- Glasgow-haskell-users

RE: flexible contexts and context reduction
by Sittampalam, Ganesh 27 Mar '08

27 Mar '08

> > Because I want to be able to make Foo values where the parameter type > > isn't in Ord, too. I just want unFoo to work on specific Foo values where it is. > but your recursive function requires a recursive constraint, which your data > type does not guarantee byconstruction, and which the Ord instances do not guarantee > due to lack of closed classes. Yeah. As you imply, I'm probably out of luck. > a rather pedestrian approach would record during construction whether or not > all parts are in Ord (see below). Unfortunately I need a (fully) polymorphic construction function, so this isn't possible. I'm trying to generalise the trick described at http://hsenag.livejournal.com/11803.html to arrows, but getting stuck with the "first" combinator. Thanks, Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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RE: flexible contexts and context reduction
by Sittampalam, Ganesh 27 Mar '08

27 Mar '08

> So the other question is whether this is useful. How often do people write stuff like this? > f :: Ord [a] => a -> a -> Bool > f x y = x>y > Nevertheless, I hadn't realised it was possible before, and now I can see it is. A closer example to what I was actually doing was this: {-# LANGUAGE GADTs #-} module Foo where data Foo a where Foo1 :: a -> Foo a Foo2 :: Foo a -> Foo b -> Foo (a, b) unFoo :: Ord a => Foo a -> a unFoo (Foo1 a) = a unFoo (Foo2 x y) = (unFoo x, unFoo y) [in the real code I did actually use the Ord constraint in the base case] The error I get is this: Foo.hs:10:20: Could not deduce (Ord a2) from the context () arising from a use of `unFoo' at Foo.hs:10:20-26 Possible fix: add (Ord a2) to the context of the constructor `Foo2' In the expression: unFoo x In the expression: (unFoo x, unFoo y) In the definition of `unFoo': unFoo (Foo2 x y) = (unFoo x, unFoo y) Foo.hs:10:29: Could not deduce (Ord b1) from the context () arising from a use of `unFoo' at Foo.hs:10:29-35 Possible fix: add (Ord b1) to the context of the constructor `Foo2' In the expression: unFoo y In the expression: (unFoo x, unFoo y) In the definition of `unFoo': unFoo (Foo2 x y) = (unFoo x, unFoo y) Which suggests that GHC has also lost track of the fact that Ord (a, b) is true. But it would certainly be necessary to get from Ord (a, b) to (Ord a, Ord b) to get that to work. Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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RE: flexible contexts and context reduction
by Sittampalam, Ganesh 27 Mar '08

27 Mar '08

> If you write the instance > instance Theta => Foo (T a b) where ... > where Theta is an arbitrary context, then, given a dictionary for (Foo (T a b)), you can get dictionaries for each constraint in Theta. Or in logic > > Theta <=> Foo (T a b) > > In your example, Theta is empty. >If you'd written > instance (Show a, Ix b) => Ord (a,b) then you could get a Show a and Ix b dictionaries from an Ord (a,b) dictionary. (But not an Ord a or Ord b > one.) I sort of see, but then what is Tom actually proposing? Replacing the body of the Ord declaration with something like OrdDict, thus forcing any possible instance for Ord (a, b) to provide OrdDict a and OrdDict b because of the definition of Super? In the current world, the possibility of instances like (Foo, Foo) make it impossible to write the code I originally asked about, so something would have to change, either with Ord or with my code, to be able to do anything like it. Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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RE: flexible contexts and context reduction
by Sittampalam, Ganesh 27 Mar '08

27 Mar '08

Well, Ord Foo doesn't hold, does it? So Ord (a, b) isn't equivalent to (Ord a, Ord b). Ganesh -----Original Message----- From: Simon Peyton-Jones [mailto:simonpj@microsoft.com] Sent: 27 March 2008 09:05 To: Sittampalam, Ganesh; 'Tom Schrijvers'; Ganesh Sittampalam Cc: glasgow-haskell-users(a)haskell.org; Martin Sulzmann Subject: RE: flexible contexts and context reduction Why "unfortunately"? Looks fine to me. Simon | | Unfortunately, GHC accepts the following: | | {-# LANGUAGE FlexibleInstances #-} | module Foo2 where | | data Foo = Foo | deriving Eq | | instance Ord (Foo, Foo) where | (Foo, Foo) < (Foo, Foo) = False ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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RE: flexible contexts and context reduction
by Sittampalam, Ganesh 27 Mar '08

