August 2003 - Haskell

ANNOUNCE: New Chameleon version (03-08)
by Martin Sulzmann 20 Aug '03

20 Aug '03

Latest version can be downloaded via http://www.comp.nus.edu.sg/~sulzmann/chameleon/ You might also want to check out http://www.comp.nus.edu.sg/~sulzmann/chr/publications.html - The Chameleon Type Debugger (Tool Demonstration) - Interactive Type Debugging in Haskell Martin

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Announce: HWordNet - A Haskell Interface to WordNet
by Hal Daume III 19 Aug '03

19 Aug '03

Briefly, http://www.isi.edu/~hdaume/HWordNet/ HWordNet is a Haskell binding to the WordNet database. You will actually need to have WordNet installed to use the Haskell interface, but you won't need any of the source (the Haskell interface is 100% pure Haskell; no crummy FFI bindings here!). However, it does of course use the WordNet database files, so you'll need those. See http://www.cogsci.princeton.edu/~wn/ for more about WordNet This interface doesn't cover all of WordNet -- for instance, it doesn't yet do morphology. This will be available in a release soon. Also, it does use implicit parameters, so you've got to have GHC or Hugs or some other compiler that supports those. - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume

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RE: more unsafePerformIO questions ([...])
by Simon Marlow 19 Aug '03

19 Aug '03

> On Tuesday, 2003-08-19, 13:18, Simon Marlow wrote: > > [...] > > > Yes, I agree that one shouldn't rely on the "no duplication of work" > > property. However, folloing this argument we arrive at the > conclusion that > > hGetContents is an invalid use of unsafePerformIO. (which > is something I've > > been saying for a while now :-). > > Can't hGetContents be implemented without unsafePerformIO but with > unsafeInterleaveIO? Wouldn't this be valid? Am I'm missing > something here? unsafeInterleaveIO = return . unsafePerformIO Well, almost. unsafeInterleaveIO should also give you the guarantee that the IO in its argument can't be performed *before* the unsafeInterleaveIO is executed, but I don't think this distinction is important to the present discussion. Actually I dislike unsafeInterleaveIO even more than unsafePerformIO, because it has an implicit laziness assumption. The only purpose of unsafeInterleaveIO is to "delay" some IO until it is "demanded". But Haskell lacks a precise definition of "demand", prefering to leave the evaluation order up to the implementation. Hence, unsafeInterleaveIO is left in limbo - it might do something useful, but that depends entirely on your implementation. GHC doesn't guarantee much about unsafeInterleaveIO at all. Cheers, Simon

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RE: more unsafePerformIO questions (is it safe to use with ReadMode Handles)?
by Simon Marlow 19 Aug '03

19 Aug '03

> >> Isn't there the possibility of inlining causing a read to > >> happen twice even if it only appears to happen once? > > > >In theory that would be a valid transformation, but in practice no > >compiler would duplicate arbitrary computations. GHC > certainly doesn't. > > I was thinking of a situation like > > let x = unsafePerformIO readFooFromDB in x+x > > I see from your "Secrets of the GHC inliner" paper that x wouldn't be > inlined by GHC, but it seems to me like a serious abuse of > the principle of > referential transparency to write programs that _assume_ that. Yes, I agree that one shouldn't rely on the "no duplication of work" property. However, folloing this argument we arrive at the conclusion that hGetContents is an invalid use of unsafePerformIO. (which is something I've been saying for a while now :-). Cheers, Simon

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RE: more unsafePerformIO questions (is it safe to use with ReadMode Handles)?
by Simon Marlow 19 Aug '03

