- Haskell-Cafe - Haskell.org

Inferring Safety
by Ivan Lazar Miljenovic 05 Jul '12

05 Jul '12

When it says up the top of Haddock docs that a module is "Safe-Infered" (which should probably Safe-Infer*r*ed), what does that actually mean? My understanding of Safe Haskell is that there's no unsafe* functions, etc. being used unless the module author swears (by using -XSafe) that the module is indeed safe... Yet as I pointed out on Google+ [1], the Data.GraphViz [2] module of my graphviz package is labelled as being safe-infered, despite having this function in there: dotizeGraph :: (Ord cl, Graph gr) => GraphvizParams Node nl el cl l -> gr nl el -> gr (AttributeNode nl) (AttributeEdge el) dotizeGraph params gr = unsafePerformIO $ graphToGraph params' gr where params' = params { fmtCluster = const [] , fmtNode = const [] , fmtEdge = const [] } This function uses `dot -Tcanon` to add in positional (as well as possibly other) information to FGL graphs. Now, as far as I know this function _is_ safe (assuming that you have Graphviz installed), but I certainly haven't proven it enough that for every possible input graph the same positional attributes will be attached back (i.e. that the graph will always be laid out the same), and thus I haven't used -XSafe. So what's going on here? [1]: https://plus.google.com/101302416956767249890/posts/3NUWxd9P6xV [2]: http://hackage.haskell.org/packages/archive/graphviz/2999.13.0.3/doc/html/D… -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Martin Odersky on "What's wrong with Monads"
by Jonathan Geddes 05 Jul '12

05 Jul '12

Cafe, I was watching a panel on languages[0] recently and Martin Odersky (the creator of Scala) said something about Monads: "What's wrong with Monads is that if you go into a Monad you have to change your whole syntax from scratch. Every single line of your program changes if you get it in or out of a Monad. They're not polymorphic so it's really the old days of Pascal. A monomorphic type system that says 'well that's all I do' ... there's no way to abstract over things. " [0, 53:45] Thoughts? --J Arthur [0] - http://css.dzone.com/articles/you-can-write-large-programs

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Using "syntactic" to implement the lambda calculus
by Alex Rozenshteyn 05 Jul '12

05 Jul '12

(This is intended as a simplification of the problem I actually need to solve.) I'm trying to implement the lambda calculus (with CBV evaluation) using the "syntactic" package, in such a way that a simple extension is also simple to implement. I am stuck on the fact that it seems that the Value type needs to be parametrized over the Expr type and I can't seem to figure out how to do it. I've read this post<http://www.haskell.org/pipermail/haskell-cafe/2011-May/091770.html>from the archives, but I still seem to be missing something. Does anyone have any suggestions? > -- Lambda calculus > type Ident = String > data Value = VLam Ident Expr > data Expr = Var Ident | Lam Ident Expr | App Expr Expr > eval :: Expr -> Value > eval e = > case e of > Var _ -> error "not closed" > Lam i e' -> VLam i e' > App e1 e2 -> > case eval e1 of > Lam i e' -> subst e' (eval e2) i > _ -> error "illegal application" > -- Lambda calculus with integers and addition > data Value = VLam Ident Expr | VInt Integer > data Expr = Var Ident | Lam Ident Expr | App Expr Expr | Plus Expr Expr > eval e = > case e of > ... > Plus e1 e2 -> > case (eval e1, eval e2) of > (VInt x1, VInt x2) -> VInt $ x1 + x2 > _ -> error "illegal addition" -- Alex R

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translation between two flavors of lexically-scoped type variables
by Kangyuan Niu 04 Jul '12

04 Jul '12

The paper "Lexically scoped type variables" by Simon Peyton Jones and Mark Shields describes two ways to give type variables lexical scoping. They state that one of the advantages of the GHC-style type-sharing approach over the SML-style type-lambda approach is that the former allows for existential quantification in addition to universal quantification. As an example, they give this code: data Ap = forall a. Ap [a] ([a] -> Int) The constructor `Ap` has the type: Ap :: forall a. [a] -> ([a] -> Int) -> Ap And one can write a function: revap :: Ap -> Int revap (Ap (xs :: [a]) f) = f ys where ys :: [a] ys = reverse xs with the annotations on `xs` and `ys` being existential instead of universal. But I'm a little confused about *why* type-lambdas don't allow this. Aren't both Haskell and SML translatable into System F, from which type-lambda is directly taken? What does the translation for the above code even look like? Why isn't it possible to write something like: fun 'a revap (Ap (xs : 'a list) f) = f ys where ys :: 'a list ys = reverse xs in SML? -Kangyuan Niu

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existential types and cast
by Corentin Dupont 04 Jul '12

04 Jul '12

Hi all, I read somewhere (here: http://stackoverflow.com/questions/2300275/how-to-unpack-a-haskell-existent…) that it's bad to try to unbox an existential type using a cast. OK, but without I really can't figure out how to do what I want: *data NewPlayer = NewPlayer deriving (Typeable, Eq) data NewRule = NewRule deriving (Typeable, Eq) class (Eq e, Typeable e) => Event e where data EventData e instance Event NewPlayer where data EventData NewPlayer = P Int instance Event NewRule where data EventData NewRule = R Int instance Typeable1 EventData where typeOf1 _ = mkTyConApp (mkTyCon "EventData") [] data EventHandler = forall e . (Event e) => EH e (EventData e -> IO ()) addEvent :: (Event e) => e -> (EventData e -> IO ()) -> [EventHandler] -> [EventHandler] addEvent e h ehs = (EH e h):ehs triggerEvent :: (Event e) => e -> (EventData e) -> [EventHandler] -> IO () triggerEvent e d ehs = do let r = find (\(EH myEvent _) -> cast e == Just myEvent) ehs case r of Nothing -> return () Just (EH _ h) -> case cast h of Just castedH -> castedH d Nothing -> return ()* How to remove the casts from triggerEvent? All that I want is to apply the handler found on the data passed in parameter. I tried to add a function apply in the class, without success: *apply :: (EventData e -> IO ()) -> (EventData e) -> IO () apply = ($)* Thanks! Corentin

