Re: [Haskell-cafe] [reactive] A pong and integrate
I that the saw sleep time at each loop is fixed (0.02). So game speed will depend on processor speed, since with a more powerful CPU frames will be computed quicklier? Yes, that's how it works. So we don't have (with the Simple branch) some way to say I want my sprite to move 100 pixels *per second* on the left, except if we provide ourselves a time signal? That's exactly the case, although I'd probably provide a delta time signal. The Simple version is a discrete stream library, and it doesn't pretend to have continuous-time abstractions. Gergely -- http://www.fastmail.fm - Access your email from home and the web ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal-install
2010/5/19 Ivan Lazar Miljenovic ivan.miljeno...@gmail.com:
Serguey Zefirov sergu...@gmail.com writes:
Why there is no switch to turn off any use of proxy in cabal-install?
Or to supply username/password pair in command line.
I have a strange situation: wget works like charm ignoring proxy (I
downloaded Cabal and cabal-install to investigate the problem using
wget), Firefox works like charm igoring proxy, and only cabal cannot
access anything, because it follows proper proxy protocol and our
proxy requires username and password.
Ignoring proxy?
Yes, they go right to internet. Firefox set to No proxy connection type.
And I don't know of proper way to supply them through http_proxy
environment variable.
export http_proxy=http://${username}:${passwo...@${proxy_url};
I tried it and it didn't work. I don't know reason, though, maybe it
was because my current password not entirely alphanumeric.
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[Haskell-cafe] TagSoup 0.9
Hello Neil , I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this error: TQ\TagSoup\TagSoupExtensions.lhs:29:17: `Tag' is not applied to enough type arguments Expected kind `*', but `Tag' has kind `* - *' In the type synonym declaration for `Bundle' Failed, modules loaded: TQ.Common.TextAndListHandling. where line 29 is the type declaration for 'bundle' in the following code: module TQ.TagSoup.TagSoupExtensions where import TQ.Common.TextAndListHandling import Text.HTML.TagSoup import Text.HTML.Download import Control.Monad import Data.List import Data.Char type Bundle = [Tag] [snip] tagsOnPage :: String - IO(String) tagsOnPage url = do tags - liftM parseTags $ openURL url let results = unlines $ map(show) $ tags return (results) extractTags :: Tag - Tag - [Tag] - [Tag] extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= fromTag ) tags extractTagsBetween :: Tag - [Tag] - [Tag] extractTagsBetween _ [] = [] extractTagsBetween markerTag tags = if startTags == [] then [] else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail startTags) where startTags = dropWhile (~/= markerTag ) tags I need to repair this code quickly. I am hoping you can quickly help me resolve this. Thanks. Ralph Hodgson, @ralphtq http://twitter.com/ralphtq ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: What do _you_ want to see in FGL?
Ivan Lazar Miljenovic wrote: Heinrich Apfelmus writes: Yes; what I mean is that you can retrofit a custom vertex type to any graph implementation that uses a fixed vertex type. So, let's say that data Gr a b = .. -- graph with vertex type Vertex Gr = Int then type Gr' node a b = CustomVertex node Gr a b data CustomVertex node gr a b = CV (gr a b) (Map node (Vertex gr)) is a graph with custom vertex type node . Sounds like it's more complicated than it's worth tbh ;-) Yup. ;) Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal-install
Serguey Zefirov sergu...@gmail.com writes:
export http_proxy=http://${username}:${passwo...@${proxy_url};
I tried it and it didn't work. I don't know reason, though, maybe it
was because my current password not entirely alphanumeric.
Shouldn't matter as long as you put it within quotes.
--
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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Re: [Haskell-cafe] TagSoup 0.9
Neil says that the API of TagSoup changed in 0.9. All usages of the type Tag should now take a type argument, e.g. Tag String. Regards, Malcolm On Wednesday, May 19, 2010, at 08:05AM, Ralph Hodgson rhodg...@topquadrant.com wrote: ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Bug with [Double]
Hello all, It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have Prelude [1,1+2/3..10] [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327] -- It is a bug! Prelude [1,5/3..10] [1.0,1.6667,2.3335,3.0,3.6665,4.333,5.0,5.667,6.334,7.001,7.668,8.336,9.004,9.671] -- correct, but... Prelude [1,5/3..4] [1.0,1.6667,2.3335,3.0,3.6665,4.333] -- ... wrong again Prelude [1,1+2/3..10] :: [Float] [1.0,1.667,2.335,3.002,3.67,4.34,5.01,5.68,6.35,7.02,7.69,8.36,9.03,9.7] -- correct Prelude [1,1+2/3..10] :: [Double] [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327] -- wrong Any comments? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
Dmitry Olshansky wrote: It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have snip Any comments? The problem you point out is not a problem with Haskell, but a problem with the whole concept of floating point arithmetic as implemented on all modern CPUs. See: http://docs.sun.com/source/806-3568/ncg_goldberg.html You would have got similar problems with just about any language running on the same hardware. Erik -- -- Erik de Castro Lopo http://www.mega-nerd.com/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
Dmitry Olshansky olshansk...@gmail.com writes: Hello all, It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have Prelude [1,1+2/3..10] [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327] -- It is a bug! No it isn't, because of the dodgy Ord instance for Float and Double values. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
2010/5/19 Erik de Castro Lopo mle...@mega-nerd.com:
Dmitry Olshansky wrote:
It seems that I saw something like this in Cafe recevtly. But I am not
sure...