19 Aug '03

> I'm finishing up my Haskell interface to WordNet > (http://www.cogsci.princeton.edu/~wn/) and have a standard > unsafePerformIO question :). > > Basically, the interface functions by first calling an initialization > function, 'initializeWordNet :: IO WordNetEnv'. WordNetEnv is > essentially just a record of a lot of Handles which we will be reading > from and doing binary search in. > > All the functions which use the WordNetEnv (i.e., every other function > in the interface) basically do the following: > > - take one handle from the WordNetEnv and do binary search > in it, read a line. > > - use that line to read another line from another of the handles > > - parse that last one > > of course, therefore, all of these functions have type '... -> IO > something'. > > However, one of the "rules of thumb" for using unsafePerformIO is when > you can imagine a functional interface doing the same thing. I can: > simply read in all the databases in initializeWordNet and then do > Data.List.lookup on the results. Does this mean it's safe to wrap all > these functions in unsafePerformIO? It sounds like your interface is pure, as long as you don't expect the contents of any of the databases to change during the run of your program. That is, it doesn't matter whether you do all the IO at the start or lazilly on demand. If the databases *do* change over time, then there are two possibilities: 1. the contents change due to external factors only 2. the contents change because this program doing the writing in (1), you can still pretend the interface is pure, by imagining that all the changes happened earlier. This works as long as you only read the external data once. In (2), you have a truly impure interface, so using unsafePerformIO would be wrong. Cheers, Simon

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RE: overlapping instances and functional dependencies
by Hal Daume 19 Aug '03

19 Aug '03

Suppose somewhere we have an instance: instance C Int Bool Int when the first instance decl you have says we also have instance C Int (x,y,Bool) Int in this case, Int + (x,y,Bool) should uniq. specify Int. however, we also have: instance C a (a,c,b) c where, if we let a=Int, b=Bool, c=Char, then we get that Int + (Int,Char,Bool) should uniq. specify Char. these two confict because if, in the first case, we instantiate x to Int and y to Char, then one says that the third param should be a Bool and the other says the third param should be a Char. (or at least this is my understanding -- someone might correct me) - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-admin(a)haskell.org > [mailto:haskell-admin@haskell.org] On Behalf Of Wolfgang Jeltsch > Sent: Friday, August 08, 2003 4:33 PM > To: The Haskell Mailing List > Subject: overlapping instances and functional dependencies > > > Hello, > > I have this code: > class C a b c | a b -> c where > f :: a -> b -> c > > instance C a b c => C a (x,y,b) c where > f a (_,_,b) = f a b > > instance C a (a,c,b) c where > f _ (_,c,_) = c > ghci -fglasgow-exts -fallow-overlapping-instances compiles it without > complaint but hugs -98 +o says: > ERROR "ClassProblem.hs":7 - Instances are not consistent > with dependencies > *** This instance : C a (a,b,c) b > *** Conflicts with : C a (b,c,d) e > *** For class : C a b c > *** Under dependency : a b -> c > Can anyone tell me what the reason for this is and, maybe, > how to avoid these > problems with Hugs? > > Wolfgang > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Re: Haddock Problems
by Dominic Steinitz 19 Aug '03

19 Aug '03

I realise I have to give an explicit type for the functions but this still doesn't get rid of the warnings. Dominic. ----- Original Message ----- From: "Dominic Steinitz" <dominic.steinitz(a)blueyonder.co.uk> To: <haskell(a)haskell.org> Sent: Monday, August 18, 2003 6:06 PM Subject: Haddock Problems > Can someone tell me why I get the following warnings and also why the > generated documentation is blank? > > Many thanks, Dominic. > > [dom@redearth crypto]$ haddock Codec/Encryption/DES.hs > Warning: module not found: Word > Warning: module not found: Codec.Encryption.DESAux > Warning: module not found: Prelude > Warning: Codec.Encryption.DES: the following names could not be resolved: > encrypt decrypt > > Here's the offending module. > > -------------------------------------------------------------------------- -- > - > -- | > -- Module : Codec.Encryption.DES > -- Copyright : (c) Dominic Steinitz 2003 > -- License : BSD-style (see the file libraries/base/LICENSE) > -- > -- Maintainer : dominic.steinitz(a)blueyonder.co.uk > -- Stability : experimental > -- Portability : portable > -- > -- Takes the DES module supplied by Ian Lynagh and wraps it so it can > -- used with the standard modes. > -- <http://web.comlab.ox.ac.uk/oucl/work/ian.lynagh/>. > -- > -------------------------------------------------------------------------- -- > - > > module Codec.Encryption.DES ( > -- * Function types > encrypt, decrypt > ) where > > import Codec.Encryption.DESAux > import Word > > -- | Basic DES encryption which takes a key and a block of plaintext > -- and returns the encrypted block of ciphertext according to the standard. > > encrypt = flip des_enc > > -- | Basic DES decryption which takes a key and a block of ciphertext and > -- returns the decrypted block of plaintext according to the standard. > > decrypt = flip des_dec > >

