- Haskell-Cafe - Haskell.org

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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Re: Set Monad Problem...
by MR K P SCHUPKE 31 Mar '04

31 Mar '04

Okay, what is wrong with using a State Monad where the state is a Set. You can then define additional functions to insert etc. Regards, Keean.

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RE: [Haskell-cafe] Adding Ord constraint to instance Monad Set?
by Simon Peyton-Jones 31 Mar '04

31 Mar '04

| > bindSet :: forall a b. (Ord a, Ord b) | > => T a -> (a -> T b) -> T b | | To translate the question from my problem to this one: | What would be if Set would be declared within Ord context, i.e. | data (Ord a) => Set a = ... | ? Now it would be safe to use Set as Monad because Sets could only | constructed from Ord types. No, the same problem arises. You could imagine that every Set contains its Ord methods. Then bind wouldn't need to pass them. But return :: Monad a => a -> m a The Set instance would have to be unitSet :: a -> Set a Where does the Ord dictionary come from? No, this won't work. Some of this stuff is discussed in a very old paper of mine http://research.microsoft.com/%7Esimonpj/Papers/classhask.ps.gz But I can't promise it covers this particular issue, and it predates functional dependencies. | "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

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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

>This syntax would also have the advantage that you can apply different >contexts to different constructors. Er, you can already do that as I just pointed out: data (Ord a,Num b) => Either a b = Left a | Right b Note that to constrain the same type variable differently for different constructors would be a type error. In other words this: data Either a = Num a => Left a Ord a => Right a Is not valid, because (forall a . Num a) is not the same type as (forall a . Ord a). So infact the current way of writing it as data (Num a) => Either a = Left a | Right a Is better because it does not allow you to make that mistake. 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

| "Constraints on datatype declarations are a misfeature of Haskell, and | have no useful effect." |=20 | 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 Might I try and pursuade you that mis-feature is a bit strong. A data type declaration creates two things - the data type itself and the constructor function. If you consider the constrains in the data type declaration to apply only to the constructor then it all makes sense. functions do not infer constrains from enclosed functions, for example: fna :: Num a => a -> a fna a = -a fnb :: a -> a fnb a = fna a In this case fna has a type error, the function is declared without the required Num a constraint. This is the same for data constructors, consider: data Num a => MyList a = MyList [a] This means that the constuctor for mylist has the (Num a) constraint. Simple! This is not a misfeature - more a mis-understood feature. So, using: print $ MyList ['a','b','c'] -- fails: No instance for (Num Char) print $ MyList [1,2,3] -- is Okay. and unexpectedly a use of the constructor does not get the constraint: fnc :: a -> MyList a fnc a = MyList [a] -- fails: Could not deduce (Num a) from the context () But if you consider the constructor to be just like a normal function this is perfectly consistant. This should definitely not be removed, as there is no other way to apply a constaint to a constructor function. 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

But that is already used for existential datatypes: data Test = forall a . Num a => Test a Maybe that lack of forall is enough to distinguish the two though. Your example would currently be written: data (Ord a,Num b) => Either a b = Left a | Right b I dont have any problem with this, just remember these are constraints on function arguments... It my superficially look like a class system, but in reality it is more like a constraint based programming language (for example Prolog) at the type level... Regards, Keean.

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RE: [Haskell-cafe] Adding Ord constraint to instance Monad Set?
by Simon Peyton-Jones 31 Mar '04

31 Mar '04

Alex | 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 I'm sorry to say that you just can't make Set an instance of Monad. To make an instance of Monad for a type constructor T you must have a function bindT :: forall a b. T a -> (a -> T b) -> T b And there just *is* no such function for Set, because of the need for ordering. There's a less polymorphic function bindSet :: forall a b. (Ord a, Ord b) => T a -> (a -> T b) -> T b To see why this is no good, consider sequence :: Monad m => [m a] -> m [a] The code for sequence calls (>>=) a lot. You can't supply bindSet as the function to call because bindSet takes two Ord parameters at run-time, whereas ordinary bind does not. It's not just a quirk of the type system -- there is a real problem! Simon

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[Haskell-cafe] RE: [Haskell] Programming language shootout (completing theHaskell entry)
by Simon Marlow 31 Mar '04

31 Mar '04

> When compiling via gcc, there's the __builtin_prefetch function: > > http://gcc.gnu.org/onlinedocs/gcc-3.3.3/gcc/Other-Builtins.html > (about 2/3rds of the way down the page) Thanks, I wasn't aware of that. > see also: > http://gcc.gnu.org/projects/prefetch.html > > I didn't read it very carefully, but it's not clear (on the > x86 cpus) if > there is a prefetch instruction that is common between the amd & intel > flavours, ie would a ghc binary with prefetch be portable between > P3/P4/Athlon. P3/P4 and Athlon have different prefetch instructions. However, Athlon64 supports the P3/P4 instructions. Cheers, Simon