In GHC 6.12.1 (Platform 2010 on Windows Vista) I have
snip
Any comments?
The problem you point out is not a problem with Haskell, but a problem
with the whole concept of floating point arithmetic as implemented on
all modern CPUs. See:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
You would have got similar problems with just about any language running
on the same hardware.
This is what used for Double list generation (Haskell Platform 2010):
--
numericEnumFromThenTo :: (Ord a, Fractional a) = a - a - a - [a]
numericEnumFromThenTo e1 e2 e3
= takeWhile predicate (numericEnumFromThen e1 e2)
where
mid = (e2 - e1) / 2
predicate | e2 = e1 = (= e3 + mid)
| otherwise = (= e3 + mid)
--
So normal C loop like for {double i = 1; i = 10; i += 1+2/3) {
insert_list(i); } won't generate the same list, as Haskell does in
[1,1+2/3..10].
PS
Rationals:
Prelude [1,1+2/3..10] :: [Rational]
[1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
3,25 % 3,9 % 1,29 % 3,31 % 3]
Same result.
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Re: [Haskell-cafe] Bug with [Double]
Ivan Lazar Miljenovic ivan.miljeno...@gmail.com writes: Dmitry Olshansky olshansk...@gmail.com writes: Hello all, It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have Prelude [1,1+2/3..10] [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327] -- It is a bug! No it isn't, because of the dodgy Ord instance for Float and Double values. And by Ord, I of course mean Enum... -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type famillies Lifting IO
I started playing with type families. I wanted to achieve, for the beginning, something like: import qualified Control.Monad.IO.Class as IOC import Control.Monad.Trans.Class import Control.Monad.Trans.Cont import Data.Functor.Identity class (Monad m, Monad (IO' m)) = MonadIO m where type IO' m :: * - * liftIO :: IO a - IO' m a liftM :: m a - IO' m a It allows to add IO to computation even if computation originally was 'pure'. First step was easy: instance MonadIO Identity where type IO' Identity = IO liftIO = id liftM = return . runIdentity instance MonadIO IO where type IO' IO = IO liftIO = id liftM = id instance MonadIO (ST r) where type IO' (ST r) = IO liftIO = id liftM = unsafeSTToIO --instance IOC.MonadIO m = MonadIO m where --type IO' m = m --liftIO = IOC.liftIO --liftM = id However I run into problems - this code doesn't want to compile: instance MonadIO m = MonadIO (ContT r m) where type IO' (ContT r m) = ContT r (IO' m) liftIO f = ContT $ \cont - liftIO f = cont liftM f = ContT $ \cont - liftM f = cont Or this: instance MonadIO m = MonadIO (ContT r m) where type IO' (ContT r m) = ContT r (IO' m) liftIO f = lift . liftIO liftM f = lift . liftIO In fact there is strange interfering types of ghci: ghci :t lift . liftIO lift . liftIO :: (m ~ IO' m1, MonadTrans t, Monad m, MonadIO m1) = IO a - t m a ghci :t lift . liftIO :: (m ~ IO' m1, MonadTrans t, Monad m, MonadIO m1) = IO a - t m a interactive:1:7: Couldn't match expected type `IO' m' against inferred type `m1' `m1' is a rigid type variable bound by an expression type signature at interactive:1:18 NB: `IO'' is a type function, and may not be injective In the second argument of `(.)', namely `liftIO' In the expression: lift . liftIO :: (m ~ (IO' m1), MonadTrans t, Monad m, MonadIO m1) = IO a - t m a What's the problem? I guess I don't understand something basic about type famillies. Regards signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
But Prelude Data.List [1,1+2/3..4] :: [Double] [1.0,1.6665,2.333,2.9996,3.666,4.332] Prelude Data.List unfoldr (\n - let n'=n+2/3 in if n' = 4 then Just (n',n') else Nothing) 1 :: [Double] [1.6665,2.333,2.9996,3.666] Prelude Data.List takeWhile (=4) $ iterate (+2/3) 1 :: [Double] [1.0,1.6665,2.333,2.9996,3.666] How 'dodgy' it should be to produce different result? How [a,b..c] works in this case? 2010/5/19 Ivan Lazar Miljenovic ivan.miljeno...@gmail.com: Dmitry Olshansky olshansk...@gmail.com writes: Hello all, It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have Prelude [1,1+2/3..10] [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327] -- It is a bug! No it isn't, because of the dodgy Ord instance for Float and Double values. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
Thanks, it's clear now.
2010/5/19 Serguey Zefirov sergu...@gmail.com:
2010/5/19 Erik de Castro Lopo mle...@mega-nerd.com:
Dmitry Olshansky wrote:
It seems that I saw something like this in Cafe recevtly. But I am not
sure...
In GHC 6.12.1 (Platform 2010 on Windows Vista) I have
snip
Any comments?
The problem you point out is not a problem with Haskell, but a problem
with the whole concept of floating point arithmetic as implemented on
all modern CPUs. See:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
You would have got similar problems with just about any language running
on the same hardware.
