Re: [Haskell-cafe] ANNOUNCE: docidx-1.0.0
With apologies, I now announce docidx-1.0.1 which fixes a problem kindly pointed out by Jack Henahan. If you tried to install docidx using cabal install over the weekend and failed, please try again now. The docidx-1.0.0.tar.gz uploaded to hackage on Friday was missing most of its source code (everything except its Main module in fact), because I hadn't added an other-modules field to docidx.cabal. I hadn't actually tried to build from that tarball, so I didn't spot this: I had assumed that if I could cabal install in my source directory, then the tarball (produced by cabal sdist) would work too. (And yes, I've tested this version before announcing it! :-) ) Apologies again, and many thanks to Jack for pointing this out. -Andy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
Patrick Browne patrick.bro...@dit.ie writes: Is it possible to model partial inheritance using Haskell type classes? For example, if the class Bird has a flying method can we represent Penguins as a sub-class of Bird without a flying method? I'm not sure the question makes sense, if fly is a method of class Bird, then it can't also be a member of class Penguin. You can of course make instances of Bird without implementing the fly method (whether they are also instances of Penguin or not), or by implementing it as undefined. -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] I confused myself with generics, and now I confused ghc too
I want to implement generic comparison on top of syb-with-class, and based on looking at syb, it seems I could base it on something like gzipWithQ from syb's Data.Generics.Twins. However, there's no Data.Generics.SYB.WithClass.Twins. So I set out to see if I can write one myself (well, one with just enough stuff to support gzipWithQ anyway) Here's what I've got so far: http://hpaste.org/49168 The problem is that GHCi seems to be somewhat confused, or at least confusing, about the type of gzipWithQ here. When I ask with :t, it reports the type as: gzipWithQ :: (Data ctx a2, Data ctx a3, Data ctx a) = Proxy ctx - (forall a0. Data ctx a0 = a0 - forall a1. Data ctx a1 = a1 - r) - a2 - a3 - [r] If I actually replace gzipWithQ's annotated type with this, GHCi will reject the program. And in any case with GenericQ being defined as type GenericQ ctx r = Data ctx a = a - r, isn't this closer to gzipWithQ's type with the type synonyms expanded? gzipWithQ :: forall ctx a r. Data ctx a = Proxy ctx - (Data ctx a0 = a0 - (Data ctx a1 = a1 - r)) - (Data ctx a2 = a2 - (Data ctx a3 = a3 - [r])) (GHCi accepts this as gzipWithQ's type so I assume it's the same one. I assume those class constraints after the = come with implicit foralls? Because it seems to mean the same thing if I put foralls in there) I have no idea how to even start figuring out how to make gzipWithQ and its type work. I only got this far by copying code from Data.Generics.Twins and passing down the proxy parameter and/or context type parameter (is it even possible for this approach to work?). What I can tell is that GHCi will still change the type like that even if gzipWithQ = undefined, so I'll have to change the type and not just the body of the function. Is this the kind of thing that all those newtypes in Data.Generics.Twins are for? Should I read up on type theory? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trying to make a calculator
Hi guys, I'm new to Haskell, I learn it for couple of months now and we've learned only simple functions like map, zip etc. But now my assignment is a lot tougher than I thought so I'll need some guidelines. I've googled my assignment and I saw a person with the exact assignment so I am gonna copy the task then I'm gonna show you what I've done so far. Here it is: Make calculator function. The function argument is a list of strings and also form such list, as each string of the argument made definite action. - If the string has the form of an arithmetic expression - calculate this expression. The string result becomes part of the list-result. If the expression contains a variable which is not assigned value, the result is displayed undefined. - If the string has the form Name = value calculated from the last expression is assigned to the variable with the corresponding name in the list, and in the result listis formed a string with type name = ... where the site stands blank corresponding value. If there is not a calculated expression to be assigned to form a string no value. - If the string is non-blank, but there is a species different from the above two case, form the string error. - If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument [3 +7 / 2, 2 + x, = s, 2 * s +4, , 2 + +4 / 5] function should provide a result-list [6, undefined, s = 6, 16, error]. = So what I've done so far is: calculator [] = [] calculator [ ] = [] calculator [c] = [c] calculator (c: :cs) = c:calculator cs calculator ( :cs) = calculator cs calculator (c: : :cs) = calculator (c: :cs) calculator (c:cs) | (head c `elem` ['a'..'z']) (head(tail c) `elem` ['+','-','*','/']) (last(take (length c-2)c) `elem` ['1','2','3','4','5','6','7','8','9']) = calculator cs | otherwise = error:calculator cs I try at first to use pattern matching for the blank strings in the first list, so that I can get rid off them in the second list. Am I even on the right way here? I found the elem function and I'm trying to check if the first char of the string is a letter or a number, then if the second char is one of the operations and so on. Problem is that I have no idea how can I check if the variable is defined already or how to save a variable and then give that variable the new value - is that even possible? :] And another thing `elem` [1..9] isn't working - how can I make a single char of a string to hold an integer.. Could you give me some guidelines or at least tell me if I'm on the right way here so I can move forward and improve my program. Thanks a lot! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trying to make a calculator
2011/7/18 Emanuil Boyanov emanuilboya...@gmail.com: Hi guys, I'm new to Haskell, I learn it for couple of months now and we've learned only simple functions like map, zip etc. But now my assignment is a lot tougher than I thought so I'll need some guidelines. I've googled my assignment and I saw a person with the exact assignment so I am gonna copy the task then I'm gonna show you what I've done so far. Here it is: Make calculator function. The function argument is a list of strings and also form such list, as each string of the argument made definite action. - If the string has the form of an arithmetic expression - calculate this expression. The string result becomes part of the list- result. If the expression contains a variable which is not assigned value, the result is displayed undefined. - If the string has the form Name = value calculated from the last expression is assigned to the variable with the corresponding name in the list, and in the result list is formed a string with type name = ... where the site stands blank corresponding value. If there is not a calculated expression to be assigned to form a string no value. - If the string is non-blank, but there is a species different from the above two case, form the string error. - If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument [3 +7 / 2, 2 + x, = s, 2 * s +4, , 2 + +4 / 5] function should provide a result-list [6, undefined, s = 6, 16, error]. = So what I've done so far is: calculator [] = [] calculator [ ] = [] calculator [c] = [c] calculator (c: :cs) = c:calculator cs calculator ( :cs) = calculator cs calculator (c: : :cs) = calculator (c: :cs) calculator (c:cs) | (head c `elem` ['a'..'z']) (head(tail c) `elem` ['+','-','*','/']) (last(take (length c-2)c) `elem` ['1','2','3','4','5','6','7','8','9']) = calculator cs | otherwise = error:calculator cs I try at first to use pattern matching for the blank strings in the first list, so that I can get rid off them in the second list. Am I even on the right way here? I found the elem function and I'm trying to check if the first char of the string is a letter or a number, then if the second char is one of the operations and so on. Problem is that I have no idea how can I check if the variable is defined already or how to save a variable and then give that variable the new value - is that even possible? :] And another thing `elem` [1..9] isn't working - how can I make a single char of a string to hold an integer.. Could you give me some guidelines or at least tell me if I'm on the right way here so I can move forward and improve my program. Thanks a lot! Hello, You should work by smaller steps. For instance, you can begin by abstracting the syntax. At the very beginning, you get some strings (a list of statements) and you transform them in something much simpler to use. Here, there is also an intermediary step to make things easier. Let's say you have 1 + 2 . You can make things simpler by working on [1, +, 2]. Here, the problem with blancs is already dealt with. You just have a list of tokens. Then, to make it simple later on to differentiate between a number and an operator (and a variable), you can define your own representation, for instance: data Token = TNum Int | TOp Op | ... data Op = Add | Sub | ... Now the list [1, +, 2] becomes [TNum 1, TOp Add, TNum 2]. Now you can use pattern matching on the Token and easily know what you have. For your variable assignment problem, you have to provide an environment, i.e. a mapping between variable name and their value. If you use a recursive function to evaluate your little language, you can provide that environement as a supplementary argument. Try to proceed that way, one step at a time, and come back if you have any problem. Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trying to make a calculator
But now my assignment is a lot tougher than I thought so I'll need some guidelines. I'm pretty sure your course instructor has already given you the exact guidelines that are required, in her lecture. There's hundreds of ways to make a calculator, and it really depends on what the teacher wants to emphasize, in the context of the lecture. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to use Yesod OAuth
