- Haskell-Cafe - Haskell.org

Cmdargs and common flags
by Magnus Therning 12 Mar '11

12 Mar '11

I'm looking for a good way of dealing with common flags in cmdargs. Currently what I do requires a bit of repetition that I'd really like to get rid of: 1. Data types data Modes = Cmd1 { foo :: String, ... } | Cmd2 { foo :: String, ... } 2. Mode specifications cmd1 = Cmd1 { foo = def &= help "..", ...} cmd2 = Cmd2 { foo = def &= help "..", ...} I have no idea how to deal with the repetition in the data types, so all suggestions are welcome. For the repetition in the mode specifications I attempted to factor the common bits into a "variable": flagFoo = "" &= help ".." but that resulted in only the first command getting the proper specification. I suppose this comes down to the impurity, but how do I work around that? (Especially, are there any good examples of using the pure part of the API?) /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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Light and fast http server
by Victor Oliveira 12 Mar '11

12 Mar '11

Hi cafe, There are a lot of http servers in hackage. I didn't have used none. I would like to know if one of them is something closer of the nginx. I need some light and fast. It don't need support all http, just the basics is fine. Suggestions? []s Victor

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Light and fast http server
by Greg Weber 12 Mar '11

12 Mar '11

> > snap or warp/yesod. maybe in a few years we will have a winner for the platform... > --dons > On Friday, March 11, 2011, Vo Minh Thu <noteed(a)gmail.com> wrote: > 2011/3/11 Victor Oliveira <rhapsodyv(a)gmail.com>: >> Hi cafe, >> >> There are a lot of http servers in hackage. I didn't have used none. >> I would like to know if one of them is something closer of the nginx. >> I need some light and fast. It don't need support all http, just the > basics is fine. >> Suggestions? > > Snap and Warp come to mind. Have a look at this reddit thread: > > http://www.reddit.com/r/programming/comments/flpao/the_haskell_high_perform… > > Cheers, > Thu To be clear, Yesod and Snap are web frameworks. If you want just a web server, then your main options are snap-server or Warp as has been pointed out. The happstack framework also has a server, but they are planning on switching it out in the future. As for speed, warp is at least twice as fast as snap-server. As for lightness- * snap-server and Warp support a similar feature set, with some differences that might be important, depending on your needs. * Warp has a much smaller code base * lightness may be a perception of how you interface with the server. I don't know how you would interface with snap-server except through the Snap framework. Warp can be used with Yesod, but runs on the WAI interface. The WAI inteface allows you to change your mind and deploy to a different server in the future. Hopefully it will also allow you to use different frameworks in the future, although this is not the case yet. But just as important, it is also designed to support easy and re-usable low-level web programming.

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cabal install yesod gives an error on ghc 7.02
by Vagif Hagverdiyev 12 Mar '11

12 Mar '11

cabal install yesod gives an error on ghc 7.02: http://hackage.haskell.org/trac/ghc/ticket/5004 One of the commenters on that ticcket says: "found that providing an unstripped version of libHsghc-7.0.2.a allows for successful linking. " Where can i get that unstripped version of libHsghc-7.0.2.a ?

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possible bug for ghc 7 + xcode 4 on snow leopard?
by Carter Schonwald 12 Mar '11

12 Mar '11

Hey All, As of installing xcode 4 earlier today, i'm getting errors of the form ld: library not found for -lcrt1.10.5.o collect2: ld returned 1 exit status for any haskell code that invokes ld in its cabal install process. 1) is this related to some problem in xcode / what the haskell code defaults to linking to? 2) is this something completely unrelated? thanks! -Carter

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Error while registering packages: cairo & glib
by Daniel Díaz 12 Mar '11

12 Mar '11

Hi, cafe, When I have updated my Haskell Platform version to 2011.2.0.0, and I try to install cairo or glib, during the registering I read the following: Registering glib-0.12.0... Installing library in C:\Documents and Settings\User\Datos de programa\cabal\glib-0.12.0\ghc-7.0.2 Registering glib-0.12.0... setup.exe: internal error: unexpected package db stack: [UserPackageDB] And then, the instalation fails. Searching in Google, I have found some links related: * http://www.mail-archive.com/haskell-cafe@haskell.org/msg81071.html * http://hackage.haskell.org/trac/hackage/ticket/731 But I never had this problem before. Thanks for any help, Daniel Díaz

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Reference monad
by Joshua Ball 12 Mar '11