This is what used for Double list generation (Haskell Platform 2010):
--
numericEnumFromThenTo :: (Ord a, Fractional a) = a - a - a - [a]
numericEnumFromThenTo e1 e2 e3
= takeWhile predicate (numericEnumFromThen e1 e2)
where
mid = (e2 - e1) / 2
predicate | e2 = e1 = (= e3 + mid)
| otherwise = (= e3 + mid)
--
So normal C loop like for {double i = 1; i = 10; i += 1+2/3) {
insert_list(i); } won't generate the same list, as Haskell does in
[1,1+2/3..10].
PS
Rationals:
Prelude [1,1+2/3..10] :: [Rational]
[1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
3,25 % 3,9 % 1,29 % 3,31 % 3]
Same result.
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Re: [Haskell-cafe] Bug with [Double]
On 19/05/2010, at 19:24, Dmitry Olshansky wrote: Prelude [1,1+2/3..10] [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327] -- It is a bug! Unfortunately, it isn't. Section 6.3.4 of the Haskell report says: For Float and Double, the semantics of the enumFrom family is given by the rules for Int above, except that the list terminates when the elements become greater than e3+i/2 for positive increment i, or when they become less than e3+i/2 for negative i. In this case, i = 2/3 so the last value in the list is 10+1/3. The same applies to the other examples. Personally, I consider the Enum class itself to be broken. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
Roman Leshchinskiy r...@cse.unsw.edu.au writes: Personally, I consider the Enum class itself to be broken. Oh? In what sense? It seems to work fine for data types representing bounded enumerable values with a proper mapping to/from Int (it's not bijective since there's no proper mapping from Int - Bool for example); the problem is the fact that Double, etc. are made instances of Enum when this is not the case for those data types. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
On 19/05/2010, at 20:36, Ivan Lazar Miljenovic wrote: Roman Leshchinskiy r...@cse.unsw.edu.au writes: Personally, I consider the Enum class itself to be broken. Oh? In what sense? Firstly, the enumFrom* family of functions shouldn't be methods and the class itself should provide enough facilities for implementing them generically. GHC, for instance, specialises them for all primitive numeric types just to get foldr/build fusion to work. That shouldn't be necessary and doesn't help with overloaded code anyway. For instance, this generates an intermediate list: foo :: Enum a = a - a - [Int] foo a b = [fromEnum x | x - [a..b]] It's even worse when you want to implement similar functionality for other data structures. In vector, I basically had to duplicate all those specialisations to get decent performance. The generic case is horribly inefficient: enumFromTo x y = fromList [x .. y] There is no other sensible definition. Secondly, it should be possible to compute the length and the nth element of [a..b] in constant time. At the moment, it's impossible to distribute [a..b] efficiently across multiple threads - you have to generate the entire list first and then split it into chunks. It's completely unclear to me what [:a .. b:] should mean in DPH, for instance. So basically, Enum only provides enough functionality to desugar [a..b] and friends and even here, it doesn't interact well with fusion. Of course, these concerns weren't relevant back when the class was designed. But it is really broken now, IMO. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Numerical Analysis
SAGE is the kind of thing that I dreamed to have available online a few years ago. To recode everithing in haskell perhaps does not worth the pain, but perhapts it would be nice to do something similar to SAGE in an advanced environment such is Google Wave, with all the collaborative facilities for free. Perhaps this would attract some developers. The utility of the whole thing perhaps would incentivate the integration of existing haskell math software and to translate/integrate foreign code. Cheers 2010/5/16 Pierre-Etienne Meunier pierreetienne.meun...@gmail.com Hello Cafe, Being a complete beginner in the field of numerical analysis, but anyway needing it to solve real problems, I wrote a few functions recently to solve systems of polynomial equations using the projected polyhedron method by Maekawa and Patrikakalis. This requires solving systems of linear equations precisely, thus the simple Gauss method was not enough, and I had to write also an algorithm for the SVD decomposition. Upon discovering the algol / fortran specifications of these :-( , heavily published in important journals, I thought it would be nice to provide the world with fast reliable implementation of these numerical methods (i.e. not simply bindings to lapack). Moreover, writing numerical things in haskell is much more pleasant than I thought at first. Here are a few random thoughts on this : - The haskell 98 norm does not require enough about IEEE-754 compliance, thus C bindings are still needed to guess for instance the machine epsilons, or manipulating ULPs. Moreover, taking advantage of hardware rounding is not easy, even if the hardware is IEEE-compliant : calling a C function from haskell screws up the speed advantages of hardware rounding, for instance. Maybe the new LLVM backend will make this possible ? - The current Array library is definitely not adapted to production code. It makes debugging tricky, requires a heavy use of Debug.Trace to actually see what happens, and does not seem as fast as one could expect. It seems that each algebra library on hackage redefines part of it, but a unified version would be nice : a discussion within the haskell community seems to be needed... - A numerical analysis library should really take advantage of the parallelism in GHC, especially with the arrival of hardware such as fermi (anyway, I do not know how much haskell is compilable to fermi code). The love for loops and side-effects among this community is hard to understand, but that's more of a cultural problem. Finally, as stated by William Stein, the creator of SAGE, of course it would take thousands of man-years to rewrite these codes in python, but if a language like haskell, and a compiler do 90% of the work, how many man-years are left ? If anyone here has got the time, the team and the will to start such a project, I'd love to contribute ! Cheers, PE ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Proof format
Is this how a rigorous Haskeller would lay out the proofs of the following
theorems? This is Bird 1.4.6.
