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Nice pedagogical examples of type classes?
by Benjamin Pierce 29 Jan '05

29 Jan '05

My Advanced Programming course is quickly approaching the lectures on type classes, and I am interested in finding a little more (beyond what's in SOE) in the way of examples that illustrate nice uses (especially of more advanced aspects of the class system). I'd be most grateful for pointers to people's favorites. Examples should be reasonably small and depend only on Haskell 98 features. I haven't discussed monads yet, but I'm also interested in good monadic examples for later in the semester. Many thanks, - Benjamin P.S. Suggestions for interesting exercises along these lines are also appreciated!

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RE: Answers to Exercises in Craft of FP
by Christian Hofer 28 Jan '05

28 Jan '05

This should have been sent to haskell-cafe... > > Am 27.01.2005 um 21:28 schrieb Paul Hudak: > >> Chris, I'm not sure that I understand your argument. How about this >> scenario, which is what I do: Students are assigned problems, >> without solutions. They are given some time to work them out and >> turn them in. Then they are given the solutions, most of which I go >> over in class. > > That is perfectly alright with me. The problem that we are discussing > is that it would be helpful to have the solutions to the exercises for > a book that I buy for studying on my own. And this is where I learn > most from. But most of those books (including the Haskell School of > Expression) do not include solutions. In contrast, Pierce's Types and > Programming Languages helps the reader by giving at least solutions to > the most important exercises. And it is just nice to check, if you > have understood the problem correctly and whether there is a more > elegant solution. > > Regards, > Chris >

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RE: [Haskell-cafe] wierd type errros with MPTCs
by Simon Peyton-Jones 28 Jan '05

28 Jan '05

The kind error seems fair enough. GHC looks at the class decl, and decides that indexVal is of kind *. Then the instance declaration uses it with kind (*->*). You may say that it should look at the instance decl too, before deciding kinds, but the instance decl might be in another file. Uncommenting insertIndex allows the compiler to see that indexVal must be of kind (*->*), since it's applied to item in the type of insertIndex. So the compiler does do this much lookahead; the class ops can't be in another module. In GHC you can say class Table table (indexVal :: * -> *) where ... if you like, to make it 100% clear. When you declare a default method, GHC makes up a definition like this: $dmunion :: Table table indexVal => table item -> table item -> table item $dmunion t1 t2 = t1 ("dm" for default method) Then, in the instance decl, if you don't give a declaration for union, GHC makes one up, as if you had written instance Table DBTable IVal where union = $dmunion Now you can see where the error arises. The call to $dmunion gives the constraint (Table DBTable ??) with no way for it to figure out what the ?? is. The type of $dmunion is ambiguous in fact. Now, I agree that the error message, mentioning $dmunion, is rather unhelpful. But I can't see an easy way to improve it. Meanwhile, I hope this helps you make progress. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of S. | Alexander Jacobson | Sent: 27 January 2005 17:53 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] wierd type errros with MPTCs | | This code gives me a Kind error because IVal isn't | applied to enough type arguments. | | import qualified Set | | class Table table indexVal | indexVal->table where | --insertIndex::item->indexVal item -> table item ->table item | union::table item -> table item -> table item | --union t1 t2 = t1 | | data DBTable item = DBTable | data IVal item = Name item | | instance Table DBTable (IVal ) where | | Weirdly, when I uncomment the insertIndex | function, things work. But, if I then uncomment | the default implementation of union, I get: | | No instance for (Table DBTable indexVal) | arising from use of `Main.$dmunion' at example.hs:13 | In the definition of `union': union = Main.$dmunion | In the definition for method `union' | In the instance declaration for `Table DBTable IVal' | | I don't know what this error even means. But it | goes away if I put the union implementation in the | instance rather than in the class. | | Bot these error messages seem unreasonable. Can | someone clarify? | | Note: I am using GHC 6.2.2 | | -Alex- | | | ______________________________________________________________ | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: Answers to Exercises in Craft of FP
by David Owen 28 Jan '05

28 Jan '05

>Gour (gour-haskell-cafe-sender-b3e10c(a)atmarama.org) wrote RE- : > > >Do you know if there are solutions to exersises available somewhere? > >Have you gone through the whole book, i.e. all the exercises? > >Sincerely, >Gour Hi Gour, Unfortuantely I don't know of anywhere that the exercise answers can be found, even after some google searching. I would definitely find them useful though as there are a couple I haven't been able to work out. I can't say I've gone through all the exercises yet but I'll get there one day! I've got as far as the abstract data types chapter. regards DaveO

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fastest Fibonacci numbers in the West
by William Lee Irwin III 28 Jan '05

