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pretty printing with comments
by Pasqualino "Titto" Assini 22 Oct '09

22 Oct '09

Hi, Is there any simple way of pretty printing haskell source code while preserving the comments? I am looking at the haskell-src-ext library. It can parse files with comments and it can pretty print but, for what I can see it cannot do both :-) (prettyPrint won't work on the structure returned by parseFileWithComments). Thanks, titto

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Real World Haskell Chapter 19 and GHC 6.10
by Michael Mossey 22 Oct '09

22 Oct '09

The examples in the "error handling" chapter (19) of RWH don't run under GHC 6.10. For instance, an example might be main = handle (\_ -> putStrLn "error") (print $ 5 `div` 0) Trying to load this results in "amigous type variable 'e' in the constraint: 'Exception e' arising from a use of 'handle' etc etc. I was able to fix this via the ludicrously complex: main2 = (handle :: (SomeException -> IO ()) -> IO () -> IO ()) (\_ -> putStrLn "Error calculating result") (print $ 5 `div` 0) Is there a more concise way to use "handle", or can someone point me to a tutorial that might explain the changes in 6.10 and in a general way how to get the RWH examples to run? Thanks, Mike

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Needing strictness?
by michael rice 22 Oct '09

22 Oct '09

Busy night. I wanted the question and response to be on the same line (see below), so I used putStr instead of putStrLn, but Haskell jumps the gun and goes to the second line to get the response before the question ever gets printed. Is this an example of needing strictness? How would I do that, or something else, to get the result I want? putStrLn "Some yes/no question (y/n)?" ans <- getLine Michael

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ANN: GPipe 1.02 and Vec-Transform 1.0.1
by Tobias Bexelius 21 Oct '09

21 Oct '09

I've uploaded new versions of these two packages to hackage: GPipe-1.0.2: * Removed rasterize and made rasterizeFront work for other primitives than triangles instead. * Fixed notB Vertex and Fragment instances Vec-Transform-1.0.1: * Added rotationLookAt Cheers Tobias Bexelius _________________________________________________________________ Windows Live: Dina vänner får dina uppdateringar från Flickr, Yelp och Digg när de skickar e-post till dig. http://www.microsoft.com/windows/windowslive/see-it-in-action/social-networ…

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How to use "bracket" properly ?
by zaxis 21 Oct '09

21 Oct '09

winSSQ count noRed noBlue = do { yesRed <- [1..33] \\ noRed; yesBlue <- [1..16] \\ noBlue; bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ count yesRed yesBlue hd1); return () } will report: Couldn't match expected type `IO ()' against inferred type `[()]' In a stmt of a 'do' expression: bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\ hd1 -> pickSSQ count yesRed yesBlue hd1) However, the following works fine: winSSQ count noRed noBlue = do let yesRed = [1..33] \\ noRed let yesBlue = [1..16] \\ noBlue bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ count yesRed yesBlue hd1) Why ? -- View this message in context: http://www.nabble.com/How-to-use-%22bracket%22-properly---tp25953522p259535… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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list -> sublists
by satorisanitarium 21 Oct '09

21 Oct '09

I just started learning haskell and I just came to my first wtf moment. I'm trying to do something like this: calling: foo 3 [1,2,3,2,4,1,6,3,6,2,3,5,2,5,2,1,6,4] returns: [[1,2,3],[2,4,1,6,3],[2,3]] but i have no idea how to put something into a sublist. How to make a list of sublists out of a list, whether they be a list of numbers or a string. TX in advance. -- View this message in context: http://www.nabble.com/list--%3E-sublists-tp25975341p25975341.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Test cases for type inference
by Peter Verswyvelen 21 Oct '09

21 Oct '09

For learning, I would like to develop my own implementation of type inference, based on the paper "Typing Haskell in Haskell". At first sight, the source code of THIH contains a small number of tests, but I was wandering if a large test set exist? Thanks, Peter

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systemtimmetypeable: Workaround for lack of deriveable data instance in either System.Time or Data.Time.
by Thomas Hartman 21 Oct '09

