- Haskell-Cafe - Haskell.org

RE: [Haskell-cafe] Template Haskell and Types
by Simon Peyton-Jones 26 Sep '05

26 Sep '05

Hmm. Q is a monad, so I think fail :: Monad m => String -> m a will do the job. 'recover' should catch the exception, and let you try something else. Simon | -----Original Message----- | From: Gracjan Polak [mailto:gracjan@acchsh.com] | Sent: 20 September 2005 10:43 | To: Simon Peyton-Jones | Cc: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] Template Haskell and Types | | Simon Peyton-Jones wrote: | > design note http://research.microsoft.com/~simonpj/tmp/notes2.ps | | | In the above paper there is something about 'giveUp'. Seems to quite | useful, but there is no such thing in ghc 6.4. | | Where did my giveUp go? And why? | | -- | Gracjan

3 2

Failure and comonads
by Einar Karttunen 26 Sep '05

26 Sep '05

Hello With the resent discussion on comonads on #haskell (mostly due to Uustalo's excellent paper) I am wondering is it possible to model failure with comonads? It seems to me that Reader,Writer and State can be implemented with both monads and comonads. IO can be implemented as a monad but OI wants linear types. Streams make fine comonads. Is there anything corresponding to Maybe or List? - Einar Karttunen

1 0

map in IO
by Tom Hawkins 26 Sep '05

26 Sep '05

What is the best way to perform a map inside IO? I defined the following function, but this must be common enough to warrant something in the standard library. What am I missing? -Tom mapIO :: (a -> IO b) -> [a] -> IO [b] mapIO _ [] = return [] mapIO f (x:xs) = do y <- f x ys <- mapIO f xs return (y : ys)

2 1

RE: Typing problems with basic arithmetic - help!
by Scherrer, Chad 25 Sep '05

25 Sep '05

> For some reason the following code is producing an error > message from ghci that the the patterns are non-exhaustive. > Does anyone have any idea why that could be, given that the > patterns are, at least in my meaning, provably exhaustive? > You forgot to match against the empty list. Try adding choosenonuniqueset n [] = [] -Chad Scherrer

1 0

Local quantification annotations
by Joel Reymont 24 Sep '05

24 Sep '05

Folks, What are the local quantification annotations they talk about here? http://www.cc.gatech.edu/~saswat/research/Summary.ps Thanks, Joel -- http://wagerlabs.com/

1 0

Typing problems with basic arithmetic - help!
by Marcin Tustin 24 Sep '05

24 Sep '05

I'm trying to do some basic arithmetic code (in fact it calculates the probability of a birthday clash given year length and group size), and GHC and Hugs are both not liking the code: [marcin@ah-m2b2 ~]$ ghci burthday.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Compiling Main ( burthday.hs, interpreted ) burthday.hs:14:37: No instance for (Fractional Integer) arising from use of `/' at burthday.hs:14:37 Probable fix: add an instance declaration for (Fractional Integer) In the definition of `pnorep': pnorep days n = (numeratorex days n) / (denominatorex days n) Failed, modules loaded: none. Prelude> :reload Compiling Main ( burthday.hs, interpreted ) Ok, modules loaded: Main. *Main> pnorep 365 30 <interactive>:1:0: Ambiguous type variable `a' in the constraints: `Fractional a' arising from use of `pnorep' at <interactive>:1:0-5 `Integral a' arising from use of `pnorep' at <interactive>:1:0-5 Probable fix: add a type signature that fixes these type variable(s) *Main> The first time is with the type annotations, the second without: import Ratio import Numeric numeratorex days n = (numdays days n) - (subtractex days n) -- :: Rational numdays days n = days * n --numdays :: Integer -> Integer -> Integer subtractex days n = (days * (days - 1)) / 2 --:: Rational denominatorex days n = (days^n) -- denominatorex :: Integer -> Integer -> Integer pnorep days n = (numeratorex days n) / (denominatorex days n) Help! Thanks!

