Re: [Haskell-cafe] number-parameterized types and heterogeneous lists
Hello, sorry for the late answer, I was off for the weekend :-) The paper "Number-parameterized types" by Oleg Kielyov is located at http://okmij.org/ftp/papers/number-parameterized-types.pdf It impressively shows what one can do with Haskell's type system. What I am after is the replacement of a "chain" of digits, like e.g. D1 $ D0 $ D0 $ Sz by a list [D1,D0,D0] so I can effectively use list operations on those "typed numbers". I also wonder if there is some kind of "generalized" foldr such that, e.g. D1 $ D0 $ D0 $ Sz = specialFoldr ($) Sz [D1,D0,D0] I think that this foldr must be some "special" foldr that augments the data type of the result in each foldr step. Would this be possible or am I just chasing phantoms ? Thanks Harald. " Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." " This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited." ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help with generalizing function
Hello leledumbo, Monday, June 23, 2008, 10:30:51 AM, you wrote: > I've successfully create a function to return lists of N-ple that satisfy the tuples may contain any combination of types, use lists instead -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help with generalizing function
Hint: use recursion. Hint 2: You can use <- in a list comprehension or list "do"-block to select a single list from a list of lists. -- ryan On Sun, Jun 22, 2008 at 11:30 PM, leledumbo <[EMAIL PROTECTED]> wrote: > > I've successfully create a function to return lists of N-ple that satisfy the > following function: > x1 + x2 + x3 + ... + xN = C > But unfortunately, it's not generic. The N is supposed to be an input, too. > I don't know how to make a dynamic N-ple (is it possible anyway?). > Currently, here's the implementation: > [code] > findAllAns c = [ (x1,x2,x3,x4,x5) | >x1 <- [0..c], >x2 <- [0..c], >x3 <- [0..c], >x4 <- [0..c], >x5 <- [0..c], >x1 + x2 + x3 + x4 + x5 == c > ] > [/code] > I tried using lists of lists, like this: > [code] > findAllAns c n = [ [x] | x <- [0..c], length [x] == n ] > [/code] > but [x] doesn't mean "a list of x", instead it's "a list that contains an x, > where x in [0..c]". Can anyone help me? > -- > View this message in context: > http://www.nabble.com/Help-with-generalizing-function-tp18063291p18063291.html > Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. > > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What is a rigid type variable?
To answer the question in the subject: >From "Simple unification-based type inference for GADTs", Peyton-Jones, et al. ICFP 2006. http://research.microsoft.com/users/simonpj/papers/gadt/ "Instead of "user-specified type", we use the briefer term rigid type to describe a type that is completely specified, in some direct fashion, by a programmer-supplied type annotation." So a rigid type is any type specified by a programmer type signature. All other types are "wobbly". Does anyone know what is going to change about the terminology with the new "boxy types" paper? http://research.microsoft.com/users/simonpj/papers/boxy/ -- ryan -- ryan On Sun, Jun 22, 2008 at 8:26 PM, Xiao-Yong Jin <[EMAIL PROTECTED]> wrote: > Hi all, > > I'm writing a short function as follows, but I'm not able to > find a suitable type signature for `go'. It uses > Numeric.LinearAlgebra from hmatrix. > > > -- | Map each element in a vector to vectors and thus form a matrix > -- | row by row > mapVecToMat :: (Element a, Element b) => > (a -> Vector b) -> Vector a -> Matrix b > mapVecToMat f v = fromRows $ go (d - 1) [] >where > d = dim v > go :: Element b => Int -> [Vector b] -> [Vector b] > go 0 vs = f (v @> 0) : vs > go !j !vs = go (j - 1) (f (v @> j) : vs) > > > If I give the type signature to go as this, I got the > following error > >Couldn't match expected type `b1' against inferred type `b' > `b1' is a rigid type variable bound by > the type signature for `go' at test.hs:36:20 > `b' is a rigid type variable bound by > the type signature for `mapVecToMat' at test.hs:31:35 > Expected type: Vector b1 > Inferred type: Vector b >In the first argument of `(:)', namely `f (v @> 0)' >In the expression: f (v @> 0) : vs > > So what is this rigid type variable all about and what is > correct type of the function `go'? > > Thanks in advance, > X-Y > -- >c/*__o/* ><\ * (__ >*/\ < > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Help with generalizing function
I've successfully create a function to return lists of N-ple that satisfy the following function: x1 + x2 + x3 + ... + xN = C But unfortunately, it's not generic. The N is supposed to be an input, too. I don't know how to make a dynamic N-ple (is it possible anyway?). Currently, here's the implementation: [code] findAllAns c = [ (x1,x2,x3,x4,x5) | x1 <- [0..c], x2 <- [0..c], x3 <- [0..c], x4 <- [0..c], x5 <- [0..c], x1 + x2 + x3 + x4 + x5 == c ] [/code] I tried using lists of lists, like this: [code] findAllAns c n = [ [x] | x <- [0..c], length [x] == n ] [/code] but [x] doesn't mean "a list of x", instead it's "a list that contains an x, where x in [0..c]". Can anyone help me? -- View this message in context: http://www.nabble.com/Help-with-generalizing-function-tp18063291p18063291.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What is a rigid type variable?
