- Haskell-Cafe - Haskell.org

Haskell DB tutorial link is broken.
by Edward Ing 10 Aug '07

10 Aug '07

Hi, The following link is broken. http://www.haskell.org/hawiki/HaskellDbTutorial Edward Ing

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Derivation of Eq given Ord
by Dougal Stanton 10 Aug '07

10 Aug '07

Is there a reason why automatic derivation of Ord without Eq doesn't do "the sensible thing" and just derive Eq anyway? > newtype Id a = Id { a :: String } > deriving (Read, Show, Eq, Ord) > newtype Ego a = Ego { b :: String } > deriving (Read, Show, Ord) Both will type check, but if you try any (in)equality operators on the second they'll be spat back at you. > *Main> let e1 = Ego "" > *Main> let e2 = Ego "" > *Main> e1 < e2 > > <interactive>:1:0: > No instance for (Eq (Ego a)) > arising from use of `<' at <interactive>:1:0-6 > Possible fix: add an instance declaration for (Eq (Ego a)) > In the expression: e1 < e2 > In the definition of `it': it = e1 < e2 It doesn't seem *much* of a hardship, but it wasn't what I expected. I'm not used to GHC accepting input and silently dropping stuff it doesn't like... Cheers, D.

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Re: [Haskell-cafe] Problem with question 3 about knights andknavesonw ikipedia
by Peter Verswyvelen 10 Aug '07

10 Aug '07

Oh boy, never mind, after a good night sleep the solution is super obvious... Grrr what a waste of time ;-) The solution is in the sentence "the logician has then enough information to solve the problem", which I yesterday read like "the problem can now be solved", which is not the same. So the second answer must have been "Yes" otherwise the logician did not have enough information to solve the problem (because two cases still remain). Gee. But it was good Haskell practice ;-) >>("John is a knave ","Bill is a knight","Yes","Yes") >----- Oorspronkelijk bericht ----- >Van: Peter Verswyvelen [mailto:bf3@telenet.be] >Verzonden: donderdag, augustus 9, 2007 11:35 PM >Aan: steve(a)fenestra.com, haskell-cafe(a)haskell.org >Onderwerp: RE: [Haskell-cafe] Problem with question 3 about knights and knaves onw ikipedia > >Indeed, I missed that. This rules out the first answer is "no" > >But I still keep the 3 other solutions then :( > >>("John is a knight","Bill is a knight","Yes","No ") >>("John is a knave ","Bill is a knight","Yes","Yes") >>("John is a knave ","Bill is a knave ","Yes","No ") > >Any more help (or just the solution, I give up) is very welcome to help this >poor man in logic hell ;-) > >Oh well, it seems I'm getting too old for this stuff ;) > >-----Original Message----- >From: haskell-cafe-bounces(a)haskell.org >[mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Steve Schafer >Sent: Thursday, August 09, 2007 8:22 PM >To: haskell-cafe(a)haskell.org >Subject: Re: [Haskell-cafe] Problem with question 3 about knights and knaves >onw ikipedia > >On Thu, 9 Aug 2007 20:07:02 +0200, you wrote: > >>("John is a knight","Bill is a knight","Yes","No ") >>("John is a knave ","Bill is a knight","Yes","Yes") >>("John is a knave ","Bill is a knave ","Yes","No ") >> >>Anyone has an idea what I missed here? > >You're missing a key element of the problem: After John answers the >first question, the Logician doesn't have enough information to solve >the problem. Think about that for a second, and you will see the light. > >Steve Schafer >Fenestra Technologies Corp. >http://www.fenestra.com/ >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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In-place modification
by Andrew Coppin 10 Aug '07