12 Mar '11

Hi, Suppose I want the following functions: newRef :: a -> RefMonad (Ref a) readRef :: Ref a -> RefMonad a writeRef :: Ref a -> a -> RefMonad () for some appropriate data Ref = ... Obviously these functions are already satisfied by IORefs and STM. But if I wanted to implement my own (for fun)... would it be possible? Particularly, in a pure way, without unsafePerformIO? runRefMonad :: RefMonad a -> a I could try to do it with a state monad, and keep all of the refs in a Data.Map, but then I would have to solve the garbage collection problem, so that doesn't really work. Josh "Ua" Ball

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Flexible Wrappers - an Introduction
by Iain Alexander 12 Mar '11

12 Mar '11

There are three flexible wrappers provided in the initial flexiwrap package [1]. (More may be required in the future.) short long wraps FW FlexiWrap values FWT FlexiWrapT unary type constructors FWCTC FlexiWrapCTC binary operators which combine two unary type constructors The short names are frequently convenient since type signatures can get quite long. The suffixes are a simple encoding of the kind of the wrapped quantity, description of which I defer to a later post. The first parameter to each of these is a phantom - it plays no part in the implementation type, but serves as an index to generate multiple distinct types from each wrapper. In addition, by using a type-level list here, we can use it to specify any number of instance implementations. By way of a rather contrived but simple example, the module Data.Flex.SmallCheck.Wrap implements and uses an instance selector called FWEqFirst to implement an Eq instance which compares pairs by their first element. It is used there by defining a type FW (FWEqFirst :*: TNil) (Bool, Bool) which has the requisite properties. The initial proof-of-concept version of the package provides only a limited number of instance selectors (with the associated machinery) for each wrapper. FW (in Data.Flex.Wrap) has only an implementation for Eq instances, with FWDefaultEq to explicitly specify the default implementation which simply delegates to the underlying wrapped type. FWT (Data.Flex.WrapT) has FWTDefaultFunctor (Functor) FWTDefaultApplicative (Applicative) FWTDefaultMonadAll (the combination of FWTDefaultMonad and FWTDefaultMonadState) FWTDefaultMonad (Monad) FWTDefaultMonadState (MonadState) which all again delegate to the underlying type. There is also a separate module Data.Flex.WrappedMonad which provides selectors to implement Functor and Applicative instances for a Monad which lacks them. FWWrapMonad (the combination of FWMonadFunctor and FWMonadApplicative) FWMonadFunctor (Monad => Functor) FWMonadApplicative (Monad => Applicative) FWCTC (Data.Flex.WrapCTC) has FWCTCDefaultFunctor (Functor) FWCTCDefaultMonad (Monad) which simply delegate to the application of the wrapped binary operator to two constructor arguments, but provides underlying machinery for these together with MonadPlus and MonadTrans. Data.Flex.Compose contains the (:.) (alias O) type composition operator as found in e.g. the TypeCompose package [2]. It contains a fairly literal translation of Mark Jones and Luc Duponcheel's scheme for monad composition [3], FWCompP - use the "prod" construction FWCompD - use the "dorp" construction FWCompS - use the "swap" construction and selectors for MonadTrans and MonadPlus FWCompDefaults (the combination of FWCompTrans and FWCompMonadPlus) FWCompTrans (MonadTrans) FWCompMonadPlus (a synonym for FWCompMonadPlusR) FWCompMonadPlusR (using the MonadPlus instance of the right-hand argument of the composition) FWCompMonadPlusL (using the MonadPlus instance of the left-hand argument of the composition) Data.Flex.FlipT contains the operator FlipT which flips the arguments of an operator such as (:.). FWFlipDefaults (the combination of FWFlipMonad and FWFlipMonadPlus) FWFlipMonad (Monad) FWFlipMonadPlus (MonadPlus) These delegate the implementation to the underlying binary operator, which it wraps with FWCTC, passing the phantom selector list along. So, to recap the example given in the package release announcement: data FWStrict = FWStrict type Strict = FW (FWStrict :*: TNil) type StrictT = FWCTC (FWFlipDefaults :*: FWCompMonadPlusL :*: FWCompDefaults :*: FWCompS :*: TNil ) (FlipT O) Strict defines a (e.g. monad, but in general constructor) transformer StrictT. FWCTC is a flexible wrapper. Its first parameter is an HList-like type-level list of instance specifications. O is the type composition operator, and FlipT flips its arguments. Strict is a user-defined wrapper type. (This is strict vs. loose, not strict vs. lazy. It is intended to be used to wrap values of a data structure which satisfy certain criteria.) FWCompMonadPlusL specifies a particular MonadPlus instance (which delegates to the left operand of the composition, which is the right operand of the flipped composition, i.e. whatever argument you pass to StrictT), and FWCompS specifies the Jones/Duponcheel SComp construction [3]. The other two items in the list (apart from the TNil terminator) specify default implementations of other instances. [1] http://hackage.haskell.org/package/flexiwrap-0.0.1 [2] http://hackage.haskell.org/package/TypeCompose [3] http://web.cecs.pdx.edu/~mpj/pubs/RR-1004.pdf