(i)
Theorem: (*) x = (* x)
Proof:
(*) x ={definition of partial application} \y - x * y =
{commutativity of *} \y - y * x ={definition of (* x)} (* x)
(ii)
Theorem: (+) x = (x +)
Proof:
(+) x ={definition of partial application} \y - x + y =
{definition of (x +)} (x +)
(iii)
Theorem: (-) x /= (- x)
Proof:
(-) x ={definition of partial application} \y - x - y /=
{definition of prefix negation, which is not a section} (- x)
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[Haskell-cafe] (no subject)
This is another proof-layout question, this time from Bird 1.4.7. We're asked to define the functions curry2 and uncurry2 for currying and uncurrying functions with two arguments. Simple enough: curry2 :: ((a, b) - c) - (a - (b - c))curry2 f x y = f (x, y) uncurry2 :: (a - (b - c)) - ((a, b) - c)uncurry2 f (x, y) = f x y The following two assertions are obviously true theorems, but how are the formal proofs laid out? 1. curry2 (uncurry2 f) x y = f x y 2. uncurry2 (curry 2 f) (x, y) = f (x, y) _ The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. http://www.windowslive.com/campaign/thenewbusy?tile=multiaccountocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_4___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov sergu...@gmail.com wrote: PS Rationals: Prelude [1,1+2/3..10] :: [Rational] [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 % 3,25 % 3,9 % 1,29 % 3,31 % 3] Same result. This sounds like a bug to me. The section of the Haskell Report that deals with the Enum class mentions Float and Double, not Rational, and there's really no sensible reason why Rationals would exhibit this behaviour given that they don't have rounding error. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
* Ben Millwood wrote: Prelude [1,1+2/3..10] :: [Rational] [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 % 3,25 % 3,9 % 1,29 % 3,31 % 3] Same result. This sounds like a bug to me. The section of the Haskell Report that deals with the Enum class mentions Float and Double, not Rational, and there's really no sensible reason why Rationals would exhibit this behaviour given that they don't have rounding error. Double is not better: Prelude [9,9+2/3..10] [9.0,9.666,10.332] Prelude [7,9 .. 10] [7,9] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
On 19/05/2010, at 23:44, Ben Millwood wrote: On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov sergu...@gmail.com wrote: PS Rationals: Prelude [1,1+2/3..10] :: [Rational] [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 % 3,25 % 3,9 % 1,29 % 3,31 % 3] Same result. This sounds like a bug to me. The section of the Haskell Report that deals with the Enum class mentions Float and Double, not Rational, and there's really no sensible reason why Rationals would exhibit this behaviour given that they don't have rounding error. From Section 12.1 of the Library Report: instance (Integral a) = Enum (Ratio a) where succ x = x+1 pred x = x-1 toEnum = fromIntegral fromEnum = fromInteger . truncate -- May overflow enumFrom = numericEnumFrom -- These numericEnumXXX functions enumFromThen = numericEnumFromThen -- are as defined in Prelude.hs enumFromTo = numericEnumFromTo -- but not exported from it! enumFromThenTo = numericEnumFromThenTo The numericEnum functions are defined in Section 8 of the Language Report and have semantics required for Float and Double. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Intuitive function given type signature
On Wed, May 19, 2010 at 04:27:14AM +, R J wrote: What are some simple functions that would naturally have the following type signatures: f :: (Integer - Integer) - Integer Well, this means f is given a function from Integer to Integer, and it has to somehow return an Integer, (possibly) using the function it is given. For example, f could just ignore its argument: f _ = 6 Or it could apply it to a particular input value: f g = g 0 I'll let you think of some other possibilities. g :: (Integer - Integer) - (Integer - Integer) g is given an Integer-Integer function and has to produce a different one. But as someone else pointed out, you can also think of this as g :: (Integer - Integer) - Integer - Integer That is, g is given an Integer-Integer function as well as an Integer, and must somehow use these things to produce another Integer. There are lots of ways to do this--I'll let you figure this one out. -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (no subject)
On Wed, May 19, 2010 at 01:37:49PM +, R J wrote:
This is another proof-layout question, this time from Bird 1.4.7.
We're asked to define the functions curry2 and uncurry2 for currying and
uncurrying functions with two arguments. Simple enough:
curry2 :: ((a, b) - c) - (a - (b - c))curry2 f x y = f
(x, y)
uncurry2 :: (a - (b - c)) - ((a, b) - c)uncurry2 f (x, y) = f
x y
The following two assertions are obviously true theorems, but how are the
formal proofs laid out?
There are lots of variations, I wouldn't say there's one right way
to organize/lay out the proofs. But here's how I might do it:
curry2 (uncurry2 f) x y
= { def. of curry2 }
uncurry2 f (x,y)
= { def. of uncurry2 }
f x y
I'll let you do the other one.
By the way, are you working through these problems just for
self-study, or is it homework for a class?
