- Haskell-Cafe - Haskell.org

Converting -fglasgow-exts into LANGUAGE pragmas
by Martijn van Steenbergen 31 Mar '09

31 Mar '09

Hello café, I want to replace the -fglasgow-exts in the snippet below by LANGUAGE pragmas. Rank2Types alone doesn't suffice. Which do I need to get the snippet to compile? > {-# OPTIONS -fglasgow-exts #-} > > module RunMud where > > import Control.Monad.State > > type Mud = StateT MudState IO > data MudState = MudState { mRunMud :: RunMud } > type RunMud = forall a. Mud a -> IO a > > getRunMud :: Mud RunMud > getRunMud = do > s <- get > return (mRunMud s) Thanks in advance, Martijn.

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Re: Exception handling in numeric computations
by John Lato 31 Mar '09

31 Mar '09

> From: Donn Cave <donn(a)avvanta.com> > Quoth John Lato <jwlato(a)gmail.com>, > >> An exception is caused by some sort of interaction with the run-time >> system (frequently a hardware issue). The programmer typically can't >> check for these in advance, but can only attempt to recover after >> they've happened. >> >> An error is some sort of bug that should be fixed by the programmer. > > I have never felt that I really understood that one. Honestly, me neither, until recently. I'm only barely starting to understand it, and I do think there's a great deal of overlap. Even if an error is a bug that can be fixed by the programmer, certain exceptional situations can also be fixed by the programmer by handling the exception, even if they can't be detected in advance. In general I would say that I agree with JCC's responses. > > What about invalid inputs? Say someone encounters a disk full error, > and the resulting partly written file is now unreadable data for its > intended application because of an invalid file encoding? Is that > an exception, or a bug that should be fixed? > > My guess is that you'll say it's a bug, i.e., that application's > file decoding result should be an Either type that anticipates that > the file encoding may be invalid. Not necessarily. I would be satisfied with a Maybe :) I've encountered malformed data frequently enough that any decoders I write (and I have done so recently) would anticipate invalid data. > > I will also guess if the file is unreadable because of an external > I/O problem like no read access to file or filesystem, you would > similarly expect this to be treated like that - I mean, ideally, e.g., > hGetLine :: Handle -> IO (Either IOError String) Not necessarily, but possibly. The big difference, of course, is that decoding can be a pure operation, while reading never is. I personally wouldn't mind if hGetLine had the type you give. The way I see it, there are two advantages to exceptions in this case. The first is that it's very easy for exceptions to trickle up and be handled at a higher level. The second 'advantage' is that the programmer doesn't need to explicitly handle exceptions, whereas an Either would require at least a pattern match to use the resulting value. John

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ghc: Var/Type length mismatch message - what does it mean?
by John Lask 31 Mar '09

31 Mar '09

consider the following types (from the paper: "Stream Fusion From Lists to Streams to Nothing at All") > data Stream a = forall s. Stream ( s ->(Step a s)) s > data Step a s = > Done > | Yield a s > | Skip s an instance of this data type is: > stream0 :: Stream () > stream0 = Stream (\ s -> Yield () s) () now consider: > data StreamM m a = forall s. StreamM ( s ->m (Step a s)) s and the following instance: > stream0IO :: StreamM IO () > stream0IO = StreamM (\ s -> return (Yield () s)) () loads ok, but as soon as we introduce the following constraint: > data Monad m => StreamM' m a = forall s. StreamM' ( s ->m (Step a s)) s > stream0IO' :: StreamM' IO () > stream0IO' = StreamM' (\ s -> return (Yield () s)) () we get the following message when we load into ghci (6.8.2) Var/Type length mismatch: [] [base:GHC.Base.(){(w) tc 40}] is this a bug? if not, what is this message telling us ?

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Unique monad?
by Andrew Coppin 31 Mar '09

31 Mar '09

Maybe I'm just being blind here, but I don't see a monad transformer (or even a monad) in the standard libraries for producing "unique" values. Have I missed something? Related, I don't see a monad for gradually consuming input. We've got the Reader monad, but that appears to just give you global access to a single monolithic value. That value could be some kind of collection, but there's no way to track where you've read up to (or efficiently access that point). We've got Parsec, which is a pretty efficient and powerful text parser. But what if you don't need full parsing capabilities? You just need to consume input, but you're not exactly sure where you're going to consume it? The Writer monad seems to let you incrimentally write values out, but there's no matching monad for incrimentally reading stuff back in. (I guess ultimately you can build all these specialised monads out of the general State monad if you want. I'm just surprised they're not already defined somewhere...)