On Mon, Jun 23, 2008 at 5:58 AM, Luke Palmer <[EMAIL PROTECTED]> wrote:
> On Mon, Jun 23, 2008 at 3:26 AM, Xiao-Yong Jin <[EMAIL PROTECTED]> wrote:
>> Hi all,
>>
>> I'm writing a short function as follows, but I'm not able to
>> find a suitable type signature for `go'. It uses
>> Numeric.LinearAlgebra from hmatrix.
>>
>>
>> -- | Map each element in a vector to vectors and thus form a matrix
>> -- | row by row
>> mapVecToMat :: (Element a, Element b) =>
>> (a -> Vector b) -> Vector a -> Matrix b
>> mapVecToMat f v = fromRows $ go (d - 1) []
>>where
>> d = dim v
>> go :: Element b => Int -> [Vector b] -> [Vector b]
>> go 0 vs = f (v @> 0) : vs
>> go !j !vs = go (j - 1) (f (v @> j) : vs)
>
> If you want to give a type signature for 'go', you need a GHC
> extension called ScopeTypeVariables (IIRC).
I was indeed correct on the name of this extension, but it would be no
help to you to know this since I made a typo :-)
The extension is called ScopedTypeVaraibles.
You probably already know that this can be enabled with:
{-# LANGUAGE ScopedTypeVariables #-}
Luke
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Re: [Haskell-cafe] What is a rigid type variable?
On Mon, Jun 23, 2008 at 3:26 AM, Xiao-Yong Jin <[EMAIL PROTECTED]> wrote: > Hi all, > > I'm writing a short function as follows, but I'm not able to > find a suitable type signature for `go'. It uses > Numeric.LinearAlgebra from hmatrix. > > > -- | Map each element in a vector to vectors and thus form a matrix > -- | row by row > mapVecToMat :: (Element a, Element b) => > (a -> Vector b) -> Vector a -> Matrix b > mapVecToMat f v = fromRows $ go (d - 1) [] >where > d = dim v > go :: Element b => Int -> [Vector b] -> [Vector b] > go 0 vs = f (v @> 0) : vs > go !j !vs = go (j - 1) (f (v @> j) : vs) If you want to give a type signature for 'go', you need a GHC extension called ScopeTypeVariables (IIRC). The problem is that the 'b' in the signature of mapVecToMat is not the same b as the one in 'go'. With this extension, you can put those variables into scope for the body of the definition using explicit 'forall' quantifiers: mapVecToMat :: forall a b. (Element a, Element b) => (a -> Vector b) -> Vector a -> Matrix b -- rest as before But the Element b constraint is redundant in the signature of 'go', since that constraint is already in place by the above signature. As far as I can tell, giving an explicit signature for 'go' is not possible without this extension. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Planet haskell
I was hoping to have my summer of code blog added to planet haskell but [EMAIL PROTECTED] no longer seems to exist. Hopefully the owner is subscribed to this list? Jamie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] What is a rigid type variable?
Hi all, I'm writing a short function as follows, but I'm not able to find a suitable type signature for `go'. It uses Numeric.LinearAlgebra from hmatrix. -- | Map each element in a vector to vectors and thus form a matrix -- | row by row mapVecToMat :: (Element a, Element b) => (a -> Vector b) -> Vector a -> Matrix b mapVecToMat f v = fromRows $ go (d - 1) [] where d = dim v go :: Element b => Int -> [Vector b] -> [Vector b] go 0 vs = f (v @> 0) : vs go !j !vs = go (j - 1) (f (v @> j) : vs) If I give the type signature to go as this, I got the following error Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the type signature for `go' at test.hs:36:20 `b' is a rigid type variable bound by the type signature for `mapVecToMat' at test.hs:31:35 Expected type: Vector b1 Inferred type: Vector b In the first argument of `(:)', namely `f (v @> 0)' In the expression: f (v @> 0) : vs So what is this rigid type variable all about and what is correct type of the function `go'? Thanks in advance, X-Y -- c/*__o/* <\ * (__ */\ < ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda and closures in PHP -- could someone please comment?