21 Oct '09

At http://osdir.com/ml/haskell-cafe@haskell.org/2009-10/msg00197.html I griped that the lack of Data-Derivable time values was causing me headache in happstack. In the proposed cabal package http://patch-tag.com/r/tphyahoo/systemtimetypeable/snapshot/current/content… I submit a workaround that, while probably not the ideal thing, has proved helpful to me. Basically, I use the type MyTime, which is data-deriveable, when working with Macid in happstack; and convert from System.Time and/or Data.Time with the accompanying utility functions when necessary. Perhaps the utility functions are useful on their own as well. I found it surprisingly hard to get from Data.Time values to System.Time values. Anyways, I am interested in what others think and, of course, patches and suggestions welcome. thomas.

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Re: [Haskell-beginners] map question
by Gregory Propf 21 Oct '09

21 Oct '09

Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On Thu, 9/17/09, Tom Doris <tomdoris(a)gmail.com> wrote: From: Tom Doris <tomdoris(a)gmail.com> Subject: Re: [Haskell-beginners] map question To: "Joost Kremers" <joostkremers(a)fastmail.fm> Cc: beginners(a)haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers <joostkremers(a)fastmail.fm> Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude> map (+ 1) [1,2,3,4] [2,3,4,5] Prelude> map (* 2) [1,2,3,4] [2,4,6,8] Prelude> map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude> map (2 /) [1,2,3,4] [2.0,1.0,0.6666666666666666,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude> map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners -----Inline Attachment Follows----- _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Re: [Haskell-cafe] Error handling package
by Michael Snoyman 20 Oct '09

20 Oct '09

(Sorry, accidently took off cafe.) On Mon, Oct 19, 2009 at 12:44 AM, Henning Thielemann < lemming(a)henning-thielemann.de> wrote: > > On Mon, 19 Oct 2009, Michael Snoyman wrote: > > Does the explicit-exception package provide what you need? >> >> http://hackage.haskell.org/package/explicit-exception >> >> >> I don't think so, but correct me if I'm wrong. I want to make it easy to >> chain together >> computations which could fail in different ways. For example, something >> like this: >> >> attemptReadInt :: String -> Attempt Int >> attemptLookup :: String -> [(String, String)] -> Attempt String >> attemptLookupInt :: String -> [(String, String)] -> Attempt Int >> attemptLookupInt k m = attemptLookup k m >>= attemptReadInt >> >> Now, in the explicit-exception package, I could- in this simple example- >> define >> something like: >> >> data MyErrors = KeyNotFound | InvalidInt >> > > > type Attempt = Exceptional MyErrors > True; that's what I meant by I could do this in my simple example. > > > But this solution would not scale. >> > > You want to add other exceptions? The idea of my package is to make > exceptions explicit in the type. Otherwise you would use > extensible-exceptions. Or you could define MyErrors using an existential > type. Which is my point. I'm trying to provide a package for non-explicit exceptions. To compare to other programming languages, I think your package is providing the equivalent of Java checked exceptions, while mine is providing (safe) unchecked exceptions. I say safe because you still need to explicitly decide to turn an Attempt into a possible runtime exception which will bring down your program. Defining MyErrors using an existential type would essentially recreate the entire attempt package; I don't see that purpose in everyone wanted unchecked exceptions needing to reinvent the wheel in non-compatible ways. If multiple libraries use attempt, they can easily have their possible-error-returning functions chain together safely. > > > Additionally, there's two immediate features I think I would miss from my >> package: >> >> 1) fail works properly, so an Attempt would be a valid monad response from >> people who >> use that function. >> > > As far as I understand, 'fail' is used/abused for reporting failed pattern > matches in do notation. If a failed pattern match indicates a programming > error, it should be a really error, and not something that must be handled > at run-time. > That's a lot of very debateable statements you just made. It might be that it's strongly encouraged to only use fail for failed pattern matching, but in practice you could use it for any monadic failure. Also, there's nothing stopping a user from re-throwing pattern match exceptions received in an Attempt. Michael

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