-Brent
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Re: [Haskell-cafe] cabal: problem building ffi shared library and significance of __stginit
On Tue, 2010-05-18 at 17:31 -0400, Anthony LODI wrote: Hello, I'm trying to build some haskell code as a .so/.dll so that it can ultimately be used by msvc. I have it working when I compile by hand (listed below) but I can't get the exact same thing built/linked with cabal. On linux everything builds fine, but when I try to link the resulting .so file, I get an error about a missing '__stginit_CInterface' reference. Indeed I couldn't find that name in any of the cabal-generated .dyn_o files. I checked the output of 'cabal build -v' and it seems to be executing about the same thing that I'm executing manually so I'm not sure what could be going wrong. On windows cabal won't even configure since '--enable-shared' seems to imply '-dynamic' (right?), and that's not currently supported. Also, when I remove the line 'hs_add_root(__stginit_CInterface);', and the corresponding forward declaration, the program runs fine! Does ghc no longer need this call or are my toy programs just being lucky sofar? For reference for other people, Anthony and I worked this out today. full example: http://pastebin.com/aLdyFMPg The difference between doing it manually and building a library via Cabal is the package name. When building directly with ghc, the default package name is aka the main package. When building a ghc/Haskell package, the package name gets set (ghc -package-name test-0.0). This package name gets encoded into the symbol names. So we get: __stginit_testzm0zi0_CInterface vs __stginit_CInterface (testzm0zi0 is the Z-encoding of test-0.0) What is bad here is that the __stginit stuff is even necessary. Anthony is going to file a ghc ticket and/or complain on the ghc users list, citing this example. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] TagSoup 0.9
Thanks Malcolm, Providing a 'String' type argument worked: type Bundle = [Tag String] extractTags :: Tag String - Tag String - Bundle - Bundle extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= fromTag ) tags From: Malcolm Wallace [mailto:malcolm.wall...@me.com] Sent: Wednesday, May 19, 2010 1:48 AM To: rhodg...@topquadrant.com Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] TagSoup 0.9 Neil says that the API of TagSoup changed in 0.9. All usages of the type Tag should now take a type argument, e.g. Tag String. Regards, Malcolm On Wednesday, May 19, 2010, at 08:05AM, Ralph Hodgson rhodg...@topquadrant.com wrote: ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Hello Neil , I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this error: TQ\TagSoup\TagSoupExtensions.lhs:29:17: `Tag' is not applied to enough type arguments Expected kind `*', but `Tag' has kind `* - *' In the type synonym declaration for `Bundle' Failed, modules loaded: TQ.Common.TextAndListHandling. where line 29 is the type declaration for 'bundle' in the following code: module TQ.TagSoup.TagSoupExtensions where import TQ.Common.TextAndListHandling import Text.HTML.TagSoup import Text.HTML.Download import Control.Monad import Data.List import Data.Char type Bundle = [Tag] [snip] tagsOnPage :: String - IO(String) tagsOnPage url = do tags - liftM parseTags $ openURL url let results = unlines $ map(show) $ tags return (results) extractTags :: Tag - Tag - [Tag] - [Tag] extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= fromTag ) tags extractTagsBetween :: Tag - [Tag] - [Tag] extractTagsBetween _ [] = [] extractTagsBetween markerTag tags = if startTags == [] then [] else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail startTags) where startTags = dropWhile (~/= markerTag ) tags I need to repair this code quickly. I am hoping you can quickly help me resolve this. Thanks. Ralph Hodgson, @ralphtq http://twitter.com/ralphtq ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] TagSoup 0.9
Forgot to add: I now need to understand the following warnings on this line import Text.HTML.Download: TagSoupExtensions.lhs:24:2: Warning: In the use of `openItem' (imported from Text.HTML.Download): Deprecated: Use package HTTP, module Network.HTTP, getResponseBody = simpleHTTP (getRequest url) TagSoupExtensions.lhs:24:2: Warning: In the use of `openURL' (imported from Text.HTML.Download): Deprecated: Use package HTTP, module Network.HTTP, getResponseBody = simpleHTTP (getRequest url) Ok, modules loaded: TQ.TagSoup.TagSoupExtensions. *TQ.TagSoup.TagSoupExtensions From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-boun...@haskell.org] On Behalf Of Ralph Hodgson Sent: Wednesday, May 19, 2010 10:30 AM To: 'Malcolm Wallace' Cc: haskell-cafe@haskell.org Subject: RE: [Haskell-cafe] TagSoup 0.9 Thanks Malcolm, Providing a 'String' type argument worked: type Bundle = [Tag String] extractTags :: Tag String - Tag String - Bundle - Bundle extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= fromTag ) tags From: Malcolm Wallace [mailto:malcolm.wall...@me.com] Sent: Wednesday, May 19, 2010 1:48 AM To: rhodg...@topquadrant.com Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] TagSoup 0.9 Neil says that the API of TagSoup changed in 0.9. All usages of the type Tag should now take a type argument, e.g. Tag String. Regards, Malcolm On Wednesday, May 19, 2010, at 08:05AM, Ralph Hodgson rhodg...@topquadrant.com wrote: ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Hello Neil , I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this error: TQ\TagSoup\TagSoupExtensions.lhs:29:17: `Tag' is not applied to enough type arguments Expected kind `*', but `Tag' has kind `* - *' In the type synonym declaration for `Bundle' Failed, modules loaded: TQ.Common.TextAndListHandling. where line 29 is the type declaration for 'bundle' in the following code: module TQ.TagSoup.TagSoupExtensions where import TQ.Common.TextAndListHandling import Text.HTML.TagSoup import Text.HTML.Download import Control.Monad import Data.List import Data.Char type Bundle = [Tag] [snip] tagsOnPage :: String - IO(String) tagsOnPage url = do tags - liftM parseTags $ openURL url let results = unlines $ map(show) $ tags return (results) extractTags :: Tag - Tag - [Tag] - [Tag] extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= fromTag ) tags extractTagsBetween :: Tag - [Tag] - [Tag] extractTagsBetween _ [] = [] extractTagsBetween markerTag tags = if startTags == [] then [] else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail startTags) where startTags = dropWhile (~/= markerTag ) tags I need to repair this code quickly. I am hoping you can quickly help me resolve this. Thanks. Ralph Hodgson, @ralphtq http://twitter.com/ralphtq ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal-install
On May 19, 2010, at 04:49 , Ivan Lazar Miljenovic wrote:
Serguey Zefirov sergu...@gmail.com writes:
export http_proxy=http://${username}:${passwo...@${proxy_url};
I tried it and it didn't work. I don't know reason, though, maybe it
was because my current password not entirely alphanumeric.
