contains 2 implementations: - John Lato's on public domain. It is based on monoid and design with short parsers in mind. - Mine on MIT. It is based on single-linked mutable list. It seems to be significantly faster for larger parsers - at least in some cases - but it requires a monad with references (such as for example IO or ST). Version 0.0.2 does not differ much from 0.0.1 except that it is up-to-date with parsec 3.1.0 (version 0.0.1 is not). Regards

1 0

type class constraints headache
by muneson 04 Mar '10

04 Mar '10

When writing a command-line interface I ran into type class conflicts I don't understand. Could anyone explain why ghc 6.10.4 compiles this > methods :: (Eq a) => [(String, a)] > methods = > [ ("method1", undefined ) > , ("method2", undefined) > ] but not the following? > methods :: (Eq a) => [(String, a)] > methods = > [ ("method1", undefined ) > , ("method2", undefined) > ] > > enumerateMethodNames :: [String] > enumerateMethodNames = map fst methods thanks, Marcus -- View this message in context: http://old.nabble.com/type-class-constraints-headache-tp27752745p27752745.h… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 7