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Enum/Bounded question
by Patrick LeBoutillier 13 Dec '10

13 Dec '10

Hi, I'm looking for a function to make a list of all the constructors in a type that derives both Enum and Bounded, i.e. enumAll :: (Bounded a, Enum a) => [a] enumAll = [minBound .. maxBound] Does this function exists in the standard modules? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Re: [Haskell-beginners] Beginners Digest, Vol 30, Issue 17
by Russ Abbott 13 Dec '10

13 Dec '10

Is there no way to make the class approach (or something like it) work? That's how I would do it in Java--define the component as an interface as far as the structure is concerned and then define the component itself to implement the interface. * -- Russ* On Sun, Dec 12, 2010 at 3:00 AM, <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Linking modules (Brent Yorgey) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sat, 11 Dec 2010 15:37:23 -0500 > From: Brent Yorgey <byorgey(a)seas.upenn.edu> > Subject: Re: [Haskell-beginners] Linking modules > To: beginners(a)haskell.org > Message-ID: <20101211203723.GB9265(a)seas.upenn.edu> > Content-Type: text/plain; charset=iso-8859-1 > > On Sat, Dec 11, 2010 at 08:56:23PM +0100, Chadda? Fouch? wrote: > > On Sat, Dec 11, 2010 at 7:37 PM, Russ Abbott <russ.abbott(a)gmail.com> > wrote: > > > I have two modules Structure and Component. ?Module Structure defines > > > ?? ? data Structure = Component ... > > > Module Component defines > > > ?? ?data Component = ... > > > I would like Structure to import Component. ?But Component > > > includes?functions?that take a Structure as an argument. So I have (or > would > > > like) something like this organization. > > > > > > [snip] > > > > > > The functions in Component are really very Component related and should > not > > > be moved to Structure. So how can I set up this circular relationship? > > > > GHC allows you to compile mutually recursive modules, see > > > http://www.haskell.org/ghc/docs/latest/html/users_guide/separate-compilatio… > > for how to do it. > > Yes, this is possible, but it has always seemed sort of fragile and > ugly to me. Another suggestion is to put the definitions of the > Structure and Component data types into a separate module called > Types, and then import Types into both the Structure module (which > defines functions over Structures) and the Component module (which > defines functions over Components). Then no circularity is needed. > > -Brent > > > > ------------------------------ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > End of Beginners Digest, Vol 30, Issue 17 > ***************************************** >

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If I only had a brain
by Amy de Buitléir 12 Dec '10

12 Dec '10

I have a class with some functions that have type signatures of the form (RandomGen g) => some parameter types -> RandT g (State [Neuron]) something For example: stimulate :: (RandomGen g) => Int -> [Double] -> RandT g (State [Neuron]) () That "RandT g (State [Neuron])" is my implementation of a "brain", so I'd like to call it that. I tried: data (RandomGen g) => Brain g a = RandT g (State [Neuron]) a But I got this error: `State [Neuron]' is not applied to enough type arguments Expected kind `?', but `State [Neuron]' has kind `* -> *' In the type `State [Neuron]' In the definition of data constructor `RandT' In the data type declaration for `Brain' Do I need higher ranked types or existential types in order to do that? Or am I going about this the wrong way? Thank you in advance, Amy

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Linking modules
by Russ Abbott 11 Dec '10

11 Dec '10

I have two modules Structure and Component. Module Structure defines data Structure = Component ... Module Component defines data Component = ... I would like Structure to import Component. But Component includes functions that take a Structure as an argument. So I have (or would like) something like this organization. Module Structure where import Component data Structure = Component ... Module Component where data Component = ... f :: Structure -> Component ->Structure The functions in Component are really very Component related and should not be moved to Structure. So how can I set up this circular relationship? I thought that one approach would be to do something like the following. But when I try I get all fouled up. Help would be appreciated. Module Structure where class Comp data Structure = Comp ... Module Component where import Structure data Component = ... instance Comp Component If I do this I get a complaint that class Comp needs a type parameter. If I give it an artificial one, that leads to all sorts of other problems. Is there a better way? Thanks. * -- Russ *

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Using derived Show most of the time
by Russ Abbott 11 Dec '10

11 Dec '10

I have a data type for which I want to generate my own show string in a few cases, e.g., *data *T = T1 ... | T2 ... | T3 ... ... I'd like to do something like this, *instance *Show T *where* show T5 ... = <something special> show t = showTheDerived t Is there a way to do something like that simply? * -- Russ *

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Fwd: Functional Dependencies
by dan portin 11 Dec '10

11 Dec '10

Hi, *unflt* was a poor choice of name. *flt2*, which flattens a datatype containing a flattenable datatype, would have been a better name. > > If I was making a flatten class, I wouldn't want Monad as a constraint > on the result container. Monad doesn't really say anything about > "containers" - there are some containers that are Monads, but there > are monads that aren't containers and there are containers that aren't > monads. > You're right. My initial class was class Monoid m => t -> m where flt :: t -> m instance (Tree a) [a] where flt = fltTree instance (Maybe a) [a] where flt = maybeToList for some tree type like `data Tree a = Nil | Node a (Tree a) (Tree a)` (for simplicity). The problem is that I couldn't figure out how to type the "flatten twice" function, which required a declaration like flt2 :: (F t' m) => t' t -> m which is clearly incorrect (since it gives rise to a kind mismatch). "Monad" and "Functor" were introduced just to get the definition of *unflt* to work, and was unsatisfactory. I need to be able to assert that *flt2* is a nested flattenable datatype, and that the return type of the nested and containing datatype is the same monoid. I can't figure out how to type this, however.