Shouldn't matter as long as you put it within quotes.
I imagine things will go wrong if it includes an @... urlencoding is
probably a smart idea.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon universityKF8NH
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Description: This is a digitally signed message part
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Re: [Haskell-cafe] TagSoup 0.9
On Wednesday 19 May 2010 19:46:57, Ralph Hodgson wrote:
Forgot to add: I now need to understand the following warnings on this
line import Text.HTML.Download:
In Text.HTML.Download, there's the following:
{-|
/DEPRECATED/: Use the HTTP package instead:
import Network.HTTP
openURL x = getResponseBody = simpleHTTP (getRequest x)
This module simply downloads a page off the internet. It is very
restricted,
and it not intended for proper use.
The original version was by Alistair Bayley, with additional help from
Daniel McAllansmith. It is taken from the Haskell-Cafe mailing list
\Simple HTTP lib for Windows?\, 18 Jan 2007.
http://thread.gmane.org/gmane.comp.lang.haskell.cafe/18443/
-}
and
{-# DEPRECATED openItem, openURL Use package HTTP, module Network.HTTP,
getResponseBody = simpleHTTP (getRequest url) #-}
So, don't use Text.HTML.Download anymore, instead use the functions from
the HTTP package.
Deprecated stuff will probably be removed in one of the next releases.
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Re: [Haskell-cafe] TagSoup 0.9
Or use things from the download-curl package, which provides a nice
openURL function.
daniel.is.fischer:
On Wednesday 19 May 2010 19:46:57, Ralph Hodgson wrote:
Forgot to add: I now need to understand the following warnings on this
line import Text.HTML.Download:
In Text.HTML.Download, there's the following:
{-|
/DEPRECATED/: Use the HTTP package instead:
import Network.HTTP
openURL x = getResponseBody = simpleHTTP (getRequest x)
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[Haskell-cafe] Re: cabal: problem building ffi shared library and significance of __stginit
I'm trying to build some haskell code as a .so/.dll so that it can ultimately be used by msvc. I have it working when I compile by hand (listed below) but I can't get the exact same thing built/linked with cabal. On linux everything builds fine, but when I try to link the resulting .so file, I get an error about a missing '__stginit_CInterface' reference. Indeed I couldn't find that name in any of the cabal-generated .dyn_o files. I checked the output of 'cabal build -v' and it seems to be executing about the same thing that I'm executing manually so I'm not sure what could be going wrong. On windows cabal won't even configure since '--enable-shared' seems to imply '-dynamic' (right?), and that's not currently supported. Okay after some help from Duncan on #haskell, the problem seems pretty straightforward. I've uploaded the details here: http://hackage.haskell.org/trac/ghc/ticket/3252#comment:4 Cheers. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] TagSoup 0.9
Hi Ralph, I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this error: TQ\TagSoup\TagSoupExtensions.lhs:29:17: `Tag' is not applied to enough type arguments Expected kind `*', but `Tag' has kind `* - *' In the type synonym declaration for `Bundle' Failed, modules loaded: TQ.Common.TextAndListHandling. My change notes have this being a change between 0.6 and 0.8. As Malcolm says, any old uses of Tag should become Tag String. The reason is that Tag is now parameterised, and you can use Tag ByteString etc. However, I should point out that Tag ByteString won't be any faster than Tag String in this version (it's in the future work pile). Forgot to add: I now need to understand the following warnings on this line import Text.HTML.Download: Everyone's comments have been right. I previously included Text.HTML.Download so that it was easy to test tagsoup against the web. Since I first wrote that snippet the HTTP downloading libraries have improved substantially, so people should use those in favour of the version in tagsoup - you'll be able to connect to more websites in more reliable ways, go through proxies etc. I don't intend to remove the Download module any time soon, but I will do eventually. Thanks, Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal-install
I tried it and it didn't work. I don't know reason, though, maybe it was because my current password not entirely alphanumeric. Shouldn't matter as long as you put it within quotes. I imagine things will go wrong if it includes an @... urlencoding is probably a smart idea. Thank you very much! I'll definitely try that. And, as I'm on it again, switch to just ignore proxy would be better. ;) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell for control system [was: [reactive] A pong and integrate]
David Leimbach wrote: I find it's often the most practical chapter that I hit a lot during writes and changes to my server process I have in Haskell in our control system code :-) Are you actually saying that you use Haskell for a control system server? Thta would be very interesting to me. Could you tell what kind of control system? Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Extensible Records and Functional References
Hi all, I just read Functional References are a cheap and cheerful technique for working with the existing (non-extensible) record system, and may be of interest to extensible record implementers. A good implementation can be found on ... on http://hackage.haskell.org/trac/ghc/wiki/ExtensibleRecords Does this mean that there is a way to implement extensible records via functional references? Günther ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: has-0.4 Entity based records