This is increasingly less relevant to Haskell, except of course to demonstrate what a nice language Haskell is. On 20 Jun 2008, at 11:34 pm, Jules Bean wrote: I think where I differ on you is how to map the semantics of a C- like language to explicit references. I would argue that the glyph "c" in a C-like language denotes the value of C, not the reference to it. C-like languages have, for the most part, value semantics, and call-by-value. The exception of course is what C-like languages called "lvalues", but lvalues are only really on the left of the = sign and a few other special positions. I think that's the exception and not the rule. No, this is back to front. C basically follows the Algol 68 idea that the lvalue is the normative thing, and that there is an IMPLICIT COERCION from a variable to its value in certain contexts. C is full of implicit coercions: perhaps the most famous is the one that says that in almost all contexts an array is quietly coerced to a pointer to its first element. The key observation is that an implicit coercion from a variable to its contents is possible, whereas an implicit coercion from a value to "the" variable that holds it is not. Only the "a variable really stands for its address" view is coherent. In C, of course, if you want to capture the reference you do it explicitly with "&c". If we can use evidence from a relative to probe such questions, the fact that you *don't* need an explicit "&" in C++ (when you find a 'reference' to a variable, you use "c", not "&c") strongly suggests that the "variable stands for location" view is the more useful one. Thankfully, Haskell saves us these perplexities. (And replaces them with other perplexities...) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] INVITATION
james: > Haskell Hall wrote: > > Haskell Hall is up and running. Haskell Hall is a mailing list, a > forum, where you can discuss Haskell, functional programming and > anything related, freely and openly with fellow enthusiasts. We welcome > people of all abilities and know-how. So, if you fancy a change from > what you get on Haskell Cafe send an email to > [EMAIL PROTECTED] with the word subscribe in the subject > field.The first 10 members will have access to the wine cellar. :-) > > > How does or will Haskell Hall differ? > > I.e., why would I want to join another list? This is a list set up by PR Stanley after he was moderated last week, during a flame war in this channel. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41413/focus=41444 Make of that what you will. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] INVITATION
Haskell Hall wrote: > Haskell Hall is up and running. Haskell Hall is a mailing list, a forum, where you can discuss Haskell, functional programming and anything related, freely and openly with fellow enthusiasts. We welcome people of all abilities and know-how. So, if you fancy a change from what you get on Haskell Cafe send an email to [EMAIL PROTECTED] with the word subscribe in the subject field.The first 10 members will have access to the wine cellar. :-) How does or will Haskell Hall differ? I.e., why would I want to join another list? James ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Safe way to parse arguments?
On Sat, 21 Jun 2008, Xiao-Yong Jin wrote: Hi, I'm wondering how usually you parse command line arguments list safely. If the given argument is wrong, the program can still print out some error information instead of giving something like Prelude.read: no parse It's generally not a good idea to call 'read' on user data. 'read' expects well-formed input. Calling 'read' with other input is a programming _error_. If the user enters garbage, this is an _exception_ and this must be handled with 'reads' or 'maybeRead' as others suggested. http://www.haskell.org/haskellwiki/Error http://www.haskell.org/haskellwiki/Exception ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] another FFI question
On Sat, 2008-06-21 at 00:20 +0400, Bulat Ziganshin wrote:
>
> probably, you don't understand differences between OOP classes and
> type classes. look at http://haskell.org/haskellwiki/OOP_vs_type_classes
> and papers mentioned there
I took the liberty to update some of the C++ code on that page so that
the code will compile and work as expected.
Reading the rest of the page I have a question regarding section 4.2,
Type classes correspond to parameterized abstract classes (Gabriel Dos
Reis). This is probably more related to C++ than Haskell, so not all
readers might find it of interest.
--- off topic ---
Looking at 4.2, it's interesting to see how type classes are translated
into C++. Admittedly C++ isn't the prettiest language to look at, but I
feel one must be fair, and the code presented at the Wiki is too
complex, using virtuals and templates where they're not needed. Normal
overloading suffices.
