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Type-level programming problem
by oleg＠pobox.com 01 May '07

01 May '07

Thomas Schilling wrote: > data T > class Foo ns a b c | ns -> a, ns -> b, ns -> c where > mkFoo :: ns > defaultA :: a > defaultB :: c -> IO b > defaultC :: [T] -> c > f :: c -> b -> a -> (b, Int) > data DefaultA > instance Foo ns a b c => Apply DefaultA ns a where > apply _ _ = defaultA > > but I get: > > Could not deduce (Foo ns1 a b1 c1) > from the context (Apply MakeAs ns a, Foo ns a b c) > arising from use of `defaultA' Indeed. Let us examine the inferred type of defaultA: *Main> :t defaultA defaultA :: (Foo ns a b c) => a We see it is a value polymorphic over four type variables: ns, a, b, and c. The type variable 'a' is also the type of the value, so we have a way to instantiate it. There is no direct way to instantiate the remaining three. If there were a functional dependency a -> ns, a->b, a->c, we could have instantiated the remaining variables. But there are no such dependencies. So, there is really no way we can ever instantiate the type variables ns, b and c -- and so the typechecker will complain. So, we need either a functional dependency a -> ns in the definition of Foo, or defaultA should have a signature defaultA :: ns -> a (and ditto for other defaults). As I understand, the function 'defaultA' can be present in different components, identified by ns. When we write 'defaultA' however, how can we say that we mean defaultA of component X rather than of component Y? There isn't any way to name the desired component... Incidentally, if we represent components by records data XRec = XRec { defaultA :: XA } then the type of defaultA is Xref -> XA. It is the function from the type of the `namespace'. This seems to suggest the signature of defaultA should be ns -> a ... BTW, there are other ways to add the name of the namespace to the signature of defaultA. For example: newtype TaggedT ns a = TaggedT a class Foo ns a b c | ... defaultA :: TaggedT ns a or class Foo ns a b c | ... defaultA :: ns a etc.

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Getting the number of seconds since epoch from System.Time.ClockTime
by Martin Percossi 01 May '07

01 May '07

Hello haskell-cafe, In System.Time, data ClockTime = TOD Integer Integer , where the first integer represents the number of seconds since epoch, and the other represents the number of picoseconds. Is there a way of retrieving the first part? (In Haskell 98, the ClockTime type is abstract). TIA Martin My music: http://www.youtube.com/profile?user=thetonegrove

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what exactly does "deriving (Functor, Monad, MonadIO)" do?
by Thomas Hartman 01 May '07

01 May '07

I was trying to follow the reasoning in Don's article on using haskell for shell scripting http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10 In the source listing at the end we is newtype Shell a = Shell { runShell :: ErrorT String IO a } deriving (Functor, Monad, MonadIO) and I don't understand it what "deriving" is doing here, nor have I been able to find documentation on it. http://en.wikibooks.org/wiki/Haskell/Class_declarations claims: "You can only use deriving with a limited set of built-in classes. They are: Eq Ord Enum Bounded Show Read " But, here we are deriving classes not in that list. So, is this a recently added feature? Or something that came in from {-# OPTIONS -fglasgow-exts #-} ? I would just like to understand this, and I can't figure out how to begin. Thanks for any help! thomas.

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FIT for Haskell
by Philipp Volgger 01 May '07

01 May '07

Who wrote FIT for Haskell on http://darcs.haskell.org/FIT/? Does anybody know if the version is stable?

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Bay Area Haskell users' group, anyone?
by Tim Chevalier 30 Apr '07

30 Apr '07

Hello all, With all the user groups that seem to have been forming lately, I figure it's high time to start a Bay Area group for people interested in Haskell (on any level). I interpret "Bay Area" broadly, since I'm in Monterey -- perhaps encompassing all of Northern California. If you're potentially interested, reply to me off-list, and I'll see what kind of response I get. I imagine that the first meeting, at least, would consist of people gathering informally over food, drink, or such. Cheers, Tim -- Tim Chevalier * chevalier(a)alum.wellesley.edu * Often in error, never in doubt Confused? See http://catamorphism.org/transition.html

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Creating pseudo terminals
by Tom Hawkins 30 Apr '07

30 Apr '07

How can I create a pseudo terminal in Linux with Haskell? Nothing is jumping out at me in the standard library. I haven't done this before in any language, so any tips would be appreciated. From what I gather, a call to posix_openpt or openpty returns a master and a slave, or alternatively opening /dev/ptmx followed by calls to grantpt and unlockpt. I'm building a load balancing and sharing system similar to Platform's LSF. The pseudo terminal is for interactive jobs running on a remote machine. Thanks for any help! -Tom

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Type-level programming problem
by Thomas Schilling 30 Apr '07

