- Haskell-Cafe - Haskell.org

How to implement `amb' operator?
by Meihui Fan 07 Apr '04

07 Apr '04

Hi, I think it's good idea to compute non-deterministic problems with the `amb' operator, just as in LISP/scheme. But how to implement it in haskell? noclouds

3 2

Newbie: Is it possible to catch _|_ ?
by Russ Lewis 07 Apr '04

07 Apr '04

Another newbie question: Is it possible to catch _|_ - that is, to encounter it, handle it and continue? I'm assuming that you all are going to say no, since I can't figure out any way to do this and retain the functional nature of Haskell.

4 5

Data structure for filesystem representation
by Gerd M 06 Apr '04

06 Apr '04

Hello! For an university project i need to design a data structure (in haskell) that's capable of representing a unix filesystem and therefore fast insertion/deletion. Support for persistent storage is also required. Principally i was thinking to use a Binary Search Tree or an AVL Tree but since this is my first real world application in Haskell I'm not so sure if this is a good (performant) solution. Any advice is welcome! Regards, Gerd. _________________________________________________________________ MSN 8 with e-mail virus protection service: 2 months FREE* http://join.msn.com/?page=features/virus

1 0

RE: [Haskell-cafe] Using -fcontext-stackXX in an OPTIONS pragma
by Simon Peyton-Jones 06 Apr '04

06 Apr '04

Contrary to the manual, -fcontext-stackN is a static flag, so you have to set it on the command line, I'm afraid. I'll fix the manual. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Anders Höckersten | Sent: 05 April 2004 13:43 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Using -fcontext-stackXX in an OPTIONS pragma | | Hi! | I am currently trying to use GHC 6.2 together with the OPTIONS pragma. | My program is generating files and needs to set the context stack | differently for different files. I try to do this using the OPTIONS | pragma, by having this line at the top of my files: | {-# OPTIONS -fglasgow-exts -fcontext-stackXX #-} | (where XX is some generated number) | However, compiling this gives me an error: | Test.hs: unknown flags in {-# OPTIONS #-} pragma: -fcontext-stack60 | | Is it supposed to be possible to set the context stack using the OPTIONS | pragma, and if so, what am I missing? | | /Anders | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

[Haskell-cafe] Fuse functions given to map and filter without RULES pragma
by Roman Beslik 05 Apr '04

05 Apr '04

I've found a funny way to fuse functions given to map and filter without RULES pragma: toSelect = (,) Just fromSelect = uncurry mapMaybe transformFirst f (x, y) = (f x, y) maybe f = transformFirst (\g x -> g x >>= f) map f = maybe (Just . f) filter f = maybe (\x -> if f x then Just x else Nothing) test :: [Int] test = fromSelect (map (*2) (filter (<10) (map (+3) (toSelect [1,3,4,6,8])))) Everything looks like obvious application of filter and map, but GHC inline (*2), (<10), (+3) (with -O flag) :^). -- Best regards, RB.

1 0

Re: [Haskell-cafe] Context for type parameters of type constructors
by Henning Thielemann 05 Apr '04

05 Apr '04

On 3 Apr 2004, Dylan Thurston wrote: > You might want to add a functional dependency, if you only have one > type of scalars per vertor space: > > > class VectorSpace v a | v -> awhere > > zero :: v > > add :: v -> v -> v > > scale :: a -> v -> v > > But then again, you might not. > > > > instance Num a => VectorSpace a a where > > > zero = 0 > > > add = (+) > > > scale = (*) I see ... functional dependency means "for each 'v' there is only one 'a' that forms a VectorSpace with 'v'". Thus the "VectorSpace a a" instance would exclude any other instance. Since this instance causes 'undecidable instance' and 'overlapping instance' errors in each constellation I tried I finally decided to content myself with special instances instance VectorSpace Double Double instance VectorSpace Float Float These will be the anchors for each type construction recursion needed for more complex VectorSpaces. But e.g. instance VectorSpace (Complex Double) (Complex Double) is still rejected as overlapping with instance (RealFloat v, VectorSpace a v) => VectorSpace a (Complex v) of course ... unfortunately. In addition I separated a class VectorSpacePure from the VectorSpace class that contains the operations that don't need the scalar type 'a', i.e. 'zero' and 'add'. Further I swapped consistently 'a' and 'v' in the class definition and in the quantity data type, i.e. "class VectorSpace a v" and "data Quantity a v". I suppose that it will more oftenly be necessary to say something about the vectors that can built from a certain scalar (i.e. Quantity a) rather than about the scalar types that can be the base of a vector type. > By the way, depending how you resolve the issue above, you might want > instead > > > instance (RealFloat a, VectorSpace b a) => VectorSpace [b] a where > > ... That's really a nice generalization! With the specialised instances for Float and Double as shown above this works. Now I can built VectorSpace over several levels, e.g. a list of complex numbers as vectors with respect to real numbers. But it prohibits functional dependency, right? Functional dependency would probably allow the compiler to detect which scalar type is associated with an vector and thus the compiler wouldn't ask me to tell it explicitly all the time. > Alternatively, if you want to consider varying the scalars, you can > add 'a' as a dummy type variable to 'Quantity': > > > data Quantity v a = Quantity v > > > > instance (Show v, Fractional a, Normed v a) => > > Show (Quantity v a) where > > show (Quantity v) = > > let nv::a = norm v > > in (show (scale (1/nv) v)) ++ "*" ++ > > (show nv) Since I can't use functional dependency I chose this way. Maybe a type class like the VectorSpace finds the way to a revised version of Prelude? Thanks a lot for the help!