10 Aug '07

I was wittering on about stream fusion and how great it is, and I got a message from Mr C++. (Mr C++ develops commercial games, and is obsessed with performance. For him, the only way to achieve the best performance is to have total control over every minute detail of the implementation. He sees Haskell is a stupid language that can never be fast. It seems he's not alone...) He said two things. The first was "it really perplexes me that Haskell insists that everything must be read-only". Naturally, I have a good answer to that one. (It's like being "perplexed" that relational databases refuse to keep table data in the same order. It's not to be awkward, it is a fundamental and important properly of the underlying theoretical foundation - the relational algebra, et al.) He also said what basically boils down to "being able to swap elements around in O(1) time and O(0) space is the whole thing that makes linked lists useful in the first place; take that away and it's rather pointless". I don't really have an answer to that one. (Lazyness and GC is probably going to kill most of the space cost. There's still a time cost - particularly the extra GC time...) I've asked this before and nobody answered, so I take it that nobody knows the answer... Does GHC *ever* do an in-place update on anything? Does the STG even have a command for that? I always assumed that the compiler tried to find instances where a new structure is created and the old one is no longer referenced, and make that be in-place. But now I'm not so sure...

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Re: [Haskell-cafe] Pure functional GUI (was
by Peter Verswyvelen 10 Aug '07

10 Aug '07

Well, if you want to make it as big as QT, yes. But even just a simple purely functionaly GUI framework that is part of the library would be nice, because right now it gives the impression it cannot be done in Haskell... E.g. Concurrent Clean seems to have a simple UI library. >----- Oorspronkelijk bericht ----- >Van: Duncan Coutts [mailto:duncan.coutts@worc.ox.ac.uk] >Verzonden: woensdag, augustus 8, 2007 09:36 PM >Aan: 'Peter Verswyvelen' >CC: 'Donn Cave', haskell-cafe(a)haskell.org >Onderwerp: re: [Haskell-cafe] Pure functional GUI (was "a regressive view of support for imperativeprogramming in Haskell") > >On Wed, 2007-08-08 at 19:14 +0000, Peter Verswyvelen wrote: > >> So could you please tell me more about the problem with pure >> functional GUIs and why this is not part of the Haskell library? I >> mean a GUI library completely written in Haskell, not wrapping a >> popular library. > >Partly because just getting the drawing and interaction behaviour to be >acceptable would be many person-years of work. > >GUI toolkits are not especially simple things. Even if you could do it >in just 1/10th of the number of lines of code of Gtk+ or Qt it'd be a >massive undertaking. > >Duncan > > >

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Re: [Haskell-cafe] Problem with question 3 about knights and knaves onw ikipedia
by Steve Schafer 10 Aug '07

10 Aug '07

On Thu, 9 Aug 2007 23:06:04 +0200, you wrote: >Is still don't get it completely... Could you give me an extra hint? I'm >getting crazy here, especially because I was really good at this stuff 20 >years ago! :) > >Here's the reasoning > >The first answer could not be "no" because from that I can infer that John >is a knight and Bill is a knave, which would mean the logician knows the >answer. > >This leaves me with 3 possibilities: > >a) Both John and Bill are knights >b) John is a knave and Bill could be anything Correct. But you forgot to recursively apply the hint. ;) The problem states that after John answers the second question, the Logician knows the solution. How can this be? What answer did John give that allows the Logician to solve the problem? Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/

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how can I get template haskell macro-expanded code from inferStartState? (repeated post, now with subject)
by Thomas Hartman 10 Aug '07

10 Aug '07

(sorry, forgot the subject on my first post) In the following code which uses template haskell, how can I get back the macro-expanded code generated from $(inferStartState ''MyState) I *can* recover the macro-expanded code for $(cnst 1 "x") using a debugging technique bulat describes on his tutorial at http://www.haskell.org/bz/th3.htm You can see what's going on in the function debugTemplates below. I'm trying to do this actually, to better understand how HAppS deals with state. It's a bit opaque now since the example on the tutorial uses TH. I think I would understand it better if I had code that didn't depend on TH. (MyState is from the happs tutorial at http://www.haskell.org/haskellwiki/HAppS_tutorial ) thanks! thomas. {-# OPTIONS -fglasgow-exts -fth #-} module MyState where import HAppS import HAppS.Protocols.SimpleHTTP2 import Data.Monoid import Data.Typeable import Control.Monad.State (get, put) import Language.Haskell.TH import Language.Haskell.TH.Syntax data MyState = MySt { appVal :: Int } deriving (Read, Show, Typeable) instance Serialize MyState where encodeStringM = defaultEncodeStringM decodeStringM = defaultDecodeStringM instance Monoid MyState where mempty = MySt 0 mappend (MySt x) (MySt y) = MySt (x+y) -- in ghci... -fth, :m + -- ghci... :t (inferStartState ''MyState) :: (Quasi m) => m [Dec] $(inferStartState ''MyState) -- boilerplate that will eventually be SYB -- ghci... :t cnst 1 "x" :: (Monad m) => m Exp cnst n s = return (LamE (replicate n WildP) (LitE (StringL s))) dumpSplice splice = runQ splice >>= putStrLn . pprint debugTemplates = do dumpSplice (cnst 1 "x") dumpSplice (inferStartState ''MyState) {- *MyState> debugTemplates \_ -> "x" Template Haskell error: Can't do `reify' in the IO monad *** Exception: user error (Template Haskell failure) -} --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Slow IO or bad code?
by Vimal 10 Aug '07