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Need help to solve this question
by Chatura Roche 11 Mar '11

11 Mar '11

Both sections relate to the case study: Index for a document of text. SECTION A: Given the attached Haskell code which produces an index of words, make the following alterations by modifying existing functions and including new functions where necessary : 3) Treat a capitalised word (one or more capitals) as being different from the word in all lower case (but they should still be sorted alphabetically) – unless it is at the start of a sentence with only the initial letter capitalised. A sentence is terminated by a ‘.’, ‘?’ or ‘!’. import Prelude type Doc = String type Line = String type Word = String makeIndex :: Doc -> [ ([Int], Word) ] makeIndex = shorten . -- [([Int], Word)] -> [([Int], Word)] amalgamate .-- [([Int], Word)] -> [([Int], Word)] makeLists . -- [(Int, Word)] -> [([Int], Word)] sortLs . -- [(Int, Word)] -> [(Int, Word)] allNumWords .-- [(Int, Line)] -> [(Int, Word)] numLines . -- [Line] -> [(Int, Line)] splitUp -- Doc -> [Line] splitUp :: Doc -> [Line] splitUp [] = [] splitUp text = takeWhile (/='\n') text : -- first line (splitUp . -- splitup other lines dropWhile (==’\n’) . -- delete 1st newline(s) dropWhile (/='\n')) text -- other lines numLines :: [Line] -> [(Int, Line)] numLines lines -- list of pairs of = zip [1 .. length lines] lines -- line no. & line -- for each line -- a) split into words -- b) attach line no. to each word splitWords :: Line -> [Word] -- a) splitWords [] = [] splitWords line = takeWhile isLetter line : -- first word in line (splitWords . -- split other words dropWhile (not.isLetter) . -- delete separators dropWhile isLetter) line -- other words where isLetter ch = (‘a’<=ch) && (ch<=’z’) (‘A’<=ch) && (ch<=’Z’) numWords :: (Int, Line) -> [(Int, Word)] -- b) numWords (number, line) = map addLineNum ( splitWords line) -- all line pairs where addLineNum word = (number, word) -- a pair allNumWords :: [(Int, Line)] -> [(Int, Word)] allNumWords = concat . map numWords -- doc pairs sortLs :: [(Int, Word)] -> [(Int, Word)] sortLs [ ] = [ ] sortLs (a:x) = sortLs [b | b <- x, compare b a] -- sort 1st half ++ [a] ++ -- 1st in middle sortLs [b | b <- x, compare a b] -- sort 2nd half where compare (n1, w1) (n2, w2) = (w1 < w2) -- 1st word less (w1 == w2 && n1 < n2) -- check no. makeLists :: [(Int, Word)] -> [([Int], Word)] makeLists = map mk -- all pairs where mk (num, word) = ([num], word) -- list of single no. amalgamate :: [([Int], Word)] -> [([Int], Word)] amalgamate [ ] = [ ] amalgamate [a] = [a] amalgamate ((n1, w1) : (n2, w2) : rest)-- pairs of pairs | w1 /= w2 = (n1, w1) : amalgamate ((n2, w2) : rest) | otherwise = amalgamate ((n1 ++ n2, w1) : rest) -- if words are same grow list of numbers shorten :: [([Int], Word)] -> [([Int], Word)] shorten = filter long -- keep pairs >4 where long (num, word) = length word > 4 -- check word >4

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Uncatchable error
by Daniel Díaz 11 Mar '11

11 Mar '11

Hi, cafe, I'm working in a program where I use many connections with Network.HTTP. Sometimes, connections are closed while my program is reading them, and an error appears: <socket: XXX>: Data.ByteString.hGetLine: invalid argument (Bad file descriptor) All I need is to handle this error. The function 'catch' doesn't work. I guess this error comes from a call to 'error' function, or something similar. What I can do? Thanks in advance, Daniel Díaz

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