June 2008 - Haskell-Cafe

do and ifthenelse indentation in GHC
by Stefan Monnier 19 Jun '08

19 Jun '08

http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse says (and my memory seems to agree) that GHC has been changed to accept things like readnums f n = do eof <- hIsEOF f if eof then return n else do l <- hGetLine f readnums f (n + read l) where the `then' and `else' are aligned with the `if' rather than being slightly more indented as the Haskell 98 standard requires. Yet, when I try it with GHCi 6.8.2 I get an error: Prelude> :load "/home/monnier/tmp/foo.hs" [1 of 1] Compiling Main ( /home/monnier/tmp/foo.hs, interpreted ) /home/monnier/tmp/foo.hs:6:18: parse error (possibly incorrect indentation) Failed, modules loaded: none. Prelude> Does anybody know what's up with that? Stefan

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Burn ACM
by Achim Schneider 19 Jun '08

19 Jun '08

for monetary reasons. I just wanted my voice to be heard, you may pass on now. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

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reactive and <<loop>>
by stefan kersten 19 Jun '08

19 Jun '08

hi, i'm having problems with a very simple example using conal elliott's 'reactive' library: module Main where import Control.Applicative import Control.Concurrent import Control.Monad import Data.Reactive import System.Random main :: IO () main = do (e, snk) <- mkEvent forkIO $ forever ((getStdRandom random :: IO Double) >>= snk >> threadDelay 10000) runE (print `fmap` withPrevE e) return () which starts to output this after a while: reactive_loop.hs: <<loop>> this is with ghc 6.8.1 on osx 10.4. any ideas what might be going wrong? many thanks, <sk>

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(Lazy) SmallCheck and peano numbers
by Levi Stephen 19 Jun '08

19 Jun '08

Hi, I have the following definitions type Zero type Succ a so that I can muck around with a Vector type that includes its length encoded in its type. I was wondering whether it was possible to use SmallCheck (or QuickCheck) to generate random Peano numbers? Is there an issue here in that what I actually want to generate is a type rather than a value? I do have reifyInt :: Int -> (forall a. ReflectNum a => a -> b) -> b but, I'm not sure if this can help me when I need to generate other values based upon that type (e.g., two vectors with the same size type) Thanks, Levi

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network-related segfaults
by Brent Yorgey 19 Jun '08

19 Jun '08

Hi all, Has anyone else experienced segfaults related to network stuff? Observe: [11:50:56 brent@xenophon:~]$ ghci -ignore-dot-ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m +Text.HTML.Download Prelude Text.HTML.Download> txt <- openURL "http://www.google.com" Loading package bytestring-0.9.1.0 ... linking ... done. Loading package mtl-1.1.0.0 ... linking ... done. Loading package parsec-3.0.0 ... linking ... done. Loading package network-2.1.0.0 ... linking ... done. Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package tagsoup-0.6 ... linking ... done. Segmentation fault (core dumped) This just started happening yesterday. I think I may have just upgraded some packages via cabal-install but I'm not sure which ones. Now is a time when I wish all my terminal sessions were automatically logged. (Hmm, perhaps I should look into that...) -Brent

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Re: Bit Streams
by Dominic Steinitz 19 Jun '08

19 Jun '08

OK, so today I tried to write my first program using the Binary library. And I've hit a snag: It appears the library will only handle data that is byte-aligned. So if I try to write three Bool values, it uses three bytes, not three bits. Before I sit down and spend 3 months designing my own library from scratch, does anybody know of an existing library that allows you to do what Binary does, but with single-bit precision? [I presume Binary is byte-aligned for efficiency...] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict

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News of the day: C fails to stay self-hosting
by Achim Schneider 19 Jun '08

19 Jun '08

http://article.gmane.org/gmane.comp.gcc.patches/165063 -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

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Design suggestion for Data.Binary.Defer
by Neil Mitchell 19 Jun '08