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Haddock Problems
by Dominic Steinitz 19 Aug '03

19 Aug '03

Can someone tell me why I get the following warnings and also why the generated documentation is blank? Many thanks, Dominic. [dom@redearth crypto]$ haddock Codec/Encryption/DES.hs Warning: module not found: Word Warning: module not found: Codec.Encryption.DESAux Warning: module not found: Prelude Warning: Codec.Encryption.DES: the following names could not be resolved: encrypt decrypt Here's the offending module. ---------------------------------------------------------------------------- - -- | -- Module : Codec.Encryption.DES -- Copyright : (c) Dominic Steinitz 2003 -- License : BSD-style (see the file libraries/base/LICENSE) -- -- Maintainer : dominic.steinitz(a)blueyonder.co.uk -- Stability : experimental -- Portability : portable -- -- Takes the DES module supplied by Ian Lynagh and wraps it so it can -- used with the standard modes. -- <http://web.comlab.ox.ac.uk/oucl/work/ian.lynagh/>. -- ---------------------------------------------------------------------------- - module Codec.Encryption.DES ( -- * Function types encrypt, decrypt ) where import Codec.Encryption.DESAux import Word -- | Basic DES encryption which takes a key and a block of plaintext -- and returns the encrypted block of ciphertext according to the standard. encrypt = flip des_enc -- | Basic DES decryption which takes a key and a block of ciphertext and -- returns the decrypted block of plaintext according to the standard. decrypt = flip des_dec

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more unsafePerformIO questions (is it safe to use with ReadMode Handles)?
by Hal Daume 18 Aug '03

18 Aug '03

Hi folks, I'm finishing up my Haskell interface to WordNet (http://www.cogsci.princeton.edu/~wn/) and have a standard unsafePerformIO question :). Basically, the interface functions by first calling an initialization function, 'initializeWordNet :: IO WordNetEnv'. WordNetEnv is essentially just a record of a lot of Handles which we will be reading from and doing binary search in. All the functions which use the WordNetEnv (i.e., every other function in the interface) basically do the following: - take one handle from the WordNetEnv and do binary search in it, read a line. - use that line to read another line from another of the handles - parse that last one of course, therefore, all of these functions have type '... -> IO something'. However, one of the "rules of thumb" for using unsafePerformIO is when you can imagine a functional interface doing the same thing. I can: simply read in all the databases in initializeWordNet and then do Data.List.lookup on the results. Does this mean it's safe to wrap all these functions in unsafePerformIO? If not, I'll probably just create a WordNet Reader monad and wrap things up like that -- if you have to be in a monad anyway, there's no reason to pass the WordNetEnv around everywhere. But still, if possible, it would be nice for these to be pure. - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume

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Re: Type class problem
by oleg＠pobox.com 18 Aug '03

18 Aug '03

> I defined type recursion and naturals as > >newtype Mu f = In {unIn :: f (Mu f)} > >data N f = S f | Z > >type Nat = Mu N > An application of Mu should be showable if the functor maps showable types > to showable types, so the most natural way to define the instance seemed > to be > instance (Show a => Show (f a)) => Show (Mu f) where > show (In x) = show x > Of course that constraint didn't work. Well, it can if we write it in a bit different way: instance (Show (f (Mu f))) => Show (Mu f) where show (In x) = show x instance Show (N (Mu N)) where show Z = "Z" show (S k) = "S "++show k *Main> show (In (S (In (S (In Z))))) "S S Z" This solution is akin to that of breaking the type recursion when defining the fixpoint combinator fix. Only here we face the recursion on constraints. I believe the same solution should work for the Observable class. You didn't post the definition of the Observable class, so I couldn't test my claim. Flags used: ghci -fglasgow-exts -fallow-undecidable-instances /tmp/a.hs

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