- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"
by Donn Cave 28 Jun '12

28 Jun '12

To be fair, while it's great that Functor et al. are available to help solve problems like that with a Tree data structure, in the present context it mostly shows that a trivial example was used to illustrate the problem. Unless I'm missing how this generic fold functionality solves the more general problem. Quoth Alberto G. Corona, > ... There is no such problem in imperative languages, which work > ever in an hardwired IO monad. Or to look at it from the other direction, the problem with Haskell is that it allows the programmer to write non-IO functions. To the extent that's a real problem, the solution would obviously be to exercise some restraint with the pure code and write more in the IO monad. If you will humor me, we understand that this code is equally satisfactory from a mathematical perspective and there's no crime in it. But does it help with the `flags problem?' Suppose that functions f and g both have IO types; f calls g, and g has optional behavior depending on some IO value computed previously, the flag. The way I understand it, I can accomplish this without passing the flag around: with an IORef. What's less clear to me is whether Haskell really supports the module level accessible IORef value I need. Also known as "top level mutable state." Is the NOINLINE pragma I've been using with GHC, really a sound and reliable practice that we're reasonably confident will be supported by other compilers? It feels to me like I'm pulling a weird trick, to do something that's trivial in other programming languages. Maybe that's part of the problem, that imperative programming receives something a little short of full support in Haskell, inasmuch as basic features like top level mutable state require relatively arcane incantations. Donn

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ghc-7.4 on CentOS-5.8 ?
by Johannes Waldmann 28 Jun '12

28 Jun '12

Dear all, I need a recent ghc on a not-so-recent (?) CentOS. The ghc binary package (7.2 or 7.4) does not work because of a mismatch in the libc version. ghc-7.0 is working but when I use it to compile 7.4, it breaks with some linker error (relocation R_X86_64_PC32 ...) it also suggests "recompile with -fPIC" but I don't see how. (In this particular case, I absolutely cannot change/update the OS.) Thanks, J.W.

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Fwd: Martin Odersky on "What's wrong with Monads"
by Strake 28 Jun '12

28 Jun '12

On 26/06/2012, Nathan Howell <nathan.d.howell(a)gmail.com> wrote: > On Tue, Jun 26, 2012 at 3:19 PM, Tillmann Rendel > <rendel(a)informatik.uni-marburg.de> wrote: >> All fine so far. Now, consider the following additional requirement: "If >> the >> command-line flag --multiply is set, the function amount computes the >> product instead of the sum." >> >> How would you implement this requirement in Haskell without changing the >> line "amount (Leaf x) = x"? > > One option is to encode the desired behavior at the type level. By > extended the data type slightly and adding a Functor instance, > selecting between a product and a sum can be done using their Monoid > newtypes: ... Better yet, use foldMap: > import Data.Monoid > import Data.Foldable > import System.Environment > data Tree a = Leaf a | Branch (Tree a) (Tree a) > instance Functor Tree where > f `fmap` Leaf x = Leaf (f x) > f `fmap` Branch x y = Branch (fmap f x) (fmap f y) > instance Foldable Tree where > foldMap f (Leaf x) = f x > foldMap f (Branch s t) = foldMap f s <> foldMap f t > main :: IO () > main = do > args <- getArgs > let val :: Tree Int > val = Branch (Leaf 8) (Leaf 18) > let getResult :: Tree Int -> Int > getResult = case args of > ["--multiply"] -> getProduct . foldMap Product > _ -> getSum . foldMap Sum > print . getResult $ val Yet better yet: > {-# LANGUAGE DeriveFunctor, DeriveFoldable #-} > import Data.Monoid; > import Data.Foldable; > import System.Environment > data Tree a = Leaf a | Branch (Tree a) (Tree a) > deriving (Functor, Foldable); > ... (^_^) -- Strake

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Data.Bitmap, readBitmap exception
by Alexander Foremny 28 Jun '12