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Re: [Haskell-cafe] Context for type parameters of type constructors
by MR K P SCHUPKE 30 Mar '04

30 Mar '04

>I didn't get the point yet why the context for 'data' >is not sufficient for the 'instance' method definition. Not sure who's relpy you're replying to, but I thought my response was pretty clear (I would though)... The problem is that to define 'zero' the type of the 'zero' must unify with the vector type. If you define vector operations as (v a) then a is a polymorphic variable. However the definition of zero implies that a is an Integral .. these are not the same type: (forall a . a) is not (forall a . Integral a => a) and thats where the type error comes from. You have two choices, drop the definition of zero, or use multi-parameter type classes. You could get rid of zero and have: class VectorSpace v where set :: Num a => a -> v a add :: Num a => v a -> v a -> v a scale :: Num a => a -> v a -> v a Should do the trick! (zero becomes "set 0") Regards, Keean.

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What are Kind errors and how do you fix them?
by S. Alexander Jacobson 29 Mar '04

29 Mar '04

Implementing Reverse from before, I am running into this weird error: type ReverseType a string = (string ->(string,a)) data Reverse a string = Reverse (ReverseType a string) instance Monad (Reverse a s) where return x = Reverse (\text -> (text,x)) (Reverse p) >>= k = Reverse p3 where p3 s0 = p2 s1 where (Reverse p2) = k a (s1,a)=p s0 Produces the error: Kind error: Expecting kind `* -> *', but `Reverse a s' has kind `*' When checking kinds in `Monad (Reverse a s)' In the instance declaration for `Monad (Reverse a s)' I have no clue what this error message means. -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Re: [Haskell-cafe] Context for type parameters of type constructors
by MR K P SCHUPKE 29 Mar '04

29 Mar '04

>Could not deduce (Num a) from the context (VectorSpace VList) The problem is in the definition: zero = VList (repeat 0) Is "0" an Int or an Integer? To define zero, instances need to be parameterised by vector type: EG: class VectorSpace v a where zero :: v a add :: v a -> v a -> v a scale :: a -> v a -> v a instance VectorSpace VList Int where zero = VList (repeat 0) ... instance VectorSpace VList Float where zero = VList (repeat 0.0) etc... >Could not deduce (Num a) from the context (VectorSpace VList) The problem is in the definition: zero = VList (repeat 0) Is "0" an Int or an Integer? To define zero, instances need to be parameterised by vector type: EG: class VectorSpace v a where zero :: v a add :: v a -> v a -> v a scale :: a -> v a -> v a instance VectorSpace VList Int where zero = VList (repeat 0) ... instance VectorSpace VList Float where zero = VList (repeat 0.0) etc... Regards, Keean.

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RE: [Haskell-cafe] looking for optimization advice
by Simon Marlow 29 Mar '04

29 Mar '04

> I think that adding the extra check to see if the pointers > are identical > sped this up enough that it's probably no longer a major > issue--I'm pretty > certain that the problem was large strings that were > identical, so every > byte had to be checked, so probably scary non-portable home-made > ForeignPtrs would not be worth the effort. (Although I'm > somewhat curious as to how it would be done...) A ForeignPtr allocated with mallocForeignPtr in GHC is heap-allocated garbage-collectable storage, and doesn't need a finalizer. The other kind of ForeignPtr created with newForeignPtr is associated with some external storage and needs a finalizer. Finalizers are expensive, and so is malloc(), but mallocForeignPtr is very cheap. This gives rise to a sum-type in the implementation of ForeignPtr: see the source code (libraries/base/GHC/ForeignPtr.hs). You could make a specialised version by ripping out the non-malloc ForeignPtr bits, and make an unboxable ForeignPtr type. A mallocForeignPtr-style ForeignPtr is essentially the same as an IOUArray. In fact, the underlying implementation type is the same (MutableByteArray#), except that a ForeignPtr is allocated in pinned (immovable) storage so that it can be passed to foreign functions. There could easily be a conversion from a "malloc" ForeignPtr to IOUArray. There is also some overlap with the StorableArray type. A StorableArray has a ForeignPtr inside it. It really looks like the framework could be simplified here, but I remember putting some thought into it and not coming up with a simple solution. Cheers, Simon

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dimension of arrays
by Fred Nicolier 29 Mar '04

29 Mar '04

Is there a way to get the number of dimension of an array ? i.e. something like : >dims :: (Ix a) => Array a b -> Int >dims = ... >a = listArray (1,10) [1,2..] >b = listArray ((1,1),(10,10)) [1,2..] >dims a -- should be equal to 1 >dims b -- should be equal to 2 The key is somewhere in the Ix class but where ? Fred -- ------------------------------------------------------- Dr. Frederic Nicolier Maitre de conferences, laboratoire LAM - URCA Animateur du projet ANITA : http://f.nicolier.free.fr/recherches/ Dept. GE&II, IUT de Troyes 9 rue de Quebec 10026 Troyes Cedex Tel: 03 25 42 71 01 Std: 03 25 42 46 46 Fax: 03 25 42 46 43 email: f.nicolier(a)iut-troyes.univ-reims.fr ---------------------------------------------------------

