- Beginners - Haskell.org

Typeclasses and generic types
by Christopher Howard 10 May '11

10 May '11

I'm learning how to use typeclasses for the first time, and I tried something like so: [CODE] data Point a = Point a a getX (Point a _) = a getY (Point _ a) = a instance Eq Point where (==) a b = getX a == getX b && getY a == getY b [/CODE] But when I try to load this in GHCI I get: [CODE] `Point' is not applied to enough type arguments The first argument of `Eq' should have kind `*', but `Point' has kind `* -> *' In the instance declaration for `Eq Point' [/CODE] This seems to be saying that Eq requires a concrete argument, which I suppose I can understand. I can fix this by changing "data Point a = Point a a" into "data Point = Point Int Int" or something like that, but that seems kind of lame: what if I want to leave my Point type more generic? (Maybe sometimes I'll need a Point Int, and other times I'll need a Point Float, or some other weird combination I haven't thought of yet.) Instead of changing the "data" declaration, can I change the "instance" declaration somehow so that it just defines an instance for a particular Point sub-type? I tried some variations like "instance Eq (Point Float)" but I haven't been able to come up with anything yet that satisfies the compiler. -- frigidcode.com theologia.indicium.us

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haskell for FPGAS
by David Blubaugh 09 May '11

09 May '11

To All, Has anyone developed HASKELL FOR FPGA development ?? Thank You, Daid Blubaugh

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Question about constraints, signatures
by Philippe Sismondi 09 May '11

09 May '11

I am trying to understand the Typeclassopedia article by Brent Yorgey in Monad.Reader. I am simultaneously working my way through Real World Haskell, so I may be doing things a bit out of order. Here is my question (which may itself be unclear, as I am a true *noob* at this): Why are there two different kinds of "constraints" in a signature, viz. those that specify types and those that specify typeclasses? For instance, we could write this: f :: (Num d) => d -> String -> Int In my example above there are two kinds of things that constrain f: (a) d must be an instance of Num; and (b) the second argument to f must be of type String, and f must return an Int. I (more or less) understand that types and typeclasses are not the same. But it seems to me that they do conflate in a way, if we think of types as defined by functions on them. So why should we not be able to write: f :: Num d -> String -> Int TIA. Best, - Phil -

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Making a generic interpreter
by Erik Helin 07 May '11

07 May '11

Hi, I have made the following (very small and basic) interpter: data Code = PUSH Integer | ADD | BRANCH [Code] [Code] | CMP deriving (Show) data StackElement = StackInteger Integer | StackBool Bool deriving (Show) type Stack = [StackElement] eval :: [Code] -> Stack -> Stack eval [] s = s eval (PUSH n:t) s = eval t (StackInteger n:s) eval (ADD:t) (StackInteger x:StackInteger y:s) = eval t (StackInteger (x + y):s) eval (CMP:t) (StackInteger x:StackInteger y:s) = eval t (StackBool (x == y):s) eval (BRANCH c1 c2:t) (StackBool cond:s) = if cond then eval (c1 ++ t) s else eval (c2 ++ t) s Now, I would like to change StackElement to be able to work with any types, that is: data StackElement a b = StackInteger a | StackInteger b deriving (Show) How would I make this change? I thought about using typeclasses, but since type a and type b in StackElement a b clearly are related (the == operation on type a must result in type b), I need to use multi-parameter type classes (and maybe also functional dependecies?). I also thought about introducing a new type Operation: data Operation a b = Operation { cmp :: a -> a -> b, add :: a -> a -> a, (more required functions) } Is any of these solutions the preferred one, or is there another solution that I don't know of (this is very likely, I'm completely new to Haskell)? Thanks for taking your time and reading this lengthy post, Erik

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Function for working with functions in data?
by Mike Meyer 06 May '11

06 May '11

I'm filling a functional itch that Haskell isn't scratching. I'm looking for a function to take a list of functions, and apply them all to a value. The signature is obvious :: [a -> b] -> a -> [b], and it's nearly trivial to write (\fs a -> map ($ a) [fs]), but Haskell has most useful list functions already provided. I couldn't turn it up in hoogle or the prelude. Am I just missing it somewhere, or is it really not available in a library? Thanks, <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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Get things from IO monad
by jianqiuchi＠gmail.com 06 May '11

06 May '11

Hi, I have a silly question, Can i get things out of IO monad? for example, function 'anotherFun' doesn't work. genFunc :: IO String anotherFunc :: (Monad m) => m String -- m is a complex monad and is not an IO monad -- m is not an instance of MonadIO so liftIO will not work anotherFunc = do genStr <- genFunc return genStr I just want to get something from IO monad and wrap it with some other monad. Is there anyway to do that? Thanks.