28 Jun '12

Hello, I am using bitmap-0.2 [1] to load a Bitmap file [2]. However, the following sample GHCi session fails for me. > Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO (Bitmap Word8) > *** Exception: Data/Bitmap/Internal.hs:(63,17)-(67,15): Non-exhaustive patterns in case I downloaded the sources of bitmap-0.2 from Hackage and modified the relevant function by appending the last line. > decodeCType :: CInt -> PixelComponentType > decodeCType k = case k of > 1 -> PctWord8 > 2 -> PctWord16 > 3 -> PctWord32 > 4 -> PctFloat > _ -> error $ "decodeCType: unexpected integer (" ++ show k ++ ")" With this modification the same GHCi session given above results in the following error. > Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO (Bitmap Word8) > *** Exception: decodeCType: unexpected integer (7077888) I don't have any idea where 7077888 comes from. Am I doing something wrong? Is my bitmap corrupted in some way? The number 7077888 has no apparent significance except that it is a perfect cube [3]. So I guess this is not a problem with endianness. In fact as far as I can tell `decodeCType` is only called on `(PixelComponent t => c_type t)` and `c_type t` only yields integer values in the (inclusive) range of 1 to 4. That is if I am not missing some orphaned instance. Could anyone please enlighten me what is happening and if I am doing something wrong? In case this is relevant: I am using bitmap-0.2 from Hackage with GHC 7.4.1 on a 3.2.20 kernel i686 architecture Linux. Regards, Alexander Foremny [1] http://hackage.haskell.org/package/bitmap [2] https://www.dropbox.com/s/pyutvni9vx6f6mo/b.bmp [3] http://www.wolframalpha.com/input/?i=7077888

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Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"
by Tillmann Rendel 28 Jun '12

28 Jun '12

Hi Rico, Rico Moorman wrote: >> data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer) >> >> amount:: Tree -> Integer >> amount (Leaf x) = x >> amount (Branch t1 t2) = amountt1 + amountt2 >> >> [...] additional requirement: "If the command-line flag --multiply is set, >> the function amount computes the product instead of the sum." >> >> How would you implement this requirement in Haskell without changing the >> line "amount (Leaf x) = x"? > The (for me at least) most obvious way to do this would be, to make the > operation to be applied to determine the amount (+ or *) an explicit > parameter in the function's definition. > > data Tree a = Leaf a > | Branch (Tree a) (Tree a) > amount :: (a -> a -> a) -> Tree a -> a > amount fun (Leaf x) = x > amount fun (Branch t1 t2) = amount fun t1 `fun` amount fun t2 I agree: This is the most obvious way, and also a very good way. I would probably do it like this. > Which drawbacks do you see besides increased verbosity? Well, you did change the equation "amount (Leaf x) = x" to "amount fun (Leaf x) = x". In a larger example, this means that you need to change many lines of many functions, just to get the the value of fun from the point where it is known to the point where you need it. > [...] I am wondering which ways of doing this in Haskell you mean. I thought of the following three options, but see also Nathan Howells email for another alternative (that is related to my option (1) below): (1) Implicit parameters: {-# LANGUAGE ImplicitParams #-} data Tree = Leaf Integer | Branch Tree Tree amount :: (?fun :: Integer -> Integer -> Integer) => Tree -> Integer amount (Leaf x) = x amount (Branch t1 t2) = ?fun (amount t1) (amount t2) (2) Lexical Scoping: data Tree = Leaf Integer | Branch Tree Tree amount :: (Integer -> Integer -> Integer) -> Tree -> Integer amount fun = amount where { amount (Leaf x) = x ; amount (Branch t1 t2) = fun (amount t1) (amount t2) } (3) UnsafePerformIO: import System.IO.Unsafe (unsafePerformIO) data Tree = Leaf Integer | Branch Tree Tree amount :: Tree -> Integer amount (Leaf x) = x amount (Branch t1 t2) = fun (amount t1) (amount t2) where fun = unsafePerformIO ... I'm not happy with any of these options. Personally, I would probably go ahead and transform the whole program just to get the value of fun to where it is needed. Nevertheless, having actually done this before, I understand why Martin Odersky doesn't like doing it :) Tillmann

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Long-running request/response protocol server using enumerator/iterator/iterIO/pipes/conduits/...
by Nicolas Trangez 28 Jun '12