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HToolkit HSQL on Windows/GHC
by ATWOOD,JOHN (HP-Corvallis,ex1) 29 Mar '04

29 Mar '04

Clive, I see that HaskellDb requires htoolkit, so that's no help. Doesn't MySql listen on a port? Is that protocol documented? I've used postgresql that way from Haskell. Another thought, which I hesitate to even mention, would be to call the commandline tool via System.Cmd, pipe the results to a file, and read the file, and parse the results. Slow and ugly, but you could have it working in an hour. Seems like we could even use the higher level abstractions of HSQL or HaskellDB, but sub in this SlowAndUglyButPortable for the lower level access. John >From: "Clive Brettingham-Moore" <c(a)brettingham-moore.net> >Subject: [Haskell-cafe] HToolkit HSQL on Windows/GHC >To: <haskell-cafe(a)haskell.org> > >I realise that this is a little technical for this list, but I thought >that I'd check here before starting on the hard-core lists. > >Is there an easy way to build the HToolkit HSQL library for GHC (6.0.1 when >I >tried, 6.2.1 now) on windows (particularly MySQL/Windows XP, but I wont >say no to postgres, ODBC & sqlite)? >I assume people have tired and succeded, but I got lost [I think] in the >nasty place between the unix/cygwin libraries in GHC and the Windows focus >of the compiled MySQL client library. > >Since haskell is mainly a hobby for me, and I don't have a lot of free >time I haven't dug very deep, but not being able to interface my two of my >favourite constructs (functional programming and relational databases) is >getting to me. > >So any takers? > >C

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operating on nested monads
by Marco Righele 29 Mar '04

29 Mar '04

Hello everyone, I have some operations that have to be done in sequence, with each one having the result of the previous as input. They can fail, so they have signature a -> Maybe b Checking for error can be quite tedious so I use monadic operations: f :: a -> Maybe b do y <- foo x z <- boo y moo z The problems arise when I try to do the same thing within the IO Monad, i.e. the functions have signature a->IO (Maybe b) How can I achieve the same effect (if it is ever possible)? I feel like it should be something almost trivial, but I really can't get it. Thanks Marco

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RE: [Haskell-cafe] looking for optimization advice
by MR K P SCHUPKE 27 Mar '04

27 Mar '04

Would it ba faster to store the strings as UArrays and do the comparison in Haskell? Regards, Keean.

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Re: [Haskell-cafe] operating on nested monads
by MR K P SCHUPKE 27 Mar '04

27 Mar '04

For operating on nested monads is do: declare a class MonadIO which contains the IO functions along with monad monad-transformer lifted versions, for example: class Monad m => MonadIO m where ioPrint :: Show a => a -> m () instance MonadIO IO where ioPrint = print instance (MonadIO m,MonadT t m) => MonadIO (t m) where ioPrint = up . ioPrint where MonadT is defined: class (Monad m,Monad (t m)) => MonadT t m where up :: m a -> t m a down :: t m a -> m a instance Runnable (m a) (m a) where run = id instance (Monad m,Monad n,MonadT t m,Runnable (m a) (n a)) => Runnable (t m a) (n a) where run = run . down instance (Monad m,MonadT t m,Monad (t m)) => Runnable (t m a) (m a) where run = down You can then define MonadMaybe similarly... you end up with code like: test :: (MonadIO m,MonadMaybe m) => m a test = do ioPrint "something" c <- ioGetChar -- note: c is automatically lifted into MonadMaybe by -- MonadMaybe's definition of up in class MonadT return Nothing Then to call it you must specify the stacking: main :: IO () main = do case run (test :: MonadMaybeT MonadIO Char) of Just x -> print x Nothing -> print "NOTHING" I have not defined MonadMaybeT here... but its fairly straightforward. Regards, Keean.

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Extreme Markup Languages Conference 2004
by srn＠coolheads.com 26 Mar '04

26 Mar '04

Two kinds of information are necessary for human life on this planet: (1) Genetic information. No batteries required. (2) Everything else, i.e., civilization. Batteries required. Please be welcome to participate in the 11th annual Extreme Markup Languages Conference --> August 2-6, 2004 <-- Hotel Europa Montreal, Canada www.extrememarkup.com As always, it's a family gathering for rubber-meets-the-road technical luminaries interested in doing a better job of supporting civilization. Come share your light, and be brightened, too. You'll be glad you did, and not only because Montreal in August is great fun. Peer-reviewed paper submissions are due --> April 16, 2004 <-- Submission guidelines: http://www.mulberrytech.com/Extreme (e-mail questions to extreme(a)mulberrytech.com) Peer Reviewer Applications are due TODAY, Friday March 26, 2004. Tutorial Proposals are due TODAY, Friday March 26, 2004. -- Steve Steven R. Newcomb, Co-chair Extreme Markup Languages 2004 -- An IDEAlliance Event www.extrememarkup.com srn(a)coolheads.com ------------------------------------------------------- Coolheads Consulting http://www.coolheads.com direct: +1 540 951 9773 main: +1 540 951 9774 fax: +1 540 951 9775 208 Highview Drive Blacksburg, Virginia 24060 USA