Operator definitions can already be placed outside of a class scope,
e.g. the many overloads of << to print something to a stream. By default
these definitions cannot access private members of their operands but
the code at the Wiki has the same problem.
Just a small example:
// --- somewhere in a file ---
struct Aap {
int x;
explicit Aap(int x) : x(x) {}
Aap add(Aap const& other) const { return Aap(x + other.x); }
};
// --- somewhere in another file ---
Aap operator+(Aap const& lhs, Aap const& rhs) {
return lhs.add(rhs);
}
Overloading functions is not different. I've omitted some other (more
complex) ways you can achieve the same (aside from normal overloads it's
also allowed to introduce new specializations later).
At the time of writing it's not possible in C++ to restrict a type to
allow a certain operation (concepts[1] will hopefully fill that gap some
day, but I think it'll be a long time before tool support is ready for
prime-time). Rather, it's common to just use the operation and let the
compiler generate a (sometimes very cryptic) error message if it cannot
find the right code.
[1]: http://www.generic-programming.org/software/ConceptGCC/
A very poor man's solution to see if an operation is defined would be:
template struct has_add;
template<> struct has_add {};
int main() {
(void)sizeof(has_add); // okay
(void)sizeof(has_add); // fails to compile, put useful comment
here
}
It's not hard to add some primitive predicate logic (at compile time) to
resemble type classes in some sense, but I think this is off-topic
enough as it is.
Regards,
Niels
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Re: [Haskell-cafe] ieee 754
On Sun, 22 Jun 2008, Lennart Augustsson wrote: Haskell does not allow you to change rounding mode, NaN signallng mode, etc. But otherwise Haskell on modern platforms conforms to IEEE 754. It just uses the machine floating point implementation, which is most oftenly IEEE. However the good news are - if someone manually writes functions for IEEE number representation, they could be used exactly the same way as Float and Double, and chances are good that with some optimizer RULES the operations can be replaced by efficient FPU operations on IEEE compliant FPUs. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] INVITATION
Haskell Hall is up and running. Haskell Hall is a mailing list, a forum, where you can discuss Haskell, functional programming and anything related, freely and openly with fellow enthusiasts. We welcome people of all abilities and know-how. So, if you fancy a change from what you get on Haskell Cafe send an email to [EMAIL PROTECTED] with the word subscribe in the subject field.The first 10 members will have access to the wine cellar. :-) _ The i’m Talkathon starts 6/24/08. For now, give amongst yourselves. http://www.imtalkathon.com?source=TXT_EML_WLH_LearnMore_GiveAmongst___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Testing
Testing Testing _ Need to know now? Get instant answers with Windows Live Messenger. http://www.windowslive.com/messenger/connect_your_way.html?ocid=TXT_TAGLM_WL_Refresh_messenger_062008___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Scripting OpenOffice with Haskell
Has someone already tried to communicate with OpenOffice via Haskell? An interface seems to exist: http://framework.openoffice.org/scripting/index.html For instance, I like to mark some cells in oocalc, preprocess them and write them to a database. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ieee 754
Haskell does not allow you to change rounding mode, NaN signallng mode, etc. But otherwise Haskell on modern platforms conforms to IEEE 754. -- Lennart On Sun, Jun 22, 2008 at 9:37 AM, Sean McLaughlin <[EMAIL PROTECTED]> wrote: > Hello, > > I'm considering using Haskell for a numerical application. However, > I need to rely > on IEEE 754 standards being implemented correctly. What is the > current state of 754 in Haskell? > The definition has this paragraph, which makes me suspect Haskell is > not appropriate for this > application: > > "The default floating point operations defined by the Haskell Prelude > do not conform to current language independent arithmetic (LIA) > standards. These standards require considerably more complexity in the > numeric structure and have thus been relegated to a library. Some, but > not all, aspects of the IEEE floating point standard have been > accounted for in Prelude class RealFloat." > > Thank you, > > Sean > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ieee 754
Hello, I'm considering using Haskell for a numerical application. However, I need to rely on IEEE 754 standards being implemented correctly. What is the current state of 754 in Haskell? The definition has this paragraph, which makes me suspect Haskell is not appropriate for this application: "The default floating point operations defined by the Haskell Prelude do not conform to current language independent arithmetic (LIA) standards. These standards require considerably more complexity in the numeric structure and have thus been relegated to a library. Some, but not all, aspects of the IEEE floating point standard have been accounted for in Prelude class RealFloat." Thank you, Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