On Thu, May 13, 2010 at 7:16 PM, HASHIMOTO, Yusaku nonow...@gmail.com wrote: Sorry for spamming, what I wanted to write is I think `has' has better interface than other record packages in types. There are many libraries to write function takes an record has Foo and Bar and returns something. But writing type of the function is still difficult. I can't write such types using HList or records without reading documents. I think, using has, There's few effort to write such types. In which manner do you need to read less documentation to write: ] f :: Has Foo r = r - ... Instead when using HList: ] f :: HasField Foo record fieldType = ... I think `has' fits the needs of Haskellers who have the good habit of writing a type of a function before its definition. What does this mean exactly in terms of the type inference possible? -- Adam ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] TagSoup 0.9
Don Stewart schrieb: Or use things from the download-curl package, which provides a nice openURL function. The openURL function from TagSoup is lazy, which the proposed replacement 'getResponseBody = simpleHTTP (getRequest x)' is not. Is the openURL function from download-curl lazy? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] TagSoup 0.9
schlepptop: Don Stewart schrieb: Or use things from the download-curl package, which provides a nice openURL function. The openURL function from TagSoup is lazy, which the proposed replacement 'getResponseBody = simpleHTTP (getRequest x)' is not. Is the openURL function from download-curl lazy? Yes, see: Network.Curl.Download.Lazy.openLazyURI though I think it is possible that I strictified the code. Have a play around with it if it doesn't meet your needs -- should be /trivial/ to ensure it is chunk-wise lazy. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Devices and Webcams, The Basics
Hi everyone, I would like to start working on a program that requires access to a camera attached to the computer probably via USB or otherwise internally. Unfortunately I don't know anything about using devices in haskell. I tried looking up how to access the microphone one too and had little success. Could someone just point me in the direction of tutorials for learning the basics of both devices in general as well as the webcam more specifically? The webcam is a bit more of a priority. -Eitan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Devices and Webcams, The Basics
Haskell has bindings to USB [1]. I don't know of any USB tutorials or any webcam specific libraries. -deech [1] http://hackage.haskell.org/package/usb On 5/19/10, Eitan Goldshtrom thesource...@gmail.com wrote: Hi everyone, I would like to start working on a program that requires access to a camera attached to the computer probably via USB or otherwise internally. Unfortunately I don't know anything about using devices in haskell. I tried looking up how to access the microphone one too and had little success. Could someone just point me in the direction of tutorials for learning the basics of both devices in general as well as the webcam more specifically? The webcam is a bit more of a priority. -Eitan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Devices and Webcams, The Basics
aditya siram aditya.si...@gmail.com writes: Haskell has bindings to USB [1]. I don't know of any USB tutorials or any webcam specific libraries. I don't know of any, but if using Linux then maybe writing a binding to v4l (video for linux) might be the best/easiest approach. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Bird problem 1.6.2 -- is there an easier method?
Bird problem 1.6.2 is: If f :: (a, b) - c, then define a function swap such that: flip (curry f) = curry (f . swap). I'd very much appreciate if someone could tell me whether there's a rigorous solution simpler than mine, which is: Since (.) :: (q - r) - (p - q) - (p - r), we have f :: q - r and swap :: p - q. Type unification of f requires q = (a, b) and r = c. Since f :: (a, b) - c and curry :: ((l, m) - n) - (l - m - n), typeunification requires l = a, b = m, and n = c. Therefore,curry :: ((a, b) - c) - (a - b - c), and (curry f) :: a - b - c. Since flip :: (s - t - u) - t - s - u, type unification requiress = a, t = b, and u = c. Therefore, flip :: (a - b - c) - b - a - c,and flip (curry f) :: b - a - c. Therefore, curry (f . swap) :: b - a - c, and p :: b - a. Therefore,swap :: b - a - (a, b), and: swap :: b - a - (a, b)swap x y = (y, x) _ Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_1___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Deducing a type signature
Bird 1.6.3 requires deducing type signatures for the functions strange and stranger. Are my solutions below correct? (i) strange f g = g (f g) Assume g :: a - b. Then f :: (a - b) - c. But since g :: a - b,f g :: a, so c = a. Therefore, f :: (a - b) - a, and g (f g) :: a.Therefore, strange :: ((a - b) - a) - (a - b) - a. (ii) stranger f = f f Assume f :: a - b. Since f f is well-typed, type unification requiresa = b. Therefore, f :: a - a, and stranger :: (a - a) - a. _ Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_2___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Deducing a type signature
On May 20, 2010, at 11:03 AM, R J wrote: stranger f = f f This doesn't have a type in Haskell. Suppose f :: a - b Then if f f made sense, a = (a - b) would be true, and we'd have an infinite type. Type the definition into a file, and try loading it into ghci: Occurs check: cannot construct the infinite type: t = t - t1 Probable cause: `f' is applied to too many arguments In the expression: f f In the definition of `stranger': stranger f = f f ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Deducing a type signature
