July 2013 - Beginners

cps in yaht
by Deng Wu 13 Jul '13

13 Jul '13

hi, everyone, I'm learning haskell by reading the book <<Yet Another Haskell Tutorial>>, and I encounter a problem when comes to the contiuation passing style. The book gives a cps fold like: cfold’ f z [] = z cfold’ f z (x:xs) = f x z (\y -> cfold’ f y xs) and gives the test result: CPS> cfold (+) 0 [1,2,3,4] 10 CPS> cfold (:) [] [1,2,3] [1,2,3] but, when I try to test this, I find there is a problem, the ghci gives: ----------------------------------------------------------------------- *Main> cfold (+) 0 [] <interactive>:8:7: Occurs check: cannot construct the infinite type: t10 = (t10 -> t10) -> t10 Expected type: t10 -> t10 -> (t10 -> t10) -> t10 Actual type: t10 -> t10 -> t10 In the first argument of `cfold', namely `(+)' In the expression: cfold (+) 0 [] In an equation for `it': it = cfold (+) 0 [] ------------------------------------------------------------------------ It makes sense to me, so I change the definition of cps to something like this: cfold f z [] = z cfold f z (x:xs) = (\y -> cfold f y xs) (f x z) And it works fine: *Main> cfold (+) 0 [1,2,3] 6 So my question comes, is it a bug in the book or something I miss here? Regards! - wudeng

5 5

(no subject)
by Francisco Gutiérrez 13 Jul '13

13 Jul '13

I jio ONU O J Enviado desde mi iPhone

1 0

Re: [Haskell-beginners] Example using gmapQ
by Adrian May 13 Jul '13

13 Jul '13

Well I meant it as (gmapQ show) thing and in the meantime I found out that I need to replace show with gshow, and AFAICT, type classes are incompatible with generic stuff. Adrian. On 13 July 2013 19:27, Dilawar Singh <dilawars(a)iitb.ac.in> wrote: > Comment out the last line (main = etc...) and do this in ghci, > :t (show thing) > > Type of (show thing) is String. This is input to gmapQ function which has > following type. > > :t gmapQ gmapQ :: Data a => (forall d. Data d => d -> u) -> a -> [u] > > First argument to this function has the type (d -> u). Surely, this can't > be String. > > -- > Dilawar EE, IITB > On Sat, Jul 13, 2013 at 04:48:49PM +0800, Adrian May wrote: > >> Hi All, >> >> I got this far: >> >> {-# LANGUAGE OverloadedStrings #-} >> {-# LANGUAGE DeriveDataTypeable #-} >> import Data.Text >> import Data.Typeable >> import Data.Data >> >> data Thing = Thing { foo :: Int, bar :: String} >> deriving (Read, Show, Typeable, Data) >> thing :: Thing >> thing = Thing 1 "wop" >> con = toConstr thing >> fields = constrFields con >> main = putStrLn $ show con ++ show fields ++ ( Prelude.concat $ gmapQ show >> thing ) >> >> But it's barfing like this: >> >> Could not deduce (Show d) arising from a use of `show' >> from the context (Data d) >> bound by a type expected by the context: Data d => d -> [Char] >> at w.hs:76:65-80 >> >> I can see why, but not how to fix it. >> >> Any help much appreciated, >> Adrian. >> > > ______________________________**_________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/**mailman/listinfo/beginners<http://www.haskell.org/mailman/listinfo/beginners> >> > > > ______________________________**_________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/**mailman/listinfo/beginners<http://www.haskell.org/mailman/listinfo/beginners> >

1 0

Example using gmapQ
by Adrian May 13 Jul '13

13 Jul '13

Hi All, I got this far: {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE DeriveDataTypeable #-} import Data.Text import Data.Typeable import Data.Data data Thing = Thing { foo :: Int, bar :: String} deriving (Read, Show, Typeable, Data) thing :: Thing thing = Thing 1 "wop" con = toConstr thing fields = constrFields con main = putStrLn $ show con ++ show fields ++ ( Prelude.concat $ gmapQ show thing ) But it's barfing like this: Could not deduce (Show d) arising from a use of `show' from the context (Data d) bound by a type expected by the context: Data d => d -> [Char] at w.hs:76:65-80 I can see why, but not how to fix it. Any help much appreciated, Adrian.