2 1

Re: [Haskell-cafe] Re: Network parsing and parsec
by Keean Schupke 24 Sep '05

24 Sep '05

Andrew Pimlott wrote: >On Tue, Sep 20, 2005 at 03:01:32PM +0100, Keean Schupke wrote: > > >> (see attachment for files) >> >> > >You didn't include all the used libraries (MonadControl, MonadState). > >Andrew > > Oops, here they are (it was extracted from a larger project), sorry about that... (Have posted this back to the mailing list incase anyone else is tying to use the libraries I posted) Regards, keean.

1 0

RE: [Haskell-cafe] killing a running thread interactively
by Simon Marlow 23 Sep '05

23 Sep '05

Haskell doesn't, but the compilers all support a kind of global mutable variable. See various lengthy discussions on the topic on this list and the Haskell list in the past. Briefly, you do it like this: {-# NOILINE var #-} var :: IORef [ThreadId] var = unsafePerformIO (newIORef []) and if you're using GHC, compile the module with -fno-cse. Cheers, Simon On 22 September 2005 17:58, S. Alexander Jacobson wrote: > Ok, I didn't think Haskell had mutable global variables. > How would this work? > > -ALex- > > On Thu, 22 Sep 2005, Simon Marlow wrote: > >> There's no getAllThreadIds or similar right now. You have to save >> the ThreadIds yourself, perhaps in a global variable. Writing to a >> file doesn't work because there's no instance Read ThreadId. >> >> Cheers, >> Simon >> >> On 21 September 2005 21:14, S. Alexander Jacobson wrote: >> >>> More particularly, is there a getAllThreadIds function somewhere? >>> >>> -Ale- >>> >>> On Wed, 21 Sep 2005, S. Alexander Jacobson wrote: >>> >>>> Is the general pattern to write all threadIds to a file, and then >>>> have a separate function that takes the file and kills them? >>>> >>>> Or is there something more clever? >>>> >>>> -Alex- >>>> >>>> On Wed, 21 Sep 2005, Simon Marlow wrote: >>>> >>>>> On 16 September 2005 20:42, S. Alexander Jacobson wrote: >>>>> >>>>>> If I am running a server interactively. (using ghci). >>>>>> Is there any way to kill its running threads without terminating >>>>>> the interpreter? >>>>> >>>>> If you can get ThreadIds for the threads, yes. GHCi doesn't >>>>> (currently) create a new thread for each expression evaluation, so >>>>> attempting to kill that thread might kill your GHCi session (it >>>>> shouldn't but that's another story). >>>>> >>>>> Cheers, >>>>> Simon >>>>> >>>>> ** CRM114 Whitelisted by: simonmar(a)microsoft.com ** >>>>> >>>> >>>> ______________________________________________________________ >>>> S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com >>>> >>> >>> ______________________________________________________________ >>> S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com >> >> ** CRM114 Whitelisted by: simonmar(a)microsoft.com ** >> > > ______________________________________________________________ > S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

1 0

Network parsing and parsec
by John Goerzen 23 Sep '05

23 Sep '05

Hello, The recent thread on binary parsing got me to thinking about more general network protocol parsing with parsec. A lot of network protocols these days are text-oriented, so seem a good fit for parsec. However, the difficulty I come up time and again is: parsec normally expects to parse as much as possible at once. With networking, you must be careful not to attempt to read more data than the server hands back, or else you'll block. I've had some success with hGetContents on a socket and feeding it into extremely carefully-crafted parsers, but that is error-prone and ugly. Here's the problem. With a protocol such as IMAP, there is no way to know until a server response is being parsed, how many lines (or bytes) of data to read. Ideally, I would be able to slrup in more data as I go, but that doesn't seem to be very practical in Parsec either. Suggestions?

9 24

Growing Trees
by Tom Hawkins 23 Sep '05

23 Sep '05

I'm porting an ML program to Haskell, but am having difficulty with particular data structure: a tree where each node has references to the children as well as the parent... data Tree a = TreeRoot { stuff :: a , children :: [Tree] } | TreeNode { stuff :: a , parent :: Tree , children :: [Tree] } But because of these bidirectional links, every time I add a node I must reconstructing the entire tree. There is also the add coding complexity of threading the tree through various functions to simulate state. What are my [monadic] options? -Tom

4 4