(i) strange f g = g (f g) Assume g :: a - b. Then f :: (a - b) - c. But since g :: a - b, f g :: a, so c = a. Therefore, f :: (a - b) - a, and g (f g) :: a. Therefore, strange :: ((a - b) - a) - (a - b) - a. Almost. The return type of strange is the same as the return type of g (the outermost function), namely b. So strange :: ((a - b) - a) - (a - b) - b. Dan R J wrote: Bird 1.6.3 requires deducing type signatures for the functions strange and stranger. Are my solutions below correct? (i) strange f g = g (f g) Assume g :: a - b. Then f :: (a - b) - c. But since g :: a - b, f g :: a, so c = a. Therefore, f :: (a - b) - a, and g (f g) :: a. Therefore, strange :: ((a - b) - a) - (a - b) - a. (ii) stranger f = f f Assume f :: a - b. Since f f is well-typed, type unification requires a = b. Therefore, f :: a - a, and stranger :: (a - a) - a. Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. See how. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_2 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Intuitive function given type signature
On May 20, 2010, at 3:18 AM, Brent Yorgey wrote: On Wed, May 19, 2010 at 04:27:14AM +, R J wrote: What are some simple functions that would naturally have the following type signatures: f :: (Integer - Integer) - Integer Well, this means f is given a function from Integer to Integer, and it has to somehow return an Integer, (possibly) using the function it is given. For example, f could just ignore its argument: f _ = 6 That would NOT give f the right type. The type would be f :: Num r = a - r Or it could apply it to a particular input value: f g = g 0 That would NOT give f the right type. The type would be f :: Num a = (a - b) - b I'll let you think of some other possibilities. The key point is the 'that would NATURALLY have', which I take to mean as a result of type inference without any forcibly imposed type signatures. What kind of definition of f, *not* including :: anywhere, would result in Haskell inferring the desired signature? The key thing is that the types *must* be fully constrained to Integer. The simplest way to get something that is guaranteed to be the specific type Integer, as far as I can see, is to use something like toInteger 0. If we have identityForA :: A - A identityForB :: B - B then we can constrain a variable x to A using identityForA $ x and we can constrain a variable g to A-B using identityForB . g . identityForA. This is, in a sense, equivalent to providing a type signature, but it meets the formal requirements of these questions. identityForInteger = (+ toInteger 0) is an example. f g = g z + z where z = toInteger 0 does the trick here. m% ghci Prelude let f g = g z + z where z = toInteger 0 Prelude :type f f :: (Integer - Integer) - Integer ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bird problem 1.6.2 -- is there an easier method?
You've been asking a lot of very tutorial-ish questions on this list. Although this isn't necessarily a *bad* thing, you may receive responses more appropriate to your skill level on the haskell-beginners list http://www.haskell.org/mailman/listinfo/beginners . I don't own the Bird book, but while reading your problem the type f :: (a, b) - c is throwing off huge warning signs. What sensible implementation could such a function have? The only way I can think of to implement it is f (_, _) = undefined. Assuming this signature is somehow valid, your reasoning for the left side of the equation flip (curry f) is correct. It's a bit verbose, but you'll learn to see the types better as you become more experienced. However, your reasoning for the right side is incorrect. First, lets look at the equalities again: flip (curry f) :: b - a - c flip (curry f) = curry (f . swap) curry (f . swap) :: b - a - c The first step is to remove the curry. Since (curry :: ((a, b) - c) - a - b - c), there's only one possible type signature for (f . swap): f . swap :: (b, a) - c The types for (.) and f are known already. There's only one reasonable definition for (.), so we can reason that: (.) f g x = f (g x) f . swap :: (b, a) - c f . swap = \x - f (swap x) From this, it should be possible to derive the type of swap easily. Good luck. 2010/5/19 R J rj248...@hotmail.com: Bird problem 1.6.2 is: If f :: (a, b) - c, then define a function swap such that: flip (curry f) = curry (f . swap). I'd very much appreciate if someone could tell me whether there's a rigorous solution simpler than mine, which is: Since (.) :: (q - r) - (p - q) - (p - r), we have f :: q - r and swap :: p - q. Type unification of f requires q = (a, b) and r = c. Since f :: (a, b) - c and curry :: ((l, m) - n) - (l - m - n), type unification requires l = a, b = m, and n = c. Therefore, curry :: ((a, b) - c) - (a - b - c), and (curry f) :: a - b - c. Since flip :: (s - t - u) - t - s - u, type unification requires s = a, t = b, and u = c. Therefore, flip :: (a - b - c) - b - a - c, and flip (curry f) :: b - a - c. Therefore, curry (f . swap) :: b - a - c, and p :: b - a. Therefore, swap :: b - a - (a, b), and: swap :: b - a - (a, b) swap x y = (y, x) Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. Learn more. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Retrospective type-class extension
We all know that class (Functor f) = Monad f is preferable but its absence is a historical mistake. We've all probably tried once: instance (Functor f) = Monad f where ... However, is there a type system extension (even proposed but not implemented) that allows me to retrospectively apply such a notion? Ideally something like this would be handy if it could somehow be retrospectively applied: Monad - Applicative - Pointed - Functor -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