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RE: [Haskell-cafe] looking for optimization advice
by Simon Marlow 26 Mar '04

26 Mar '04

> > data PackedString = PS !(ForeignPtr Word8) !Int !Int A ForeignPtr is a sum type, so it can't be unboxed unfortunately. This is undoubtedly one cause of inefficiency in the code. Depending on how you're using ForeignPtrs, you could possibly define a specialiased kind of ForeignPtr that would be unboxable. This would require using low-level non-portable stuff though. Are you using mallocForeignPtr? Or adding extra finalizers to ForeignPtrs? Cheers, Simon

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Implementing and indexing 2 dimen arrays
by Alex Gontcharov 25 Mar '04

25 Mar '04

I am writing a function, It's my first Haskell program in output function I will be working on [[3,4],[1,2,3,2],[3,9,9,4],[8,3,3,4] type structures I'd like to know how to index a particular element at run-time as I don't know on which element of which sublist I am at the moment in pullHelper function which starts with values !! ind p xs !! in C it would look something like values[][10]={{....}.{....}..}; int index[4]={3,4,6,7}; int i; i for(i = 0; i<4; i++) { int c = index[i]; if(values[i][c-1] == 0) /* do something */ else /*other things*/ } Any help would be appreciated --Stack for the digits of numbers, a modulo b, stack :: Int->Int->[[Int]] stack x y = reverse (split (subStack x y)) where --Split the digits further and constructs lists of lists split :: [Int]->[[Int]] split xs | length xs == 0 = [] | otherwise = (reverse (subStack (head xs) 10)): (split (tail xs)) subStack :: Int->Int->[Int] subStack x y | x == 0 = [] | otherwise = (mod x y):(subStack (div x y) y) -- It should work for 999999999 range but since I am using -- Int maximum is 2^31-1 output :: Int->[Char] output n = loop (stack n 10000) where -- goes through sets of at most 4 digits in the list, loop loop :: [[Int]]->[Char] loop xs | length xs == 0 = [] | otherwise = (strCon (head xs)) : (loop (tail xs)) strCon :: [Int]->[Char] strCon xs | length xs == 0 = [] | otherwise = (pullStr (head xs)) : (strCon (tail xs)) pullStr :: Int->[Char] pullStr s | s == 0 = [] | otherwise = pullHelper s --Helping to pull elements from the set, nested if pullHelper :: Int->[Char] pullHelper p | values !! ind p xs !! p - 1 == 0 = ones !! p - 1 ++ values !! ind p xs !! 0 | otherwise = values !! ind p xs !! p - 1 --Nasty !

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zip comprehensions and local decls
by Christian Sievers 25 Mar '04

25 Mar '04

Hello! This is about a misconception I had about zip comprehensions and a possible extension to get the desired result. As zip compr. are implemented in hugs and in ghc (though I only tried hugs), I thought that haskell-cafe might be a good place for it. In hugs mode, I expected something like the following to work: [ (a,a*x) | x <- [1..5] | y <- [10..], let a=x+y ] but hugs says: ERROR - Undefined variable "x" I think I can see why: The expression is rewritten to [ (a,a*x) | (x,y) <- zip [ x | x<-[1..5] ] [ y | y <- [10..], let a=x+y ] ] This scheme is useful if one has ...|y<-list, let a=..., sometest a which drops some elements from the list before zipping. But it would be nice to have a way to drop already zipped tuples and to allow such usages of local declarations. Maybe a final '|' branch without generator, so to get what was intended with the expression above one might write [ (a,a*x) | x <- [1..5] | y <- [10..] | let a=x+y ] or, as another example, one could have [ (x,y) | x <- [1..5] | y <- [10..] | even x ] giving [(2,11),(4,13)]. So the idea is to rewrite [ e | oldstyle parallel list comprehension | declsandpreds ] to [ e | zipN <as before...> , declsandpreds ] Is this clear enough? Does it sound useful? Can you think of a nicer syntax? All the best Christian Sievers

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looking for optimization advice
by David Roundy 25 Mar '04