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Haskell GUI library
by Nathan Hüsken 05 May '11

05 May '11

Hey, looking on the haskell homepage, there are a lot of gui libraries (or at least bindings) available. One big difference seems to be that some are based on "Functional Reactive Programming" (I do not know what that is, just read it). So if I want to write a gui in haskell, which library should I use? Which is stable enough? Has anyone experience with some of these gui libraries? Thanks! Nathan

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Return a Foldable instance
by Federico Mastellone 05 May '11

05 May '11

Hi, I want to make a function that returns a Foldable instance, briefly, something like this: import Data.Foldable test :: Foldable f => f Int test = [1,2,3,4] But I get this error: Couldn't match expected type `f' against inferred type `[]' `f' is a rigid type variable bound by the type signature for `test' at Test.hs:3:17 In the expression: [1, 2, 3, 4] In the definition of `test': test = [1, 2, 3, ....] How can I make a function like the one above? I'm creating different implementations of a multimap, using a Set and a [], and instead of providing functions getValuesList and getValuesSet that return a [] and a Set respectively I want to provide a more generic one, getValues that returns a Foldable instance. Thanks!!! -- Federico Mastellone Computer Science Engineer - ITBA ".. there are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult." Tony Hoare, 1980 ACM Turing Award Lecture.

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Re: [Haskell-beginners] Return a Foldable instance
by Antoine Latter 04 May '11

04 May '11

On May 4, 2011 1:33 AM, "Federico Mastellone" <fmaste(a)gmail.com> wrote: > > Sorry, the function type was incorrect, is for a part of the multimap, > not the entire multimap. But it's the same problem, a function that > returns a Foldable. > > getValues :: Foldable f => k -> MultiMap k v -> f v > > When the user asks for the values that a certain key contains I want > to return something that is generic and easy to play with while also > taking advantages of the underlying structure functions to avoid > repeated code. Well, "generic and easy to play with" is pretty much the definition of a list. If the only thing the caller is doing with the list is folding over it, the compiler should get rid of the intermediary list anyway, if things work out right. So I think you're worrying too soon. But if this something you want to do anyway, I still don't see what is wrong with: > newtype LookupResult a = L [a] > deriving (Foldable) Then when you change your implementation of the multimap you can change your newtype without affecting the caller. Antoine > > On Wed, May 4, 2011 at 2:51 AM, Chaddaï Fouché <chaddai.fouche(a)gmail.com> wrote: > > On Wed, May 4, 2011 at 7:18 AM, Federico Mastellone <fmaste(a)gmail.com> wrote: > >> Yes, I got confused between haskell type classes and OO classes. > >> > >> I want to create different implementations of a multimap, for example > >> using lists and using Data.Set, and instead of providing functions > >> getValuesList and getValuesSet that return a [] and a Set respectively > >> I want to provide a more generic one, getValues that returns a > >> Foldable and avoid unnecessary conversions to and from lists. This way > >> the user folds directly the underlying structure, without having to > >> worry about which function is the best to fold the values. But I don't > >> know how to do this without an extra intermediate data type. > >> > >> One implementation returns a list and the other a set, both Foldables. > >> But I can't make a function like the one below for this two > >> getValues :: Foldable f => MultiMap k v -> f v > >> > > > > Writing this function given your intention (doing different thing > > depending on the type) is equivalent to writing an instance of > > Foldable for your (MultiMap k). If the underlying structure of your > > multimaps is always foldable, it doesn't make sense to make a function > > like this, the multimaps themselves should be Foldable. This is not > > hard to do, refer to the doc ( > > http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.1.0/Data… > > ) to see how to write a minimal instance of Foldable. > > > > -- > > Jedaï > > > > > > -- > Federico Mastellone > Computer Science Engineer - ITBA > ".. there are two ways of constructing a software design: One way is > to make it so simple that there are obviously no deficiencies, and the > other way is to make it so complicated that there are no obvious > deficiencies. The first method is far more difficult." > > Tony Hoare, 1980 ACM Turing Award Lecture.

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fmap versus bind
by Sean Perry 03 May '11

03 May '11

i came across hlint today and ran it on my experiments. It pointed out places where I was overly cautious with parens or used a $ and forget to remove it later. But one suggestion it made I was curious about. Why does it recommend using fmap over >>=? Idiomatically does fmap make more sense when the Monad is more like a collection? For instance I had "(applyOp op x y) >>= (flip (:) ds)". This seems more clear than "fmap ((:) ds) (applyOp op x y)".

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