28 Jun '12

Hello Cafe, Some time ago I tried to implement a network service using iteratee (or enumerator, can't remember), but gave up in the end. More recently I wanted to create something similar (a similar protocol), but failed again. So I'm looking for some example code or something similar (Google only helped slightly). First of all, I don't care which API/library to use, I guess for my purpose all of enumerator, iteratee, iterIO, pipes, conduits,... are OK, so all feedback is welcome. Here's the catch. Most examples out there implement some server which accepts a single client request, interprets it, creates a response, returns this, and closes the connection (or something alike, think HTTP). The protocol I'd like to implement is different: it's long-running using repeated requests & responses on a single client connection. Basically, a client connects and sends some data to the server (where the length of this data is encoded in the header). Now the server reads & parses this (binary) data, sets up some initial state for this client connection (e.g. opening a file handle), and returns a reply. Now the client can send another request, server parses/interprets it using the connection state, sends a reply, and so on. Might sound easy (and actually it's pretty easy in most other languages I know, including an OCaml implementation), yet I fail to figure out how to get this done using some enumerator-style library. Thanks for any help, I'll most likely write up something if I get things working for future reference. Nicolas

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Re: [Haskell-cafe] Data.Bitmap, readBitmap exception
by Alexander Foremny 28 Jun '12

28 Jun '12

Hello Serge, thank you very much for you response. Unfortunately this does not enlighten me. But maybe this is helpful to someone else who could given an answer. Regards, Alexander Foremny 2012/6/28 Serge Le Huitouze <serge.lehuitouze(a)gmail.com>: > Hi Alexander, > >> >> I am using bitmap-0.2 [1] to load a Bitmap file [2]. However, the >> following sample GHCi session fails for me. >> >> (...) >>> Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO (Bitmap Word8) >>> *** Exception: decodeCType: unexpected integer (7077888) >> >> I don't have any idea where 7077888 comes from. Am I doing something >> wrong? Is my bitmap corrupted in some way? The number 7077888 has no >> apparent significance except that it is a perfect cube [3]. So I guess >> this is not a problem with endianness. > > Sorry, no personal experience with Data.Bitmap... > > In case it's a hint, decimal 7077888 is 0x006C0000, and 6C is ascii > code for "l". > > HTH (?). > > --Serge

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Template-haskell loading packages
by Christopher Done 28 Jun '12