2 1

RE: [Haskell-cafe] Building Haddock on Windows
by Bayley, Alistair 05 Apr '04

05 Apr '04

Trying to invoke Haddock with --read-interface=c:\ghc\ghc-6.0\doc\html\base\base.haddock Initially I get a stack overflow, so I added: +RTS -K2M -RTS ... and now I get: Fail: end of file Action: Data.Binary.getWord8 Is the --read-interface option specified correctly? This is where my GHC installation is, and the base.haddock file exists in there. I assume this is the interface file. Is this a compatibility problem? i.e. haddock file distributed with GHC 6.0 too old for Haddock 0.6? > -----Original Message----- > From: Simon Marlow [mailto:simonmar@microsoft.com] > Sent: 17 March 2004 15:29 > To: Bayley, Alistair > Cc: haskell-cafe(a)haskell.org > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > > > - why doesn't it know what Int, String, Float, IO, Monad, > > Show, etc are? > > What invocation option do I need to ensure it links to > > Prelude/hierarchical-library stuff? (I think it's > > --read-interface; must I > > generate interface files for the Prelude and libraries though?) > > You get the interfaces for the libraries with a GHC > installation. Just > use the --read-interface flag to tell Haddock about them. > > Cheers, > Simon ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

1 0

Using -fcontext-stackXX in an OPTIONS pragma
by Anders Höckersten 05 Apr '04

05 Apr '04

Hi! I am currently trying to use GHC 6.2 together with the OPTIONS pragma. My program is generating files and needs to set the context stack differently for different files. I try to do this using the OPTIONS pragma, by having this line at the top of my files: {-# OPTIONS -fglasgow-exts -fcontext-stackXX #-} (where XX is some generated number) However, compiling this gives me an error: Test.hs: unknown flags in {-# OPTIONS #-} pragma: -fcontext-stack60 Is it supposed to be possible to set the context stack using the OPTIONS pragma, and if so, what am I missing? /Anders

1 0

Partial Specification of Right-Hand Side
by Russ Lewis 05 Apr '04

05 Apr '04

Hi, everybody. I'm really new to Haskell, but am really fascinated by it all! I have been thinking about its uses, and have a newbie question. I was thinking about the process of mapping imperative languages to functional ones. It occurred to me that you can restate an imperative function as a functional function which takes the old state of the global values as an input and returns a new set of globals and output: int foo(char arg1) {...}; foo arg1 globalsIn = ( retval , globalsOut ) Now, some imperative functions truly are functional. So I was wanting to write something like this in Haskell: foo _ globalsIn = ( <unspecified> , globalsIn ) ...other foo defintions below... Basically, the idea would be that, no matter what the argument was, the output globals would be identifal to the input globals. So, if the calling function only cared about the output globals, then no more calculation would be needed. But if the retval was required, then the thunk would cause more searches of 'foo' to be performed. Of course, you can implement it with a second function: foo x globalsIn = ( fooInternal x , globalsIn ) ...define fooInternal below... But I was wondering if Haskell had a way to do this with a single function.

1 0

Context for type parameters of type constructors
by Henning Thielemann 03 Apr '04

03 Apr '04

Sorry for sending this twice, but it seems to me that the newsgroup fa.haskell only logs the discussion of haskell and haskell-cafe. ---------- Forwarded message ---------- Date: Mon, 29 Mar 2004 01:18:27 -0800 From: googlegroups(a)henning-thielemann.de (Henning Thielemann) Newsgroups: fa.haskell Subject: Context for type parameters of type constructors NNTP-Posting-Host: 134.102.210.249 Message-ID: <a17fb6f7.0403290118.786ede0d(a)posting.google.com> I have a problem with type classes that can be illustrated with the following example: I want to declare a class for vector like data. 'Vector' is not meant as a synonyme for 'array' but a 'vector' shall be a mathematical object that allows for some linear operations, namely summing and scaling. Thus I setup a type constructor VectorSpace in the following way: > module VectorSpace > where > > class VectorSpace v where > zero :: v a > add :: v a -> v a -> v a > scale :: a -> v a -> v a I haven't added context requirements like (Num a) to the signatures of 'zero', 'add', 'scale' because I cannot catch all requirements that instances may need. The problematic part is the 'scale' operation because it needs both a scalar value and a vector. Without the 'scale' operation 'v' could be simply a type (*) rather than a type constructor (* -> *). Now let's try some instances: > data (Num a) => VList a = VList [a] > > instance VectorSpace VList where > zero = VList (repeat 0) > add (VList x) (VList y) = VList (zipWith (+) x y) > scale s (VList x) = VList (map (s*) x) > > data (Num a) => VFunc b a = VFunc (b->a) > > instance VectorSpace (VFunc b) where > zero = VFunc (\_ -> 0) > add (VFunc f) (VFunc g) = VFunc (\x -> (f x) + (g x)) > scale s (VFunc f) = VFunc (\x -> s*(f x)) But now GHC complains: $ ghc -c VectorSpace.lhs VectorSpace.lhs:37: Could not deduce (Num a) from the context (VectorSpace VList) arising from the literal `0' at VectorSpace.lhs:30 Probable fix: Add (Num a) to the class or instance method `zero' In the first argument of `repeat', namely `0' In the first argument of `VList', namely `(repeat 0)' In the definition of `zero': zero = VList (repeat 0) ... I hoped that when I declare VList within the context (Num a) then it is always asserted that a VList is built on a Num type. If it is necessary to add (Num a) somewhere in the instance declaration - then where? Btw. I'm using $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.0

2 4