27 Mar '08

Tom Schrijvers wrote: > On Wed, 26 Mar 2008, Ganesh Sittampalam wrote: >> On Wed, 26 Mar 2008, Ross Paterson wrote: >> >>> On Wed, Mar 26, 2008 at 08:52:43PM +0000, Ganesh Sittampalam wrote: >>>> I'm a bit confused about why the following program doesn't compile >>>> (in any of 6.6.1, 6.8.1 and 6.9.20080316). Shouldn't the Ord (a, b) >>>> context be reduced? >>> >>> To use bar, you need (Ord a, Ord b). You're assuming that Ord (a, b) >>> implies that, but it's the other way round. > Logically, the implication holds. There's an equivalence: > > Ord a /\ Ord b <=> Ord (a,b) Unfortunately, GHC accepts the following: {-# LANGUAGE FlexibleInstances #-} module Foo2 where data Foo = Foo deriving Eq instance Ord (Foo, Foo) where (Foo, Foo) < (Foo, Foo) = False Cheers, Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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flexible contexts and context reduction
by Ganesh Sittampalam 27 Mar '08

27 Mar '08

Hi, I'm a bit confused about why the following program doesn't compile (in any of 6.6.1, 6.8.1 and 6.9.20080316). Shouldn't the Ord (a, b) context be reduced? Cheers, Ganesh {-# LANGUAGE FlexibleContexts #-} module Test2 where foo :: Ord (a, b) => (a, b) -> (a, b) foo = bar bar :: (Ord a, Ord b) => (a, b) -> (a, b) bar = id

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Inconsistent .hi files with associated types?
by Niklas Broberg 25 Mar '08

25 Mar '08

Hi all, I'm getting a weird warning/error message from GHC that I don't understand: ===================== $ runhaskell Setup build Preprocessing library hsp-hjscript-0.3.4... Building hsp-hjscript-0.3.4... [1 of 1] Compiling HSP.HJScript ( HSP/HJScript.hs, dist\build/HSP/HJScript.o ) C:\Program\Haskell\hsp-0.3.5\ghc-6.8.2/HSP/Monad.hi Declaration for $f35 Unfolding of HSP.Monad.$f35: Can't find interface-file declaration for type constructor or class HSP.Monad.:CoF:R32XML Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error C:\Program\Haskell\hsp-0.3.5\ghc-6.8.2/HSP/Monad.hi Declaration for $f6 Unfolding of HSP.Monad.$f6: Can't find interface-file declaration for type constructor or class HSP.Monad.:CoF:R5SetResult Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error C:\Program\ghc-6.8.2\bin\ar.exe: creating dist\build\libHShsp-hjscript-0.3.4.a ===================== Can anyone explain to me what's up here? I have no .hi-boot files., and -ddump-if-trace gives no further info. Both XML and SetResult are associated types. Both of the instances in question are defined in a different package, which compiles without error. The classes that define the types are in yet another package, which also compiles without error. I note that the compilation of the package continues, so I suppose these are error messages only, even though they sound quite fatal. But what effect will this have on programs using the module? Cheers, /Niklas

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Announce: HXQ: A Compiler from XQuery to Haskell
by Leonidas Fegaras 24 Mar '08

24 Mar '08

I would like to announce the release of HXQ: A Compiler from XQuery to Haskell. HXQ is a fast and space-efficient translator from XQuery (the standard query language for XML) to embedded Haskell code. The translation is based on Haskell templates. HXQ takes full advantage of Haskell's lazy evaluation to keep in memory only those parts of XML data needed at each point of evaluation, thus performing stream-based evaluation. This results to an implementation that is as fast and space-efficient as any stream-based implementation based on SAX filters or finite state machines. Furthermore, the coding is far simpler and extensible since its based on XML trees, rather than SAX events. For example, an XQuery against the DBLP XML database (420MB) runs in 1.5 minutes. You can get more information at: http://lambda.uta.edu/HXQ/ Leonidas Fegaras U. of Texas at Arlington http://lambda.uta.edu/

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Scripting an application in haskell: howto expose application api ?
by Marc Weber 24 Mar '08

24 Mar '08

Hi, I'd like to add haskell scripting (ghc) support to vim. Passing the cmd to a .hsc module already works fine. But I can't imagine yet how to expose vim functions to the session started with newSession from the GHC API. Of course one way to go is interprocess communication (opening kind of pipe sending events/ commands replies..) But maybe you can think of someting easier ? Sincerly Marc Weber

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Re: [Haskell-cafe] Type checking of partial programs
by ac 23 Mar '08

23 Mar '08

So a number of people responded with various ways this is already possible. Of course GHC can already do this... it's type inference. The part I'm interested in working on is exposing the functionality in GHC's API to make this as easy as possible. -Abram

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