2 1

running shelly script from root's crontab
by Miguel Negrão 12 Jul '13

12 Jul '13

Hi I have a shelly script that runs fine from my user account which I would to run every night. It needs to sudo to call some other processes so I need to add it to the root's crontab (sudo crontab -e). Unfortunately the script will fail to run because it can't find several modules, since it's running from the root account. Is there a way to make this work without having to install the packages globally (sudo cabal install --globally) ? I just tried that and manage to completely ruin my haskell installation, so I had to revert to previous backup. thanks, -- Miguel Negrão http://www.friendlyvirus.org/miguelnegrao

3 3

Up-to-date HaskellDB sample or tute
by Adrian May 10 Jul '13

10 Jul '13

Hi All, All the tutorials and samples I can find for HaskellDB seem to be bitrotten. Does anybody know of a newish one? I just want to connect to a postgres DB and run some simple queries. Or is HaskellDB superseded now? TIA, Adrian.

2 2

Cabal hell with haskelldb-th and haskelldb-hdbc-postgresql
by Adrian May 10 Jul '13

10 Jul '13

Hi All, Help! Why is this happening... ─ cabal install haskelldb-hdbc-postgresql Resolving dependencies... In order, the following would be installed: convertible-1.0.11.1 (new package) HDBC-2.3.1.2 (new package) HDBC-postgresql-2.3.2.1 (new package) haskelldb-2.2.2 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 haskelldb-hdbc-2.2.2 (new package) haskelldb-hdbc-postgresql-2.1.2 (new package) *cabal: The following packages are likely to be broken by the reinstalls: haskelldb-th-2.1.0 *Use --force-reinstalls if you want to install anyway. cabal install --force-reinstalls haskelldb-hdbc-postgresql ... Installed haskelldb-hdbc-postgresql-2.1.2 ─ cabal install haskelldb-th Resolving dependencies... In order, the following would be installed: haskelldb-2.2.2 (reinstall) changes: mtl-2.1.1 -> 2.0.1.0 haskelldb-th-2.1.0 (reinstall) changes: haskelldb-2.2.2 added *cabal: The following packages are likely to be broken by the reinstalls: haskelldb-hdbc-postgresql-2.1.2 haskelldb-hdbc-2.2.2 *Use --force-reinstalls if you want to install anyway. Adrian.

1 0

Getting warp and blaze to like each other
by Adrian May 10 Jul '13

10 Jul '13

Hi All, I just cobbled together the code below from a couple of samples, but got the types matched up by trial and error. I don't really understand when things are getting converted between lazy, strict, utf8, ascii, etc. I don't want ascii in the served page at all. Is it optimal? TIA, Adrian {-# LANGUAGE OverloadedStrings #-} module Main where import Network.Wai (Application, Response (ResponseBuilder)) import Network.HTTP.Types (status200) import Network.HTTP.Types.Header (hContentType, hContentLength, hConnection) import Network.Wai.Handler.Warp (run) import Blaze.ByteString.Builder (fromByteString, fromLazyByteString) import qualified Data.ByteString.Char8 as BS (pack, length) import qualified Text.Blaze.Html5 as H import qualified Text.Blaze.Html5.Attributes as A import Text.Blaze.Html.Renderer.Utf8 import qualified Data.ByteString.Lazy as LB application:: Application application _ = return $ ResponseBuilder status200 [(hContentType, BS.pack "text/html"), (hContentLength, BS.pack bodyLen), (hConnection, BS.pack "keep-alive")] $ fromLazyByteString body where body = root bodyLen = show. LB.length $ body root = renderHtml rooth rooth :: H.Html rooth = H.docTypeHtml $ do H.body $ do H.h1 "Hello" main:: IO () main = run 8080 application

2 3

Filter to a list of function
by Igor Pinheiro Leão 07 Jul '13

07 Jul '13

Hi Guys, sorry in upsetting you again. Is there a way in which I can filter one list of function, one function at time, to a list of elements, declaring this and only this function. It would be exactly like this: specifcFilter :: [(a->Bool)] -> [a] -> [a] where for each element of type 'a' on the second list it would exist a function on the first list that would be applied to it filtering. Kind regards, Igor -- Igor Vinícius Graduando em Ciência da Computação

4 6

FP course @ NICTA
by Tony Morris 07 Jul '13

07 Jul '13

Hello, The National ICT of Australia is offering a two day functional programming course in Sydney, Australia in late August. There is no cost involved. If you are interested, please join the mailing list linked below and answer a couple of questions. We will be responding to expressions of interest later this week. Thanks! https://groups.google.com/forum/#!msg/nicta-fp/rQLXgvg9SQY/YnkIMg3OVFkJ -- Tony Morris http://tmorris.net/

1 0