10 Aug '07

Hi I am practicing writing code in haskell, by solving problems at this site. http://spoj.pl. The problem http://spoj.pl/problems/FASHION , is pretty simple. 1. Given two lists A,B , of N numbers, sort them and take sum of products. i.e. Sum ai * bi I wrote a code, but seems to give "Time limit exceeded"! >> Beginning of CODE loop t function | t == 1 = do function | otherwise = do { function; loop (t - 1) function } prod [] [] = 0 prod (a:as) (b:bs) = a*b + prod as bs to_int :: [String] -> [Integer] to_int [] = [] to_int (x:xs) = (read x) : to_int xs doit = do n <- getLine a <- getLine b <- getLine let la = to_int (words a); lb = to_int (words b); in print (prod la lb) main = do t <- getLine loop (read t) doit << END OF CODE I would love to see if there is any improvement that can be done, to the code ... Thanks! Vimal

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Operator overloading
by rodrigo.bonifacio 10 Aug '07

10 Aug '07

Hi all. I want to overload the operator "^" for working instead of the following "+++" operator: (+++) :: String -> [[String]] -> [[String]] x +++ y = [ x:e | e<-y ] How can I overload the "^" operator? Thanks a lot. Rodrigo.

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(no subject)
by Thomas Hartman 10 Aug '07

10 Aug '07

In the following code which uses template haskell, how can I get back the macro-expanded code generated from $(inferStartState ''MyState) I *can* recover the macro-expanded code for $(cnst 1 "x") using a debugging technique bulat describes on his tutorial at http://www.haskell.org/bz/th3.htm You can see what's going on in the function debugTemplates below. I'm trying to do this actually, to better understand how HAppS deals with state. It's a bit opaque now since the example on the tutorial uses TH. I think I would understand it better if I had code that didn't depend on TH. (MyState is from the happs tutorial at http://www.haskell.org/haskellwiki/HAppS_tutorial ) thanks! thomas. {-# OPTIONS -fglasgow-exts -fth #-} module MyState where import HAppS import HAppS.Protocols.SimpleHTTP2 import Data.Monoid import Data.Typeable import Control.Monad.State (get, put) import Language.Haskell.TH import Language.Haskell.TH.Syntax data MyState = MySt { appVal :: Int } deriving (Read, Show, Typeable) instance Serialize MyState where encodeStringM = defaultEncodeStringM decodeStringM = defaultDecodeStringM instance Monoid MyState where mempty = MySt 0 mappend (MySt x) (MySt y) = MySt (x+y) -- in ghci... -fth, :m + -- ghci... :t (inferStartState ''MyState) :: (Quasi m) => m [Dec] $(inferStartState ''MyState) -- boilerplate that will eventually be SYB -- ghci... :t cnst 1 "x" :: (Monad m) => m Exp cnst n s = return (LamE (replicate n WildP) (LitE (StringL s))) dumpSplice splice = runQ splice >>= putStrLn . pprint debugTemplates = do dumpSplice (cnst 1 "x") dumpSplice (inferStartState ''MyState) {- *MyState> debugTemplates \_ -> "x" Template Haskell error: Can't do `reify' in the IO monad *** Exception: user error (Template Haskell failure) -} --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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