28 Jan '05

Inspired by a discussion on freenode #haskell, I tried to write the fastest Fibonacci number function possible, i.e. given a natural number input n to compute F_n. mlton seems to enjoy substantially better speed despite equivalent algorithms; it may be enlightening to determine the causes of this, at least for those concerned about performance and the inner workings of the Haskell runtime system(s). In general, I am not usually very concerned about performance, nor am I in this case. But it's something of a mindless microbenchmark or similar. The things I'd normally think of to speed this up would be getting some primitives to find the highest bit of an integer (floor . lg) and to test a given bit of an integer (something vaguely like fromEnum . fromIntegral . (`mod` 2) . (/2^k)), both in some lightweight O(1) with low-hidden-constant manner. This doesn't appear to be a factor in the testing I did, as they're largely 1:1 translations of each other. Still, such things would be useful in various other algorithms, of far greater importance than this one. For the moment, mlton-generated binaries crash computing fib (10^8-1), and there is a 6:1 speed difference for fib (10^7-1) between the two, where mlton-generated binaries take just under 1 minute, and ghc- generated binaries take just under 6 minutes. Anyway, thoughts on how to improve all this from the programmer's point of view, or otherwise explaining what's going on or ameliorating whatever effect is at work here would be appreciated. -- wli

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wierd type errros with MPTCs
by S. Alexander Jacobson 28 Jan '05

28 Jan '05

This code gives me a Kind error because IVal isn't applied to enough type arguments. import qualified Set class Table table indexVal | indexVal->table where --insertIndex::item->indexVal item -> table item ->table item union::table item -> table item -> table item --union t1 t2 = t1 data DBTable item = DBTable data IVal item = Name item instance Table DBTable (IVal ) where Weirdly, when I uncomment the insertIndex function, things work. But, if I then uncomment the default implementation of union, I get: No instance for (Table DBTable indexVal) arising from use of `Main.$dmunion' at example.hs:13 In the definition of `union': union = Main.$dmunion In the definition for method `union' In the instance declaration for `Table DBTable IVal' I don't know what this error even means. But it goes away if I put the union implementation in the instance rather than in the class. Bot these error messages seem unreasonable. Can someone clarify? Note: I am using GHC 6.2.2 -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Question on "case x of g" when g is a function
by yeoh＠cs.wisc.edu 28 Jan '05

28 Jan '05

Can a kind soul please enlighten me on why f bit0 and f bit1 both return 0? > bit0 = False > bit1 = True > f x = case x of > bit0 -> 0 > bit1 -> 1

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Converting from Int to Double
by Dmitri Pissarenko 28 Jan '05

28 Jan '05

Hello! I have a list of integer numbers (grayscale values from 0 to 255) and want to convert them to a list of double numbers, so that each number is 0 <= x <= 1, where 0 is completely black and 1 is completely white. Before I convert the numbers, I need to convert them to a list of double values (then I can use map to re-scale each element from 0..255 to 0..1). How can I convert an Int into a Double? TIA Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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RE: [Haskell-cafe] File path programme
by Simon Marlow 28 Jan '05

28 Jan '05

On 25 January 2005 19:45, Duncan Coutts wrote: > On Tue, 2005-01-25 at 19:12 +0000, Ben Rudiak-Gould wrote: >> My concern here is that someone will actually use the library once it >> ships, with the following consequences: >> >> 1. Programs using the library will have predictable >> (exploitable) bugs in pathname handling. >> >> 2. It will never be possible to change the current weird >> behavior, because it might break legacy code. The System.FilePath >> library will >> have to remain in GHC forever in its current form, enticing >> programmers with its promise of easy pathname handling and then >> cruelly breaking its contract. >> >> If no one uses it in production code then we can fix it at our >> leisure, and having it out there with "experimental" status isn't >> necessarily a bad thing in that case. It just feels like we're >> playing a dangerous game. > > That's a sufficiently persuasive argument for me! > > Could we just punt this library for this release. After all we can add > libraries in a later point release (eg 6.4.1) you just can't change > existing APIs. We can't add libraries in a point release, because there's no way for code to use conditional compilation to test the patchlevel version number. This seems to be a common misconception, probably brought about by the fact that the time between major releases of GHC is getting quite long. Perhaps I should stop writing email and get some work done :) Cheers, Simon

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RE: [Haskell-cafe] Problem with Hyperlinking Haddock-documentation
by Simon Marlow 28 Jan '05

28 Jan '05

On 27 January 2005 16:54, daniel.is.fischer(a)web.de wrote: > BTW, would it be worthwhile to let haddock use a list of default > interfaces, say on installation it would create a file with > -i /usr/...../base.haddock > -i /usr/...../parsec.haddock &cetera > in it, later a user might add other often used interfaces ? > > Okay, writing one's own script isn't too hard, so there's no real > need for it, but it would be nice for dummies like me. I'd like Haddock to know about installed packages, and in fact we're part of the way there in GHC 6.4 because the package database now includes the locations of the .haddock file and the HTML for each package. I just have to modify Haddock to query the package database... Cheers, Simon

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