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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!

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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. *****************************************************************

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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

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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.

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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

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Functional Programming and AI
by GAYLE, Orrett Orville 01 Apr '04

01 Apr '04

I was wondering if I could be directed to resources for Artificial Inteligence where Hakell (or any other functional language for that matter) is the language of implementation. I am fairly new to the feild and that should be a concideration when sending the listings. Thanks in advance.

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Adding Ord constraint to instance Monad Set?
by S. Alexander Jacobson 01 Apr '04

01 Apr '04

Following the declaration of "instance Monad []" in the prelude, and puzzling over the absence of its equivalent from Data.Set, I naively typed: instance Monad Set where m >>= k = concatSets (mapSet k m) return x = unitSet x fail s = emptySet concatSets sets = foldl union emptySet (setToList sets) instance (Eq b,Ord b) => Ord (Set b) where compare set1 set2 = compare (setToList set1) (setToList set2) and got the following error: Could not deduce (Ord b) from the context (Monad Set) arising from use of `concatSets' at dbMeta3.hs:242 Probable fix: Add (Ord b) to the class or instance method `>>=' In the definition of `>>=': >>= m k = concatSets (mapSet k m) In the definition for method `>>=' In the instance declaration for `Monad Set' Since I obviously can't modify the class declaration for Monad, the question arises: How does one add (Orb b) to the instance method '>>='? (Aside: it be really nice if the error messages suggested textual changes or at least provide sample syntax in addition to the conceptual recommendation.) -Alex- PS I assume the reason that Set is not declared as a Monad in Data.Set is oversight rather than incompatibility.... PPS I want to thank everyone who has been taking the time to answer all of my questions. I'll try to collect my various learnings into a useful beginners page once I reach the point where I think I can create a useful document. _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Re: Adding Ord constraint to instance Monad Set?
by John Hughes 01 Apr '04

01 Apr '04

| "Constraints on datatype declarations are a misfeature of Haskell, and | have no useful effect." | | Is this the final conclusion? Yes, it is, I believe. Constraints on data type declarations are a mis-feature. I tried to get them removed, but there was some argument that they could be made useful (see John Hughes's paper "Restricted data types in Haskell", Haskell workshop 1999), and they stayed. Simon Made useful in a way that would solve the problem in this thread, I might add. The paper is at http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps for the curious. John

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Re: [Haskell-cafe] Adding Ord constraint to instance Monad Set?
by MR K P SCHUPKE 31 Mar '04

31 Mar '04

>My intention was that the context before a data constructor shall mean that >the variable (a) shall be constrained only in the type of the constructor >function, not in the type definition. Okay, functional laguages have the notion that binding a variable is true for all time (like in mathematics)... Therefore if "a" is "1" then it has type Integer - forever. In this case the type of a in the first definition is "forall a . Num a". This is true for all references to a where we mean the same a. Because a is on the left-hand-side of the equals we assume all a's on the right are the same (it is the only rational choise) and hence we end up with two different types for a single variable. you could say something like (this doesn't work by the way) data Either a = (forall a . Num a => Left a) (forall a . Ord a => Right a) But here the a in each set of brackets has nothing to do with the a on the left hand side. This is different from: (data Either a = Num a => Left a | Ord a => Right a) where it is clear a == a but Ord a /= Num a Doing anything different would break the scoping rules for variables, and it is much more important IMHO to keep these consistant. Good languages are: Consistant, Symmetric, and Complete. Regards, Keean.

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Re: [Haskell-cafe] Adding Ord constraint to instance Monad Set?
by MR K P SCHUPKE 31 Mar '04

31 Mar '04

> data Either a =3D > Num a =3D> Left a > Ord a =3D> Right a This is a mistake because the same variable is being constrained in two different ways. The only way both constrains can be satisfied is if a is both in Num and Ord. This can be written: data (Num a,Ord a) => Either a = Left a | Right a Which makes it a lot clearer that the constaints apply to both the Left and Right constructor. You have to remember that a is a polymorphic variable... now the first line [Num a => Left a] says that a has the type (forall a . Num a) but the second line [Ord a => Right a] says a has the type (forall a . Ord a) These are not the same type, so when they are combined (remember a scopes over the whole type definition) there is a type error because: (forall a . Ord a) is not the same as (forall a . Num a) Regards, Keean.

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