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do
by PR Stanley 12 Dec '07

12 Dec '07

Hi I've probably asked about the do construct, if that's the right label. Unfortunately I'm still not quite sure of its role and more specifically its syntax. Something to do with generators perhaps? A description plus some examples would be most gratefully received. Thanks, Paul

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Re: [Haskell-cafe] class default method proposal
by David Menendez 12 Dec '07

12 Dec '07

On Dec 11, 2007 1:29 PM, apfelmus <apfelmus(a)quantentunnel.de> wrote: > Without the automatic search, this is already possible > > class Functor f where > fmap :: (a -> b) -> f a -> f b > > class Functor m => Monad m where > return :: a -> m a > (>>=) :: m a -> (a -> m b) -> m b > > -- aka liftM > fmapDefault :: Monad m => (a -> b) -> m a -> m b > fmapDefault f m = m >>= (return . f) > > instance Monad [] where > return x = [x] > (>>=) = flip concatMap > > instance Functor [] where > fmap = fmapDefault > > fmap is already written for you, the instance declaration is only > boilerplate. I first saw this in Data.Traversable . > This is pretty much how I define Functor and Applicative instances for my monads. It is admittedly irritating to have to write out the boilerplate, but it doesn't seem irritating enough to require a language extension to eliminate. -- Dave Menendez <dave(a)zednenem.com> <http://www.eyrie.org/~zednenem/>

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unsubscribe
by Tim- tigre11 11 Dec '07

11 Dec '07

Very suspicious extension of attachment Sender: "Gwern Branwen" <gwern0(a)gmail.com> Recipient: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Software Tools in Haskell --- avast! Antivirus: Outbound message clean. Virus Database (VPS): 071210-0, 10/12/2007 Tested on: 12/12/2007 9:33:52 AM avast! - copyright (c) 1988-2007 ALWIL Software. http://www.avast.com

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grammars and generics
by Greg Meredith 11 Dec '07

11 Dec '07

Haskellians, Here is an idea so obvious that someone else must have already thought of it and worked it all out. Consider the following grammar. N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 N1 ::= T3 N0 where Ti (0 <= i < 4) are understood to be terminals. Using generics we can translate each production independently of the others. Like so: [| N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 |] = data N0 n1 = T0 n1 | T1 n1 (N0 n1) | T2 (N0 n1) (N0 n1) deriving (Eq, Show) [| N1 ::= T3 N0 |] = data N1 n0 = T3 n0 deriving (Eq, Show) Then, we can compose the types to get the recursive grammar. data G = N0 (N1 G) deriving (Eq, Show) This approach has the apparent advantage of treating each production independently and yet being compositional. Now, let me de-obfuscate the grammar above. The first production should be very familiar. Term ::= Var Name | Abstraction Name Term | Application Term Term The generics-based translation of this grammar yields something we already know: we can use lots of different types to work as identifiers. This is something that the nominal of Gabbay, Pitts, et al, have factored out nicely. The second production can be treated independently, but composes well with the first. Name ::= Quote Term This illustrates that a particularly interesting class of names is one that requires we look no further than our original (parametric) data type. So, my question is this. Does anyone have a reference for this approach to translation of grammars? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: [Haskell-cafe] grammars and generics
by Greg Meredith 11 Dec '07