28 Jun '12

'Ello, I'm using TH in a big project and whenever TH starts-up in the GHC (6.4.2) compilation process it loads a number of packages: [119 o 119] Compiling Main ( src/Main.hs, dist/build/eudl/eudl-tmp/Main.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package array-0.4.0.0 ... linking ... done. Loading package strict-0.3.2 ... linking ... done. Loading package split-0.1.4.3 ... linking ... done. Loading package safe-0.3.3 ... linking ... done. Loading package bytestring-0.9.2.1 ... linking ... done. Loading package deepseq-1.3.0.0 ... linking ... done. Loading package containers-0.4.2.1 ... linking ... done. Loading package binary-0.5.1.0 ... linking ... done. Loading package cereal-0.3.5.2 ... linking ... done. Loading package entropy-0.2.1 ... linking ... done. Loading package largeword-1.0.1 ... linking ... done. Loading package tagged-0.4.2.1 ... linking ... done. Loading package crypto-api-0.10.2 ... linking ... done. Loading package pureMD5-2.1.0.3 ... linking ... done. Loading package dlist-0.5 ... linking ... done. Loading package old-locale-1.0.0.4 ... linking ... done. Loading package data-default-0.4.0 ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package utf8-string-0.3.7 ... linking ... done. Loading package printf-mauke-3000.0.5.0.1 ... linking ... done. Loading package haskell-lexer-1.0 ... linking ... done. Loading package pretty-show-1.2 ... linking ... done. Loading package parallel-3.2.0.3 ... linking ... done. Loading package transformers-0.2.2.0 ... linking ... done. Loading package mtl-2.0.1.0 ... linking ... done. Loading package text-0.11.2.1 ... linking ... done. Loading package parsec-3.1.3 ... linking ... done. Loading package unix-2.5.1.1 ... linking ... done. Loading package network-2.3.0.14 ... linking ... done. Loading package old-time-1.1.0.0 ... linking ... done. Loading package HTTP-4000.2.3 ... linking ... done. Loading package base64-bytestring-0.1.1.1 ... linking ... done. Loading package blaze-builder-0.3.1.0 ... linking ... done. Loading package blaze-html-0.4.3.4 ... linking ... done. Loading package filepath-1.3.0.0 ... linking ... done. Loading package directory-1.1.0.2 ... linking ... done. Loading package syb-0.3.6.1 ... linking ... done. Loading package hs-bibutils-4.12 ... linking ... done. Loading package json-0.5 ... linking ... done. Loading package pandoc-types-1.9.1 ... linking ... done. Loading package time-1.4 ... linking ... done. Loading package xml-1.3.12 ... linking ... done. Loading package citeproc-hs-0.3.4 ... linking ... done. Loading package extensible-exceptions-0.1.1.4 ... linking ... done. Loading package regex-base-0.93.2 ... linking ... done. Loading package regex-pcre-builtin-0.94.2.1.7.7 ... linking ... done. Loading package highlighting-kate-0.5.1 ... linking ... done. Loading package process-1.1.0.1 ... linking ... done. Loading package random-1.0.1.1 ... linking ... done. Loading package tagsoup-0.12.6 ... linking ... done. Loading package temporary-1.1.2.3 ... linking ... done. Loading package texmath-0.6.0.6 ... linking ... done. Loading package digest-0.0.1.1 ... linking ... done. Loading package zlib-0.5.3.3 ... linking ... done. Loading package zip-archive-0.1.1.8 ... linking ... done. Loading package pandoc-1.9.4.1 ... linking ... done. Loading package convertible-1.0.11.1 ... linking ... done. Loading package HDBC-2.3.1.1 ... linking ... done. Loading package HDBC-postgresql-2.3.2.1 ... linking ... done. Loading package haskelldb-2.1.3 ... linking ... done. Loading package haskelldb-hdbc-2.1.0 ... linking ... done. Loading package haskelldb-hdbc-postgresql-2.1.0 ... linking ... done. Loading package QuickCheck-2.4.2 ... linking ... done. Loading package has-0.5.0.1 ... linking ... done. Loading package feed-0.3.8 ... linking ... done. Loading package MonadCatchIO-mtl-0.3.0.4 ... linking ... done. Loading package xhtml-3000.2.1 ... linking ... done. Loading package cgi-3001.1.8.2 ... linking ... done. Loading package fastcgi-3001.0.2.3 ... linking ... done. Loading package ranges-0.2.4 ... linking ... done. Loading package email-validate-0.2.8 ... linking ... done. Loading package digestive-functors-0.2.0.0 ... linking ... done. Loading package digestive-functors-blaze-0.2.1.0 ... linking ... done. Loading package curl-1.3.7 ... linking ... done. Loading package csv-0.1.2 ... linking ... done. Loading package MonadRandom-0.1.6 ... linking ... done. Loading package HUnit-1.2.4.3 ... linking ... done. Loading package hslogger-1.1.5 ... linking ... done. Loading package regex-posix-0.95.2 ... linking ... done. Loading package regex-compat-0.95.1 ... linking ... done. Loading package MissingH-1.1.1.0 ... linking ... done. Loading package ConfigFile-1.1.1 ... linking ... done. The TH is declared in a separate module, e.g. {-# LANGUAGE TemplateHaskell, QuasiQuotes #-} module Test where import Language.Haskell.TH alias :: String -> Name -> Q [Dec] alias name typ = do d <- newtypeD ctx conName [] con instances return [d] where instances = [''Show,''Num,''Eq,''Ord,''Enum,''Real,''Integral] ctx = return [] conName = mkName name con = recC conName [field] field = return (mkName ("un"++name), NotStrict, ConT typ) aliases :: Name -> [String] -> Q [Dec] aliases typ names = fmap concat $ mapM (flip alias typ) names if I put import Test aliases ''Int ["UserId","EventId"] in the Main module of my project and build the project, I get the above things being loaded. I'm not sure why the modules need to be dynamically linked considering they're not actually being used in the TH. And the module is clearly staged separate to the context in which it's used. I get that I can reify names to things, like “alias "Foo" ''SomePackage'sType” but those types exist at compile time, surely it can't require the actual packages? Can anyone explain? Ciao!

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Long-running request/response protocol server ...
by oleg＠okmij.org 28 Jun '12