25 Mar '04

Hello everyone, I've got a function that I can't think how it can be optimized any more, and perhaps it can't, but just in case, I figured I'd run it by you all, since it's currently taking 70% of my CPU time. The main problem is an unrelated scaling issue (i.e. it's getting called too many times... this is a separate issue I'm working on), but even in "reasonable" cases, it takes 20% of the processor time. The function is a simple packed string compare, and I basically just call the C standard library function memcmp for this. Without further ado: data PackedString = PS !(ForeignPtr Word8) !Int !Int instance Eq PackedString where (==) = psniceq foreign import ccall unsafe "static string.h memcmp" c_memcmp :: Ptr Word8 -> Ptr Word8 -> Int -> IO Int {-# INLINE psniceq #-} psniceq :: PackedString -> PackedString -> Bool psniceq a b | nullPS a && nullPS b = True psniceq (PS x1 s1 l1) (PS x2 s2 l2) = (l1 == l2 &&) $ unsafePerformIO $ withForeignPtr x1 $ \p1-> withForeignPtr x2 $ \p2 -> liftM (==0) $ c_memcmp (p1 `plusPtr` s1) (p2 `plusPtr` s2) l1 Is there anything obvious that I might have missed? Or is there anything incredibly subtle that I missed? :) BTW, I compile this with -O -funbox-strict-fields, so the packed string contents are unboxed, if that makes any difference. -- David Roundy http://www.abridgegame.org

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Indentation-sensitive Parsec components?
by Graham Klyne 25 Mar '04

25 Mar '04

I'm wondering if anyone has implemented any indentation-sensitive parser components that can be used with Parsec. I'm thinking of components along these lines: [[ blockStart :: Parser () -> Parser Block blockSep :: Block -> Parser () -> Parser () blockEnd :: Block -> Parser () -> Parser () ]] which might be used (somewhat self-referentially) like this: [[ doSequence :: Parser () doSequence = do { symbol "do" ; b <- blockStart (symbol "{") ; e1 <- expression ; es <- many ( do { blockSep b (symbol ";") ; expression } ; blockEnd b (symbol "}") ; dostuff (s1:ss) } ]] The intent is that blockStart would look for the separator component, and if not present would determine the appropriate indentation level from the input character stream. The other components would match or fail according to the way that the corresponding blockStart was matched. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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HToolkit HSQL on Windows/GHC
by Clive Brettingham-Moore 25 Mar '04

25 Mar '04

I realise that this is a little technical for this list, but I thought that I'd check here before starting on the hard-core lists. Is there an easy way to build the HToolkit HSQL library for GHC (6.0.1 when I tried, 6.2.1 now) on windows (particularly MySQL/Windows XP, but I wont say no to postgres, ODBC & sqlite)? I assume people have tired and succeded, but I got lost [I think] in the nasty place between the unix/cygwin libraries in GHC and the Windows focus of the compiled MySQL client library. Since haskell is mainly a hobby for me, and I don't have a lot of free time I haven't dug very deep, but not being able to interface my two of my favourite constructs (functional programming and relational databases) is getting to me. So any takers? C

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Implementing C program in Haskell
by Alex Gontcharov 24 Mar '04

24 Mar '04

Hi, Is anyone bored and wants to rewrite a small C program in Haskell?:) http://users.tpg.com.au/galutva/jap.c I am new to Haskell and it'd take me sometime to do it all myself. Thanks

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Where's the error in this snippet of code?
by Alex Gontcharov 24 Mar '04

24 Mar '04

Ignore the layout I can't find the error, running it gives parse error during compile on pStack, it is not very descriptive and I don't what is wrong. --Stack for the digits for numbers, a modulo b digStack :: Integer->Integer->[Integer] digStack a b | a == 0 = [] | otherwise = (mod a b):(digStack (div a b) b) --Takes a list of remainders and pStack :: [Integer] -> Integer -> [Integer] pStack [] _ = [] pStack (x:xs) a = digStack x a ++ pStack xs I think I will never like Haskell :-)

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Re: [Haskell-cafe] Where's the error in this snippet of code?
by Duncan Coutts 24 Mar '04

24 Mar '04

On Wed, 2004-03-24 at 18:00, Alex Gontcharov wrote: > I don't want to hide it within the function, I need the functions > to have file scope. Oh, ok. I assumed from the indentation (you see layout matters!) that it was a nested function. Haskell (like Python) is layout sensitive, the indentation makes a difference. All function definitions in the same scope have to line up in the same column. So for global/file scope functions they should not be indented at all. So the problem with your example was that your second function was indented. Write: globalFunc1 = ... globalFunc2 = ... and not: globalFunc1 = ... globalFunc2 = ... > So, If you want to call a function from another function b I have to > nest it using where? No, only if you want it nested rather than whole file scope. Duncan

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RE: [Haskell-cafe] loop in C , recursion in haskell
by Simon Marlow 24 Mar '04