11 Dec '07

Dusan, Excellent point. To close it off, you need to add an "empty" alternative. Thus, the corrected form would be N0 ::= TEmpty | T0 N1 | T1 N1 N0 | T2 N0 N0 N1 ::= T3 N0 In the lambda calculus, this would show up as a constant term, say 0, that would have to be treated in the operational semantics. See my ltu posting<http://lambda-the-ultimate.org/node/1930>of a year ago. Best wishes, --greg On Dec 11, 2007 11:33 AM, Dušan Kolář <kolar(a)fit.vutbr.cz> wrote: > Hello, > > I can't help you with the Haskell question as I'm not really in that > much. Nevertheless, your grammar is non-generating one - that means, it > cannot generate any sentence. If the starting non-terminal is N0 then > there is no production generating a string of terminals, thus, both > non-terminals (even if reachable from the staring one) are so called > non-generating ones (they can be freely removed from the grammar and all > involved productions too). > Thus, you get empty grammar. Isn't that the problem? Shouldn't the > grammar be like: > > N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 > N1 ::= T3 T4 > > ? > > Best regards > > Dusan > > > Greg Meredith wrote: > > Haskellians, > > > > Here is an idea so obvious that someone else must have already thought > > of it and worked it all out. Consider the following grammar. > > > > N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 > > N1 ::= T3 N0 > > > > where Ti (0 <= i < 4) are understood to be terminals. > > > > Using generics we can translate each production independently of the > > others. Like so: > > > > [| N0 ::= T0 N1 | T1 N1 N0 | T2 N0 N0 |] > > = > > data N0 n1 = T0 n1 | T1 n1 (N0 n1) | T2 (N0 n1) (N0 n1) deriving (Eq, > > Show) > > > > [| N1 ::= T3 N0 |] > > = > > data N1 n0 = T3 n0 deriving (Eq, Show) > > > > Then, we can compose the types to get the recursive grammar. > > > > data G = N0 (N1 G) deriving (Eq, Show) > > > > This approach has the apparent advantage of treating each production > > independently and yet being compositional. > > > > Now, let me de-obfuscate the grammar above. The first production > > should be very familiar. > > > > Term ::= Var Name | Abstraction Name Term | Application Term Term > > > > The generics-based translation of this grammar yields something we > > already know: we can use lots of different types to work as > > identifiers. This is something that the nominal of Gabbay, Pitts, et > > al, have factored out nicely. > > > > The second production can be treated independently, but composes well > > with the first. > > > > Name ::= Quote Term > > > > This illustrates that a particularly interesting class of names is one > > that requires we look no further than our original (parametric) data > > type. > > > > So, my question is this. Does anyone have a reference for this > > approach to translation of grammars? > > > > Best wishes, > > > > --greg > > > > > > -- > > L.G. Meredith > > Managing Partner > > Biosimilarity LLC > > 505 N 72nd St > > Seattle, WA 98103 > > > > +1 206.650.3740 > > > > http://biosimilarity.blogspot.com > > ------------------------------------------------------------------------ > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > -- > > Dusan Kolar tel: +420 54 114 1238 > UIFS FIT VUT Brno fax: +420 54 114 1270 > Bozetechova 2 e-mail: kolar(a)fit.vutbr.cz > Brno 612 66 > Czech Republic > > -- > > -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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c2hs and cabal
by Stefan Kersten 11 Dec '07

11 Dec '07

hi, i'm looking for a way to integrate c2hs (single .chs file) in a cabal build setup; can anybody give me some hints? thanks, <sk>

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FFI: "foreign label" declaration
by Ricardo Herrmann 11 Dec '07

11 Dec '07

Hi everyone, I'm trying to get HBDD compiled under GHC 6.8.1 and now I'm stuck with a .chs file which uses a "foreign label" syntax, which seems deprecated, since the parser can't even recognize it: foreign label "bdd_reorder_stable_window3" bdd_reorder_stable_window3 :: FunPtr (BDDManager -> IO ()) Is there new syntax for this ? Or is it a deprecated functionality ? Thanks in advance, -- Ricardo Guimarães Herrmann "In a concurrent world, imperative is the wrong default!" -- Tim Sweeney (Epic Games) -- View this message in context: http://www.nabble.com/FFI%3A-%22foreign-label%22-declaration-tp14279625p142… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: distinguish functions from non-functions in a class/instances
by oleg＠pobox.com 11 Dec '07

11 Dec '07

Philipp N. wrote: > i'm trying to wrap functions (a -> b -> ... -> z) of any arity to functions > of type ([String] -> y), where list of strings replaces the typed arguments. > the problem is, that you cannot distinguish type (x->y) from z, so these > instances are overlapping. to which apfelmus replied > Exactly. What you want to do is most likely impossible, In fact, that distinction is possible. The following article How to write an instance for not-a-function http://okmij.org/ftp/Haskell/typecast.html#is-function-type specifically describes a method of writing an instance which is selected only when the type in question is NOT a function. The method is quite general and has been extensively used (for example, to implement deep monadic join).

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Waiting for thread to finish
by Maurício 11 Dec '07

11 Dec '07

Hi, After I have spawned a thread with 'forkIO', how can I check if that thread work has finished already? Or wait for it? Thanks, Maurício

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Need help please.
by Ryan Bloor 11 Dec '07

11 Dec '07

hi I am writing a basic Parser from scratch. So far I have functions;# removeSpaces# match - which checks if a string is a substring of another# orParser which combines two parser's abilities# Basic pasrers like... parseInt, parseTrue, parseFalse, parseBoolusing the orParser on True and False.What I want to do now is have a parseBinaryOp that recognises:parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")]So I think that I have to split the initial string into four parts."+" becomes op'(' becomes tokenF')' becomes tokenB"5" becomes e1"2" becomes e2parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) =I am not sure how to go about separating the string for how I need itusing my other functiuons. Ryan _________________________________________________________________ Celeb spotting – Play CelebMashup and win cool prizes https://www.celebmashup.com

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