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About 'Curry'
by 吴兴博 11 Dec '10

11 Dec '10

pl a b = a ++ b pl a = a ++ The second definition cannot be accept by neigher ghci nor ghc. What's the problem? (ghc 6.12.3) -- ---------------- WU XingBo

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state and randomness, Rand, RandT
by Amy de Buitléir 11 Dec '10

11 Dec '10

I'm experimenting with different ways of working with both state and randomness at the same time. I put together this simple little example as a starting point, where the state in question is a list of Ints, and I want to be able to push a random number onto the stack. {-# LANGUAGE PackageImports #-} import "mtl" Control.Monad.State import Control.Monad.Random type Stack = [Int] -- push a random number onto the stack pushRandom:: State (Stack, StdGen) () pushRandom = do (xs,gen) <- get (r,gen') <- return $ randomR (1,100) gen put(r:xs,gen') That implementation works just fine. GHCi> execState pushRandom ([5,4,3,2,1], mkStdGen 0) ([46,5,4,3,2,1],40014 40692) But... I think it would be nicer to use Rand. Unfortunately, I can't figure out how to write the correct type signature. (Compiler message is: `Rand g' is not applied to enough type arguments Expected kind `*', but `Rand g' has kind `* -> *'). pushRandom2 :: (RandomGen g) => State (Stack, Rand g) () pushRandom2 = do (xs,gen) <- get (x,gen') <- getRandomR (0,100) gen put (1:xs,gen) And I'd really like to try using RandT, because this seems like the right situation for it. This compiles just fine. pushRandom3 :: (RandomGen g) => RandT g (State Stack) () pushRandom3 = do xs <- get r <- getRandomR (1,100) put (r:xs) But I don't know if it works, because I can't figure out the magic incantation to get it to run! GHCi> evalRandT $ pushRandom3 (mkStdGen 0) [5,4,3,2,1] <interactive>:1:12: Couldn't match expected type `StdGen -> [t] -> RandT g m a' against inferred type `RandT g1 (State Stack) ()' In the second argument of `($)', namely `pushRandom3 (mkStdGen 0) [5, 4, 3, 2, ....]' In the expression: evalRandT $ pushRandom3 (mkStdGen 0) [5, 4, 3, 2, ....] In the definition of `it': it = evalRandT $ pushRandom3 (mkStdGen 0) [5, 4, 3, ....] To summarise: Q1: How can I fix pushRandom2? Q2: How can I run pushRandom3? Thank you in advance for anyone who can help. Amy

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Relying on memoization
by Russ Abbott 10 Dec '10

10 Dec '10

(Is that even the right term?) Let's suppose I have a fairly complex computation to perform on a data structure. Normally I would cache the computed value in a field of the structure. The next time I need it I don't have to compute it again. (Of course if the structure changes, I have to recompute the value and store it again.) In Haskell is it the case that caching of this sort is redundant since Haskell does it for me? That is, if I call f someStructure multiple times at different places in my code and if both "f" and "someStructure" refer to the same things each time, can I rely on Haskell not to perform the computation multiple times but to look up the result it previously computed? Is there some limit to that? If every computation is stored, doesn't that create a very large collection of stored results? * -- Russ *

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Functional Dependencies
by dan portin 10 Dec '10

10 Dec '10

I was trying to make a class of "flattenable" data structures in Haskell. My first thought was to use multi-parameter type classes to assert a relation between two data types, one which is flattened and another which holds flattenable elements. Such a class is the class *F* below without the functional dependency. When given an instance declaration like *F Tree []*, applying *flt* to a structure fails with messages like *no instance for `F Tree m`*. I ended up writing the following (where *foldT* is fold over trees, etc.), in order to avoid said errors: class (Functor m, Monad m) => F t m | t -> m where flt :: t a -> m a unflt :: F t' m => t' (t a) -> m a unflt = join . fmap flt . flt instance F Tree [] where flt = foldT (\a b c -> a : b ++ c) [] instance F Maybe [] where flt = maybeToList Now, I would like to have *F* be a relation between a flattenable data type and, for instance, a monoid in which I can accumulate the flattened data type. Two things: I can't get a monoid constraint to operate properly over the variable *m* in *unflt*. This fails with messages like *no instance for `Monoid Tree [Integer]`*, and so on. I'm not sure why. Secondly, I would like to be able to have *flt* be able to return different "container structures", e.g., a custom sequence or a list. But the functional dependency I introduced prohibits this. How would you handle this problem? Thanks.

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