28 Jun '12

Nicolas Trangez wrote > The protocol I'd like to implement is different: it's long-running using > repeated requests & responses on a single client connection. Basically, > a client connects and sends some data to the server (where the length of > this data is encoded in the header). Now the server reads & parses this > (binary) data, sets up some initial state for this client connection > (e.g. opening a file handle), and returns a reply. Now the client can > send another request, server parses/interprets it using the connection > state, sends a reply, and so on.'' That is very simple to implement in any Iteratee library; I will use IterateeM for concreteness. The desired functionality is already implemented, in decoding of chunk-decoded inputs. Your protocol is almost the same: read a chunk of data (tagged with its size), and do something about it. After the chunk is handled, read another chunk. The iteratee library takes care of errors. In particular, if the request handler finished (normally or with errors) without reading all of the chunk, the rest of the chunk is read nevertheless and disregarded. Otherwise, we deadlock. The complete code with a simple test is included. The test reads three requests, the middle of which causes the request handler to report an error without reading the rest of the request. module SeveralRequests where import IterateeM import Prelude hiding (head, drop, dropWhile, take, break, catch) import Data.Char (isHexDigit, digitToInt, isSpace) import Control.Exception import Control.Monad.Trans -- Tell the iteratee the stream is finished and write the result -- as the reply to the client -- If the iteratee harbors the error, write that too. reply :: MonadIO m => Iteratee el m String -> Iteratee el m () reply r = en_handle show (runI r) >>= check where check (Right x) = liftIO . putStrLn $ "REPLY: " ++ x check (Left x) = liftIO . putStrLn $ "ERROR: " ++ x -- Read several requests and get iter to handle them -- Each request is formatted as a single chunk -- The code is almost identical to IterateeM.enum_chunk_decoded -- The only difference is in the internal function -- read_chunk below. -- After a chunk is handled, the inner iteratee is terminated -- and we process the new chunk with a `fresh' iter. -- If iter can throw async errors, we have to wrap it -- accordingly to convert async errors into Iteratee errors. -- That is trivial. reply_chunk_decoded :: MonadIO m => Enumeratee Char Char m String reply_chunk_decoded iter = read_size where read_size = break (== '\r') >>= checkCRLF iter . check_size checkCRLF iter m = do n <- heads "\r\n" if n == 2 then m else frame_err (exc "Bad Chunk: no CRLF") iter check_size "0" = checkCRLF iter (return iter) check_size str@(_:_) = maybe (frame_err (exc ("Bad chunk size: " ++ str)) iter) read_chunk $ read_hex 0 str check_size _ = frame_err (exc "Error reading chunk size") iter read_chunk size = take size iter >>= \r -> checkCRLF r $ reply r >> reply_chunk_decoded iter read_hex acc "" = Just acc read_hex acc (d:rest) | isHexDigit d = read_hex (16*acc + digitToInt d) rest read_hex acc _ = Nothing exc msg = toException (ErrorCall $ "Chunk decoding exc: " ++ msg) -- If the processing is restarted, we report the frame error to the inner -- Iteratee, and exit frame_err e iter = throwRecoverableErr (exc "Frame error") (\s -> enum_err e iter >>= \i -> return (return i,s)) -- Test -- A simple request_handler iter for handling requests -- If the input starts with 'abc' it reads and returns the rest -- Otherwise, it throws an error, without reading the rest of the input. request_handler :: Monad m => Iteratee Char m String request_handler = do n <- heads "abc" if n == 3 then stream2list else throwErrStr "expected abc" test_request_handler :: IO () test_request_handler = run =<< enum_pure_1chunk input (reply_chunk_decoded request_handler >> return ()) where input = -- first request "6"++crlf++ "abcdef" ++ crlf++ -- second request "8"++crlf++ "xxxdefgh" ++ crlf++ -- third request "5"++crlf++ "abcde" ++ crlf++ "0"++crlf++ crlf crlf = "\r\n" {- *SeveralRequests> test_request_handler REPLY: def ERROR: expected abc REPLY: de -}

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Scribd's AI challenge has Haskell support
by aditya bhargava 27 Jun '12

27 Jun '12

Hey all, I work at scribd.com. A few weeks ago we had released an AI game. It allowed you to use Javascript to program a bot. I've been wanting to add Haskell as a supported language for some time, and it's finally done! I'm pretty excited since I think we are the first big company to do something like this. Supporting Haskell was trivial thanks to GHC 7.4's Safe Haskell extension. I hope more companies support Haskell in the future! Here's the game: *http://www.scribd.com/jobs/botrace* Here's the tutorial I wrote for Haskell: * http://egonschiele.github.com/robot-fruit-hunt/* Here's the Haskell API: * https://github.com/egonSchiele/robot-fruit-hunt/blob/master/haskell/Haskell… * Here's a sandbox that allows you to test bots locally: * https://github.com/egonSchiele/robot-fruit-hunt* Comments and feedback welcome! Adit -- adit.io

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