19 Jun '08

Hi, I'm in the process of updating the Deferred Binary library, http://www-users.cs.york.ac.uk/~ndm/binarydefer/. The idea is that its does serialisation, but certain elements can be marked as deferred - instead of being written in the current file stream, they are merely pointed at and if needed, that pointer will be followed. Example:("hello","world"), where the first field is marked as deferred would write out: [6]"world""hello" i.e. [skip 6 characters if you want hello], the "world" data, the "hello" data we previously promised to put here. When reading, the "hello" would only be seeked to and read if necessary. So, its like binary, but some fields are lazy. The question is how to indicate which fields are lazy. There are three schemes I can think of: == Simple Instances == put (a,b) = putDefer a >> put b get = do a <- getDefer; b <- get; return (a,b) If the put/get and putDefer/getDefer items do not line up perfectly it will go very wrong at runtime - probably resulting in random values being created. You also can't derive the instances automatically, with something like Derive or DrIFT. == Complex Instances == This is the current scheme, based on lazy pattern matching and exceptions - very confusing, probably low performance. deferBoth = [\~(a,b) -> unit (,) <<~ a << b] Where <<~ a means write out the a field lazily, and << b means write out the b field strictly. The advantage over the simple instances is that a field being deferred is declared once. == Lazy Data Type == Instead of customizing the instance, you can write a data Defer a = Defer a type, and then instead of the original tuple write: (Defer "hello","world") But now the code must unwrap the Defer before accessing "hello", but the instance becomes much simpler, and can be derived. == The Question == Is there a simple way of tagging fields in a constructor as deferred, just once for reading and writing, and ideally outside the instance definition and not requiring additional code to unwrap? I can't think of any, but there may be something I have missed. Thanks Neil

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ANN: dataenc 0.11 and omnicodec 0.2
by Magnus Therning 19 Jun '08

19 Jun '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 A few days ago I uploaded dataenc 0.11[1] and omnicode 0.2[2]. The short description of the (most visible) changes would be the added support for Base85 encoding and decoding in dataenc. The dataenc package (version 0.11) Data encoding library currently providing Uuencode, Base64, Base64Url, Base32, Base32Hex, Base16, and Base85. Other versions0.9, 0.10.1, 0.10.2 Dependenciesbase LicenseLGPL CopyrightMagnus Therning, 2007 AuthorMagnus Therning Maintainermagnus(a)therning.org CategoryCodec Exposed modulesCodec.Binary.DataEncoding, Codec.Binary.Base16, Codec.Binary.Base32Hex, Codec.Binary.Base32, Codec.Binary.Uu, Codec.Binary.Base64Url, Codec.Binary.Base64, Codec.Binary.Base85 Upload dateThu Jun 12 15:18:19 PDT 2008 Uploaded byMagnusTherning Built onghc-6.8 The omnicodec package (version 0.2) Two simply command line tools built on dataenc. Other versions0.1 Dependenciesbase, dataenc (>=0.11), directory, filepath, haskell98 LicenseGPL CopyrightMagnus Therning, 2007 AuthorMagnus Therning Maintainermagnus(a)therning.org CategoryCodec Executablesodec, oenc Upload dateThu Jun 12 15:19:07 PDT 2008 Uploaded byMagnusTherning Built onghc-6.8 /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc [2]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/omnicodec -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkhaKtsACgkQiMWTaatN+6Ru6wCdFBrMfmFWbqTbkWhml8idgbb7 e8sAoN58Xil4ZnbFmSH7xPUtrPUCyZq9 =vlsA -----END PGP SIGNATURE-----

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Haskell's type system
by Ron Alford 18 Jun '08

18 Jun '08

I'm trying to wrap my head around the theoretical aspects of haskell's type system. Is there a discussion of the topic separate from the language itself? Since I come from a rather logic-y background, I have this (far-fetched) hope that there is a translation from haskell's type syntax to first order logic (or an extension there-of). Is this done? Doable? -Ron

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