30 Apr '07

Hi, I have a type class similar to this one. data T class Foo ns a b c | ns -> a, ns -> b, ns -> c where mkFoo :: ns defaultA :: a defaultB :: c -> IO b defaultC :: [T] -> c f :: c -> b -> a -> (b, Int) The idea is, that I define classes of components where the data types of the methods are component-specific. I therefore use a namespace to define the component types (associated types would be nicer, but are not widely supported). Given a few sample instances, I can define a configuration: data X; data XA = XA; data XB = XB; data XC = XC instance Foo X XA XB XC where mkFoo = undefined defaultA = XA defaultB XC = return XB defaultC _ = XC f _ b _ = (b,0) mkX = mkFoo :: X data Y; data YA = YA; data YB = YB; data YC = YC instance Foo Y YA YB YC where mkFoo = undefined defaultA = YA defaultB YC = return YB defaultC _ = YC f _ b _ = (b,1) mkY = mkFoo :: Y config = mkX .*. mkY .*. HNil Using this configuration, I now want to define various functions that work on As, Bs or Cs, respectively. For example, I'd like to have a :: XA :*: XB :*: HNil a = hMap DefaultA config but I fail to figure out how to define DefaultA. I tried with this one: data DefaultA instance Foo ns a b c => Apply DefaultA ns a where apply _ _ = defaultA but I get: Could not deduce (Foo ns1 a b1 c1) from the context (Apply MakeAs ns a, Foo ns a b c) arising from use of `defaultA' at /Users/nominolo/Devel/Haskell/testcase1.hs:126:16-23 Possible fix: add (Foo ns1 a b1 c1) to the class or instance method `apply' In the expression: defaultA In the definition of `apply': apply _ _ = defaultA In the definition for method `apply' I guess I need some type class that also binds b and c. Or maybe I need to do some things completely different at all. Ideally, I'd hope to keep the work for adding new Foo instances as low as possible, though. I attached the relevant parts of the HList implementation and the code. Any suggestions? /Thomas

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RE:Cross-over from Haskell.org [Haskell] Re: Newbie: what are the advantages of Haskell?
by Taillefer, Troy (EXP) 30 Apr '07

30 Apr '07

Udo, I am cross posted/migrated this to Haskell-cafe per Simons request sorry for the late response I don't check this email on weekends I really dislike Perl as a programming language but I have to strongly disagree about your statements about CPAN and the quality of its contents. I have worked professional in Perl (Feel free to feel sorry for me) for over a year and I have always been impressed with what I could find in CPAN. Perl obviously has a niche and is "well supported by its community". Perl is popular so it must have some merit. I don't subscribe to the flawed reasoning that Perl Hackers just don't know any better or that they are dumb, or intellectual inferior in some way. Programmers have very good and valid reason for choosing their tools and I will have words for those who irrational state otherwise especially in a belittling manner I am pro choice. I am going to borrow some Ebonics "Don't language Hate Appreciate" Troy -----Original Message----- From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Udo Stenzel Sent: Saturday, April 28, 2007 7:06 AM To: haskell(a)haskell.org Subject: Re: [Haskell] Re: Newbie: what are the advantages of Haskell? Michael T. Richter wrote: > I wish I knew the language better so I could start working on those > libraries. Which ones? "those libraries" cannot come into existence until someone says what's actually missing. (The bulk of CPAN is crap and is certainly not worth being reimplemented.) -Udo -- "Object-oriented programming is an exceptionally bad idea which could only have originated in California." -- E. W. Dijkstra

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Indexing of content at haskell.org by search engines
by Henning Thielemann 30 Apr '07

30 Apr '07

I have noticed that only parts haskell.org are indexed by Google. E.g. mail archives of Haskell-Cafe and HaskellWiki are not indexed, but cvs.haskell.org, darcs.haskell.org and hackage.haskell.org are.

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Re: [Haskell] Re: Newbie: what are the advantages of Haskell?
by Rafael 30 Apr '07

30 Apr '07

I'll study these ways to debugging Haskell, and in accordance with my final work plan I'll decide the better way to follow.. For the moment i'm studing Haskell things, but in the next times I'll might decide it. A lot of thanks Neil and Simon. On 4/30/07, Simon Marlow <simonmarhaskell(a)gmail.com> wrote: > Rafael wrote: > >> Hi > >> > >> HTTP. We also need better availability of libraries, and a more > >> standard and reliable way to install them and specify their > >> dependencies. We could also do with a good debugger. These are being > >> addressed by the Google Summer of Code project. > > > > Hi Neil, a good debugger ? What is in Google Summer of Code about it ? > > > > I just found it: > > http://hackage.haskell.org/trac/summer-of-code/ticket/6 > > that havent student related. > > > > I was studing about debuging techs for haskell to build a nice > > debugging enviorment in HIDE or some other IDE. It's my bacharelor > > final work. > > We're building some debugging features into GHCi. This started with Pepe > Iborra's Summer of Code project last year, and more recently Bernie Pope > reworked the breakpoint support during his internship here at MSR Cambridge. We > are actively working on polishing what we have for inclusion in the next major > GHC release. > > The GHCi debugger is based on imperative debugging techniques: breakpoints, > single stepping, etc. It's a live debugger rather than post-mortem. We > focussed on accessibilty, rather than functionality: so the debugger is always > on, and it works with everything that you can compile in GHCi. It lacks some of > the advanced debugging features you'll find in Hat, for example, but we hope it > makes up for that by being more broadly accessible. > > More information on the debugger is here: > > http://hackage.haskell.org/trac/ghc/wiki/NewGhciDebugger > > although note that development is ongoing and more features have been added > since the wiki was last updated. > > All the debugging functionality is exposed by the GHC API, so it's certainly > possible to build debugging support into an IDE based on GHC. > > Cheers, > Simon >

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