24 Mar '04

> On Thu, Mar 25, 2004 at 01:38:44AM +1100, Alex Gontcharov wrote: > > Hi, > > > > I am new to haskell and would look to write a function equivalent > > to the following loop in C > > > > int value = 500000; > > int part_stack[4]; > > int *part_ptr = part_stack; > > for (; value; value /= 10000) > > *part_ptr++ = value % 10000; > > part_stack :: [Int] > part_stack = [0,50] > > Note that I've performed a memoization optimization--this > makes the code both smaller, faster and easier to read! :P Ignore David, he's pulling your leg. Here's the proper translation: do alloca $ \value -> do poke value (500000::Int) allocaArray 4 $ \part_stack -> do alloca $ \part_ptr -> do poke part_ptr part_stack let loop = do val <- peek value if val == 0 then return () else do p <- peek part_ptr poke p (val `rem` 10000) poke part_ptr (p `plusPtr` 1) poke value (val `quot` 10000) loop loop Cheers, Simon

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loop in C , recursion in haskell
by Alex Gontcharov 24 Mar '04

24 Mar '04

Hi, I am new to haskell and would look to write a function equivalent to the following loop in C int value = 500000; int part_stack[4]; int *part_ptr = part_stack; for (; value; value /= 10000) *part_ptr++ = value % 10000; Thanks

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Re: [Haskell-cafe] What are Kind errors and how do you fix them?
by MR K P SCHUPKE 24 Mar '04

24 Mar '04

Erm... okay my mistake thats not the problem... although it is to with state... When decaring a monad the return type is included in the class definition (it is a constructor class) This means the return value MUST be the last parameter to the type, so you need: type ReverseType a string = (string ->(string,a)) data Reverse string a = Reverse (ReverseType a string) instance Monad (Reverse s) where return x = Reverse (\text -> (text,x)) (Reverse p) >>= k = Reverse p3 where p3 s0 = p2 s1 where (Reverse p2) = k a (s1,a)=p s0 I was confused by the unusual format for bind... I would wtite this: instance Monad (Reverse s) where return x = Reverse $ \s -> (s,x) (Reverse p) >>= k = Reverse $ \s -> case p s of (s',a) -> (\(Reverse r) -> r) (k a) s' or even better define the data type as: data Reverse string a = Reverse { runReverse :: ReverseType a string } instance Monad (Reverse s) where return x = Reverse $ \s -> (s,x) (Reverse p) >>= k = Reverse $ \s -> case p s of (s',a) -> runReverse (k a) s' Regards, Keean.

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Re: [Haskell-cafe] What are Kind errors and how do you fix them?
by MR K P SCHUPKE 24 Mar '04

24 Mar '04

I dont understand what you are trying to do: type ReverseType a string = (string ->(string,a)) takes a string an returns a string and another value (looks like the state monad with state=String. data Reverse a string = Reverse (ReverseType a string) this just declares a type wrapper... why have both of the above, you only need: data Reverse a s = Reverse (s -> (s,a)) NOw for your class definition: return x = Reverse (\text -> (text,x)) this is the standard return from a state monad. (Reverse p) >>= k = Reverse p3 where p3 s0 = p2 s1 where (Reverse p2) = k a (s1,a)=p s0 the second parameter (k) needs the state passed as well after unwrapping like: p2 = (\(Reverse x) -> x) (k a) s1 Regards, Keean.

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Re: [Haskell-cafe] class Runnable vs fromJust (was Re: haskell idiom for reversible computations)
by MR K P SCHUPKE 23 Mar '04

23 Mar '04

The problem with Runnable is that it is not ewasily implementable for all monads... So purists rather than implement it for some don't implement it at all. Here's some examples: class Runnable x y where run :: x -> y instance Runnable (m a) (m a) where run = id instance Runnable (s -> m a) (s -> m a) where run = id instance (Monad m,Monad n,MonadT t m,Runnable (m a) (n a)) => Runnable (t m a) (n a) where run = run . down instance (Monad m,MonadT t m,Monad (t m)) => Runnable (t m a) (m a) where run = down These declare runnable for monad-transformers for which "down" is definable down is the functional opposite of lift (or "up" as it is sometimes called) The state monad transformer needs an instance of Runnable if you want to pass a value in: instance (MonadState st (StateT st m),Monad m,Monad n,Runnable (st -> m s) (st -> n s)) => Runnable (StateT st m s) (st -> n s) where run = run . (\(ST m) s -> do (_,a) <- m s return a) instance (MonadState st (StateT st m),Monad m) => Runnable (StateT st m s) (st -> m s) where run = \(ST m) s -> do (_,a) <- m s return a Whereas defining Runnable for the continuation-monad-transformer is quite a challenge: instance (MonadT (ContT r) m,Runnable ((r -> m r) -> m r) ((r -> n r) -> n r)) => Runnable (ContT r m r) ((r -> n r) -> n r) where run = run . (\(CT m) kappa -> m kappa) instance MonadT (ContT r) m => Runnable (ContT r m r) ((r -> m r) -> m r) where run = (\(CT m) kappa -> m kappa) Have Fun. Keean.