Hello, I'm trying to use Yesod oAuth plugin to a SNS site which is very similar to Twitter. I add oAuth into the authPlugin and I'm able to see the auth URL at login page. I click the URL and it forwards me to a page ask me input that SNS site account. I fill in my account then the browser navigate to the authorize page display a authorised code (8 digits). Then, how I suppose to continue? (I think this is the 2nd phase of the oauth process) I expect there will be a text box in my App where I can fill in the authorised code. Or even the oAuth plugin automatically get the authorised code and continue to the next oauth phase. Appreciate your help! Haisheng ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
On 18/07/2011 13:52, Ketil Malde wrote: I'm not sure the question makes sense, if fly is a method of class Bird, then it can't also be a member of class Penguin. I am actually doing a language comparison and I was checking out a paper that said: Type classes allow for partial inheritance, so that penguins can be birds without flying behavior. My question is concerned with the language constructs to do this rather than the semantic validity or desirability of such construct. But as pointed out by Jerzy my question is silly because can penguins can fly: On 17/07/2011 11:14, Jerzy Karczmarczuk wrote: PS. Penguins DO fly. http://www.telegraph.co.uk/news/worldnews/1583517/Flying-penguins-found-by-BBC-programme.html This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
Patrick Browne : I was checking out a paper that said: Type classes allow for partial inheritance, so that penguins can be birds without flying behavior. ... as pointed out by Jerzy my question is silly because can penguins can fly ... No, the question is not silly because of that crazy Adelie tribe. The question is doubtful, because the Haskell type classes is not an object-oriented system of classes, subclasses, etc. When you declare Rationals, you don't inherit the multiplication for them as Numbers. This inheritance you wish may manifest itself when you specify the *instances*. That's why I suggested how you might do that: for some datatypes, say the Emperors, you specify some special flying method (e.g. dummy or bombing), or you don't specify it at all. And the Emperors won't fly. Jerzy Karczmarczuk ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Failure to install Network.CGI
On 11-07-17 12:29 AM, william murphy wrote: command line: cannot satisfy -package-id network-2.3.0.2-24fdc6b92867c7236e81708f93cae7d0 Look at the output of ghc -v and be very horrified. The problem is not lacking packages or being outdated. The problem is possessing too many packages and upgrading too much. The solution is nuking. See my http://www.vex.net/~trebla/haskell/sicp.xhtml for complete information and more ways to be horrified. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: cabal-dev 0.8
We're happy to announce the release of cabal-dev 0.8! This version is available on hackage now, and contains many bug fixes and improvements, as outlined in the full release notes below. --Rogan cabal-dev release 0.8 == The 0.8 release of `cabal-dev` fixes many of the bugs that have been reported by `cabal-dev` users. It also provides more flexible, easier configuration and improves tool support for working within the `cabal-dev` sandbox (ghc-pkg suport and better ghci support). Upgrading is recommended for all users. Please report any bugs or submit enhancement requests to https://github.com/creswick/cabal-dev/issues. Much of the work on this release (and previous releases) of `cabal-dev` was funded by Galois, Inc. http://www.galois.com/ About cabal-dev == `cabal-dev` is a Haskell build tool that augments Cabal and cabal-install to make it convenient to work with a sandboxed set of packages. It limits the effect of your build on unrelated parts of your system. Among other things, `cabal-dev` satisfies the following needs: * Avoid conflicts between libraries by working only with the set of packages that are needed for the current project. * Enable depending on a patched dependency without that patched dependency accidentally being used by another project's build. * Enable the simultaneous development of multiple interdependent packages without the development versions leaking to other project build environments. Significant effort has been made to ensure that `cabal-dev` works across platforms (Mac, Windows, and Linux) and with the versions of GHC that are most commonly in use (6.10-7.0). What's new == Details can be found on github at https://github.com/creswick/cabal-dev. Bugs fixed: * `cabal-dev` no longer passes `--verbose` to cabal-install commands that do not accept it. https://github.com/creswick/cabal-dev/issues/18 * `cabal-dev` now recognizes and passes through short flags to cabal-install. In particular, this bug frequently came up when attempting to pass a flag (`-f`) to `cabal-dev