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ANN: new release of HTTP, version 4000.0.5
by Sigbjorn Finne 31 Mar '09

31 Mar '09

Hi, a new version of the HTTP package - 4000.0.5 has been uploaded, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP Includes a bunch of fixes and cleanups (thanks to all that have reported&contributed), along with some (shock, horror) API documentation. Let me know if it appears to be well behaved and dependable. thanks --sigbjorn

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abou the Godel Numbering for untyped lambda calculus
by Algebras Math 31 Mar '09

31 Mar '09

Hi all, I am reading the book "The lambda calculus: Its syntax and Semantics" in the chapter about Godel Numbering but I am confused in some points. We know for Church Numerals, we have Cn = \fx.f^n(x) for some n>=0, i.e. C0= \fx.x and C 1 = \fx.fx. >From the above definition, I could guess the purpose of this kind of encoding is trying to encode numeral via terms. How about the Godel Numbering? From definition we know people say #M is the godel number of M and we also have [M] = C#M to enjoy the second fixed point theorem : for all F there exists X s.t. F[X] = X. What the mapping function # is standing for? How could I use it? What the #M will be? How to make use of the Godel Numbering? Thank you very much! alg

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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 31 Mar '09

31 Mar '09

Thanks for the information. Michael --- On Mon, 3/30/09, Ryan Ingram <ryani.spam(a)gmail.com> wrote: From: Ryan Ingram <ryani.spam(a)gmail.com> Subject: Re: [Haskell-cafe] Rational and % operator remix To: "michael rice" <nowgate(a)yahoo.com> Cc: "Ketil Malde" <ketil(a)malde.org>, haskell-cafe(a)haskell.org Date: Monday, March 30, 2009, 1:21 PM 2009/3/30 michael rice <nowgate(a)yahoo.com>: > I'm still not sure what some of the error messages I was getting were about. > As I wrote the function I tried to be aware of what "mixed mode" operations > were kosher ala This is a mistake, but understandable given your lispy background; there aren't really "mixed mode" operations in Haskell. One thing that might be confusing you: Numeric literals are really calls to "fromInteger"; that is, "5" is really "fromInteger (5 :: Integer)" Generally you will find that :t in the interpreter is your friend: ghci> :t 1 1 :: (Num t) => t This says that the expression "1" can be of any type that is an instance of Num. ghci> :t (\a b -> a - b) (\a b -> a - b) :: (Num a) => a -> a -> a "-" takes two arguments that are of the same type, and returns something of that type. ghci> :m Data.Ratio ghci> :t (%) (%) :: (Integral a) => a -> a -> Ratio a "%" lifts objects from an integral type into a type that represents the ratio of two integers. ghci> :t floor floor :: (RealFrac a, Integral b) :: a -> b So "floor" is a "cast" operation that converts between any fractional type to any integral type. This is why you need to either use "fromInteger" or "%" on the result of "floor" to get it back as a Rational. -- ryan

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Re: [Haskell-cafe] Internationalization of Haskell code
by Henning Thielemann 30 Mar '09

30 Mar '09

Sorry, my reply got lost in a private mailbox. :-) On Sat, 28 Mar 2009, Henning Thielemann wrote: > On Sat, 28 Mar 2009, Martijn van Steenbergen wrote: > >> Hi Henning, >> >> Henning Thielemann wrote: >>> Thanks for working on that issue! The German translation is fine. I have >>> recently written a Wiki article on how to avoid IO in Haskell code without >>> resorting to unsafePerformIO where I incidentally chose gettext as an >>> example: >>> http://progandprog.blogspot.com/2009/03/i18n-and-haskell.html >> >> I don't think this is the link you meant to post. :-) > > Right, this would be too much of an incidence. I meant this one: > http://www.haskell.org/haskellwiki/Avoiding_IO

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Really need some help understanding a solution
by Günther Schmidt 30 Mar '09

30 Mar '09

Hi guys, I tried for days now to figure out a solution that Luke Palmer has presented me with, by myself, I'm getting nowhere. He has kindly provided me with this code: import Data.Monoid newtype IntTrie a = IntTrie [a] deriving Show singleton :: (Monoid a) => Int -> a -> IntTrie a singleton ch x = IntTrie $ replicate ch mempty ++ [x] ++ repeat mempty lookupTrie :: IntTrie a -> Int -> a lookupTrie (IntTrie xs) n = xs !! n instance (Monoid a) => Monoid (IntTrie a) where mempty = IntTrie (repeat mempty) mappend (IntTrie xs) (IntTrie ys) = IntTrie (infZipWith mappend xs ys) infZipWith f ~(x:xs) ~(y:ys) = f x y : infZipWith f xs ys test = mconcat [singleton (n `mod` 42) [n] | n <- [0..]] `lookupTrie` 10 It's supposed to eventually help me group a list of key value pairs and then further process them in a linear (streaming like) way. The original list being something like [('a', 23), ('b', 18), ('a', 34) ...]. There are couple of techniques employed in this solution, but I'm just guessing here. The keywords I've been looking up so far: Memmoization, Deforestation, Single Pass, Linear Map and some others. Can someone please fill me in? Günther

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Record updates
by Andrew Coppin 30 Mar '09

30 Mar '09

Haskell's record syntax is quite nice, for a number of reasons. However, suppose I have some record: data Foobar = Foobar {foo1, foo2, foo3...} Now suppose that foo3 :: [Int], and I want to prepend 5 to it. I end up having to write something like let v1 = v0 {foo3 = 5 : (foo3 v0)} If the field name isn't "foo" but something more descriptive, and the transformation to be applied to it is more intricate, you end up with quite a bit of code. In summary, record syntax gives you a nice way of replacing the value of one field with something else, but no easy way to *modify* the existing value somehow. Does anybody know of a way around this? Is there some trick I'm not seeing? Is there an extension or proposal that fixes this?

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