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asking for a favor
by garroyo＠dsic.upv.es 23 Mar '04

23 Mar '04

Hi, Does someone has the Haskell XML Toolbox, lets say working in Windows environment?. I have tried install them in Hugs 98 for Windows but there are Hexadecimal characters escape out of range. One choice, I know, is go to LINUX. I am working in CNC programming and I am in the begining of the a research. Thanks in advace. Gustavo. ------------------------------------------------- This mail sent through IMP: http://horde.org/imp/

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haskell idiom for reversible computations
by S. Alexander Jacobson 23 Mar '04

23 Mar '04

If I am writing a rener/parser for e.g. HTTP Requests and Responses, I would like to be able to guarantee that the parser really is the inverse of the generator. I assume that if I define a pair of computations as reversible then I can put those computations in some monad such that I can do: parse = inverse generate and parse2 = inverse do {generate;generate2} Actually, I assume this is an arrow rather than a monad because a render is a computation that takes a value and produces a value rather than simply a computation that produces a value. Whether its an arrow or not, I also assume this problem is sufficiently common that there is a well worked out idiom for this such that I don't have to hand-code and prove inverse functions for all my data structures. I just don't know what it is. -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Seen in ACM technews...
by Graham Klyne 22 Mar '04

22 Mar '04

[[ # "Learning Functional Programming Through Multimedia" Slashdot (03/16/04); Jones, Isaac Haskell is a purely functional programming language and the book, "The Haskell School of Expression: Learning Functional Programming Through Multimedia," is a great introduction to the platform now being used for cutting-edge programming language research. The Haskell community is still rather small, but working on tremendously innovative research such as the meta-programming environment Template Haskell implemented by the Glasgow Haskell Compiler, while Haskell can also be extended with Embedded Domain-Specific Languages for Web authoring, parsing, and controlling humanoid robots without changing the language itself. Paul Hudak's book prepares the reader to begin writing tools or participate in ongoing programming language research. Haskell prepares readers for new developments such as the arrows concept used by the Yale Haskell group; Yale-developed Yampa, a "Functional Reactive Programming" framework, shares concepts with the animation described in "The Haskell School of Expression." The book is written in a tutorial style with alternating conceptual and application chapters. The provided code examples may be insufficient for those who want to jump ahead, but the Web site provides fill-in code. Readers completing the examples given in the book will also need the "November 2002" version of the Hugs interpreter and the User's Guide. "The Haskell School of Expression" eases programmers who are unfamiliar with functional programming into complex components such as recursion; falling back on non-functional style is not allowed with Haskell. The book also tackles the difficult issue of monads by getting readers to use them very early on and explaining the concepts behind them later in the book. Although there is a lot of drawing and music involved in "The Haskell School of Expression," the author includes mathematical background such as the algebraic reasoning for shapes and music, as well as a chapter on proof by induction. Click Here to View Full Article http://books.slashdot.org/books/04/03/12/221232.shtml?tid=126&tid=156&tid=1… ]] -- http://www.acm.org/technews/articles/2004-6/0322m.html#item12 ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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????Pattern match(es) are overlapped???
by S. Alexander Jacobson 22 Mar '04

22 Mar '04

I am implementing HTTP and I have something like: newtype Method = Method String getMethod = Method "GET" putMethod = Method "PUT" [...] doMeth getMethod = ... doMeth putMethod = ... GHC gives me a patter matches are overlapped warning. And when I run, I discover that pattern matching is not actually happening. getMethod is always executed even though the method passed is putMethod.... What am I doing wrong? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Dividing integers?
by Matias Hernandez 18 Mar '04

18 Mar '04

Hello there! Although you might not believe it since I'm going to ask a very very very basic question, I have been using Haskell for educational purposes for a couple semesters now. Nevertheless, I don't think I have ever worked with non-Int number types! :) I don't understand why I get this error. I mean, apparently "/" is not defined for integers but I don't know how to "cast" the result of the length function into a Double... Prelude> (length [1,2]) / 3 <interactive>:1: No instance for (Fractional Int) arising from use of `/' at <interactive>:1 In the definition of `it': it = (length [1, 2]) / 3 Besides, a simple integer division works: Prelude> 2 / 3 0.6666666666666666 But I would guess that's because '2' and '3' are parsed as Floats because they are used in a divide function. Any help will be appreciated! Best regards, - Matías

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RE: [Haskell-cafe] Building Haddock on Windows
by Simon Marlow 17 Mar '04

17 Mar '04

> Has anyone recently built Haddock on Windows? I'm invoking > configure and > make from Cygwin bash. Configure takes a couple of hours, and > then make produces a load of errors (see below). I'm ashamed to say that Haddock, like Alex and Happy don't really support native Windows too well. You can use them in a Cygwin (and possibly MSYS) environment, by doing the usual ./configure && make install, which will install the tool by default in /usr/local/bin. Cheers, Simon

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RE: [Haskell-cafe] Building Haddock on Windows
by Simon Marlow 17 Mar '04