install`. https://github.com/creswick/cabal-dev/issues/6 * `cabal-dev` now supports all cabal-install commands, including `test`. https://github.com/creswick/cabal-dev/issues/10 * We also made changes that should address inconsistencies with the remote repository (Hackage) cache location, but those issues remain open until we have confirmation from end users (you!) that these changes actually fix those issues. https://github.com/creswick/cabal-dev/issues/15 and https://github.com/creswick/cabal-dev/issues/14 Enhancements and new features: * `cabal-dev ghci` now invokes ghci with the exact same set of flags that would be used in a cabal-install build (using a trick copied from Leksah). This means that it will now have the appropriate package visibility, language settings, and source directories set when it is invoked. Note that in order for `cabal-dev ghci` to work, all dependencies must already be installed in the sandbox, and the package must already be configured. * `cabal-dev buildopts` will print the set of build options that would be passed to GHC when invoking `cabal build`. You can then use these flags for other tools that invoke GHC (e.g. the doctest package http://hackage.haskell.org/package/doctest). * `cabal-dev ghc-pkg` will invoke `ghc-pkg` with the appropriate flags to examine or alter the package database inside the `cabal-dev` sandbox. * `cabal-dev` now has more flexible support for cabal-install configuration: * By default `cabal-dev` will now use any settings from your cabal-install configuration file, except those that must be overridden in order to sandbox the build. This makes it more convenient to e.g. use an alternate remote repository with `cabal-dev`. * `cabal-dev` now supports merging of additional cabal-install configuration settings, to make it easy to work with multiple cabal-install configurations (`--extra-config-file=...`). * `cabal-dev` now has a flag to pass explicit options to cabal-install in case there are any conflicts between `cabal-dev` flags (`--cabal-install-arg`). * `cabal-dev` now has a flag to specify a particular cabal-install executable to use (`--with-cabal-install=...`). smime.p7s Description: S/MIME Cryptographic Signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Failure to install Network.CGI
On Mon, Jul 18, 2011 at 11:45 AM, Albert Y. C. Lai tre...@vex.net wrote: On 11-07-17 12:29 AM, william murphy wrote: command line: cannot satisfy -package-id network-2.3.0.2-24fdc6b92867c7236e81708f93cae7d0 Look at the output of ghc -v and be very horrified. The problem is not lacking packages or being outdated. The problem is possessing too many packages and upgrading too much. The solution is nuking. Once you've reinstalled, or nuked, you can use cabal-dev to never run into this problem again: http://www.reddit.com/r/haskell/comments/f3ykj/psa_use_cabaldev_to_solve_dependency_problems/ Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: cabal-dev 0.8
On Mon, Jul 18, 2011 at 1:10 PM, Rogan Creswick cresw...@galois.com wrote: We're happy to announce the release of cabal-dev 0.8! This version is available on hackage now, and contains many bug fixes and improvements, as outlined in the full release notes below. Wonderful! This is absolutely one of those indispensable tools for build automation and sanity preservation. I use cabal-dev to manage the builds for all of my Haskell projects, under many different configurations: https://jenkins.serpentine.com/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
On 18/07/2011 19:14, Jerzy Karczmarczuk wrote: That's why I suggested how you might do that: for some datatypes, say the Emperors, you specify some special flying method (e.g. dummy or bombing), or you don't specify it at all. And the Emperors won't fly. -- Here is my attempt data Emperor = Emperor data Robin = Robin class Bird a where fly :: a - a - a walk :: a - a - a instance Bird Robin where fly x y = y walk x y = x instance Bird Emperor where -- No fly method walk x y = y class (Bird a) = Penguin a where -- I do not think that I can override the walk method in the sub-class -- must be done in instance of Penguin instance Penguin Emperor where -- How can I override the walk method in the instance Penguin? -- walk x y = x Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
Oh, I got it: You want to have: class Bird b where class Penguin p where instance (Penguin b) = Bird b where fly = -- fly method for penguins 2011/7/19 Patrick Browne patrick.bro...@dit.ie On 18/07/2011 19:14, Jerzy Karczmarczuk wrote: That's why I suggested how you might do that: for some datatypes, say the Emperors, you specify some special flying method (e.g. dummy or bombing), or you don't specify it at all. And the Emperors won't fly. -- Here is my attempt data Emperor = Emperor data Robin = Robin class Bird a where fly :: a - a - a walk :: a - a - a instance Bird Robin where fly x y = y walk x y = x instance Bird Emperor where -- No fly method walk x y = y class (Bird a) = Penguin a where -- I do not think that I can override the walk method in the sub-class -- must be done in instance of Penguin instance Penguin Emperor where -- How can I override the walk method in the instance Penguin? -- walk x y = x Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
On Mon, Jul 18, 2011 at 3:14 PM, Jerzy Karczmarczuk jerzy.karczmarc...@unicaen.fr wrote: That's why I suggested how you might do that: for some datatypes, say the Emperors, you specify some special flying method (e.g. dummy or bombing), or you don't specify it at all. And the Emperors won't fly. Instead of flying, they'll throw exceptions around. =/ -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
On Tue, 2011-07-19 at 01:13 +0200, Yves Parès wrote: Oh, I got it: You want to have: class Bird b where class Penguin p where instance (Penguin b) = Bird b where fly = -- fly method for penguins I haven't followed the thread carefully but why does the bird have to be a penguin? -- As a side note - I agree with Christopher Done that the answer is that you shouldn't require for bird to fly. Regards signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
On 19/07/2011, at 5:09 AM, Patrick Browne wrote: On 18/07/2011 13:52, Ketil Malde wrote: I'm not sure the question makes sense, if fly is a method of class Bird, then it can't also be a member of class Penguin. I am actually doing a language comparison and I was checking out a paper that said: Type classes allow for partial inheritance, so that penguins can be birds without flying behavior. That would be An Image-Schematic Account of Spatial Categories by Werner Kuhn, I take it? His e-mail address is at the top of the paper, so why not ask him? I cannot find anything in the Haskell 2010 report that says anything about partial inheritance. In his example, class BUILDING building where specify the behavior of buildings here, if any class BUILDING house = HOUSE house where specify additional behavior of houses here, if any any instance of HOUSE *will* have in its interface everything that any instance of BUILDING will. As has already been pointed out, the implementation of something may be (or call) 'undefined', but this is rather boring; even Smalltalk-80 let you plug in self shouldNotImplement as the definition of a method. If you want a language where a subclass can remove features from the interface(s) of its superclass(es), you probably want Eiffel, not Haskell. But as pointed out by Jerzy my question is silly because can penguins can fly: On 17/07/2011 11:14, Jerzy Karczmarczuk wrote: PS. Penguins DO fly. http://www.telegraph.co.uk/news/worldnews/1583517/Flying-penguins-found-by-BBC-programme.html And what a fine April Fool's joke that was. But even in that joke, flying was something no other penguins can do. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
On 19/07/2011, at 11:09 AM, Patrick Browne wrote: data Emperor = Emperor data Robin = Robin class Bird a where fly :: a - a - a walk :: a - a - a instance Bird Robin where fly x y = y walk x y = x instance Bird Emperor where -- No fly method walk x y = y Note that no 'fly' method is visibly presented is not at all the same thing as no 'fly' method EXISTS. From the report: The default class method for v sub i is used if no binding for it is given in a particular instance declaration (see Section 4.3.2). so the absence of a method in 'instance Bird Emperor' doesn't mean that it doesn't _have_ one, just that it inherits the default class method. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] partial inheritance
Quoth Richard O'Keefe o...@cs.otago.ac.nz, [ ... re Werner Kuhn An Image-Schematic Account of Spatial Categories ... ] class BUILDING building where specify the behavior of buildings here, if any class BUILDING house = HOUSE house where specify additional behavior of houses here, if any any instance of HOUSE *will* have in its interface everything that any instance of BUILDING will. But ... for those who might be overly influenced by a superficial similarity to OOP here ... the idea that the above relationship makes House a subclass of Building isn't really a natural way to look at it. It's true that a House instance will also have in its interface everything that any instance of Building will, but only because the programmer is constrained to do that. To consider the whole outline: class Building building where ... specify the behavior of buildings here, if any class Building house = House house where ... specify additional behavior of houses here, if any data Shed = Shed shed instance Building Shed where ... define Building functions for Shed instance House Shed where ... define House functions for Shed The Building = House constraint doesn't add anything, it just checks that each instance of House also is an instance of Building. If you omit this constraint, everything will work the same - even the constraint will still be there, implicitly, albeit not at the same level. To me, it's somewhat misleading to use words like inheritance and subclass for the relationship between Building and House, unless there's some feature of the system I missed. Donn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