17 Mar '04

> Got it built after realising that my PATH didn't include ghc > (thanks Andy > Moran). I renamed haddock.bin to haddock.exe and copied to > into ghc's bin > for convenience (ghc's bin is already in my path). Now I just > have to figure out how to invoke it... > > Where can I find instructions for use? I see haddock.sgml in > haddock-0.6/haddock/doc but reading sgml hurts. Is there an > html version of this somewhere? HTML docs are on the web: http://www.haskell.org/haddock/docs/haddock.html Haddock is designed to be installed via 'make install', and then run using the script named 'haddock' in $prefix/bin. You *can* run the binary directly, but you'll need to pass it some extra arguments so that it can find its files. > I have a few questions: > - can you invoke it on a folder and have it generate docs > for all of the > source files in it (and sub-folders)? > - does it work on .lhs source containing bird-track style > code? It would > appear not, as I get a parse error. These Qs are answered in the docs. > - 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

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RE: [Haskell-cafe] Building Haddock on Windows
by Bayley, Alistair 17 Mar '04

17 Mar '04

http://www.haskell.org/haddock/docs/haddock.html Ahh, sorry, somehow I missed that. Yes, that is the html generated from the sqml. > -----Original Message----- > From: Arjan van IJzendoorn [mailto:afie@cs.uu.nl] > Sent: 17 March 2004 14:00 > To: Bayley, Alistair > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Building Haddock on Windows > > > Hello Alistair, > > > Where can I find instructions for use? I see haddock.sgml in > > haddock-0.6/haddock/doc but reading sgml hurts. Is there an > html version > of > > this somewhere? > > Haddock is very well documented at http://www.haskell.org/haddock/ > > Greetings, Arjan ----------------------------------------- ***************************************************************** 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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RE: [Haskell-cafe] Building Haddock on Windows
by Bayley, Alistair 17 Mar '04

17 Mar '04

Got it built after realising that my PATH didn't include ghc (thanks Andy Moran). I renamed haddock.bin to haddock.exe and copied to into ghc's bin for convenience (ghc's bin is already in my path). Now I just have to figure out how to invoke it... Where can I find instructions for use? I see haddock.sgml in haddock-0.6/haddock/doc but reading sgml hurts. Is there an html version of this somewhere? I have a few questions: - can you invoke it on a folder and have it generate docs for all of the source files in it (and sub-folders)? - does it work on .lhs source containing bird-track style code? It would appear not, as I get a parse error. - 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?) Thanks, Alistair. > -----Original Message----- > From: Simon Marlow [mailto:simonmar@microsoft.com] > Sent: 17 March 2004 13:22 > To: Bayley, Alistair; haskell-cafe(a)haskell.org > Subject: RE: [Haskell-cafe] Building Haddock on Windows > > > > > Has anyone recently built Haddock on Windows? I'm invoking > > configure and > > make from Cygwin bash. Configure takes a couple of hours, and > > then make produces a load of errors (see below). > > I'm ashamed to say that Haddock, like Alex and Happy don't really > support native Windows too well. > > You can use them in a Cygwin (and possibly MSYS) environment, by doing > the usual ./configure && make install, which will install the tool by > default in /usr/local/bin. > > 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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infix naming
by David Roundy 17 Mar '04

17 Mar '04

[moving to haskell-cafe due to an extreme shift in topic] On Wed, Mar 17, 2004 at 08:55:22AM +0100, Ketil Malde wrote: > > So it is agreed: we use "isXXXX" :-) > > Hmm... > > s1 `isSubsetOf` s2 vs s1 `isSubset` s2 > isSubsetOf s1 s2 isSubset s1 s2 > > I'm not sure I have any clear preference. ..Of reads slightly clearer > in infix notation, but is arguably a bit more confusing in prefix. This seems to be a common problem: a name that makes sense (i.e. it is obvious what it does) in prefix notation very often is completely wrong in infix notation, particularly when partially applied. "e `isElementOf` list" makes perfect sense, while "isElementOf e list" seems a bit wrong, and "isElementOf e" (of type [a] -> Bool) reads completely wrong. I guess "`isElementOf` list" (which is of type a -> Bool) does make sense. I guess perhaps names just have be chosen to either make sense in infix notation, or in prefix notation, but not both? Is this a general principle, and if so, that seems bad, since the choice of infix vs prefix notation seems like it shouldn't have to be made at the API level. Does anyone know why it was originally decided to make infix notation work as it does? To me it seems like it would make sense for (`f` x) to be the same as (f x) rather than the opposite. e.g. (`is_less_than` 2) and (is_less_than 2) sound the same way when I pronounce them, and it's not entirely pleasant that the second one is in fact the same as (>= 2). On the other hand, if you don't think of it as (is_less_than a) b, one might naturally think that (is_less_than a b) should be (a < b). -- David Roundy http://www.abridgegame.org

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