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haskell.org website ?
by Qi Qi 04 Jul '12

04 Jul '12

Hi, Haskell.org website seems dropping offline at this moment. Anyone know what's happening to it? Thanks, Qi

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Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"
by Facundo Domínguez 04 Jul '12

04 Jul '12

> That precedent supports the view that e.g. a command-line flag > shouldn't affect behavior without the type reflecting it, e.g. by > doing IO, but the de facto use of the unsafe IO trick means not > everyone agrees. For those interested, here's [1] a case where treating command line arguments as top level constants went wrong. Look specifically at the section named "Crime Doesn't Pay". Best, Facundo [1] http://www.aosabook.org/en/ghc.html > Date: Tue, 3 Jul 2012 17:49:48 -0400 > From: Alvaro Gutierrez <radical(a)google.com> > Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with > Monads" > To: Dominique Devriese <dominique.devriese(a)cs.kuleuven.be> > Cc: haskell-cafe(a)haskell.org > Message-ID: > <CAC6k%2BtpuiDrtaC_cCFA9ewawDpG1adaht96rgMc_S5wtVJwLVw(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On Thu, Jun 28, 2012 at 2:53 PM, Dominique Devriese > <dominique.devriese(a)cs.kuleuven.be> wrote: >> 2012/6/27 Tillmann Rendel <rendel(a)informatik.uni-marburg.de>: >>> How would you implement this requirement in Haskell without changing the >>> line "amount (Leaf x) = x"? >> >> I may be missing the point here, but having worked on large code bases >> with a wide variety contributors before, I find it very advantageous >> that programmers are prevented from writing an amount function whose >> behaviour depends on command line arguments without at least an >> indication in the type. The fact that the function can not perform >> stuff like that is precisely the guarantee that the Haskell type gives >> me... > > I don't think there's an answer that's uniformly right; it depends on > whether you think of the input to the program, e.g. the environment, > command-line arguments, etc. as 'constant' and in some sense, pure. > The latter are constant in the sense that they never change, but they > are not fixed at compile-time. Other languages effectively treat them > as pure (by passing them directly to main), whereas Haskell chooses > not to, which is probably the reason why getArgs has IO in its type > (something that seems unintuitive at first.) > > That precedent supports the view that e.g. a command-line flag > shouldn't affect behavior without the type reflecting it, e.g. by > doing IO, but the de facto use of the unsafe IO trick means not > everyone agrees. >

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Cabal error installing Yesod and nehe-tuts on Haskell Platform 2012.2.0.0
by Eric Macaulay 02 Jul '12

02 Jul '12

Hello all! I trust tried to install yesod using "cabal install yesod-platform" but the installation aborted with the following error: cabal: Error: some packages failed to install: authenticate-1.2.1.1 depends on zlib-conduit-0.4.0.2 which failed to install. http-conduit-1.4.1.10 depends on zlib-conduit-0.4.0.2 which failed to install. wai-extra-1.2.0.5 depends on zlib-conduit-0.4.0.2 which failed to install. xss-sanitize-0.3.2 failed while unpacking the package. The exception was: user error (truncated tar archive) yesod-1.0.1.6 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-auth-1.0.2.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-core-1.0.1.2 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-default-1.0.1.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-form-1.0.0.4 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-json-1.0.0.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-persistent-1.0.0.1 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-platform-1.0.5 depends on zlib-conduit-0.4.0.2 which failed to install. yesod-routes-1.0.1.2 failed while unpacking the package. The exception was: user error (truncated tar archive) yesod-static-1.0.0.3 depends on zlib-conduit-0.4.0.2 which failed to install. zlib-conduit-0.4.0.2 failed while unpacking the package. The exception was: user error (truncated tar archive) Any idea what's wrong?

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how to check thunk
by Kazu Yamamoto 02 Jul '12

02 Jul '12

Hello, Are there any ways to see if a value is a thunk or memorized? I would like to have a function like: isThunk :: a -> IO Bool I'm implementing some data structures described in "Purely Functional Data Structures" and want to check my implementation is correct. E.g. https://gist.github.com/2987425 Thanks. --Kazu

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Re: [Haskell-cafe] Data.Bitmap, readBitmap exception
by Balazs Komuves 02 Jul '12

02 Jul '12

Hello, Author of Data.Bitmap here. The issue is caused by the fact that Data.Bitmap does *not* support bmp files at all. This should be clear from the documentation. The functions you tried to used use their own file format, which a really very simple format; they are only added for simple experimentation. (again, this is explicit in the docs). The same documentation points to the stb-image [1] package, which supports loading of some common formats (png, jpg, bmp). In general, I suggest that with package-specific question, unless it is a widely used package, mail the maintainer directly; you have better chance to get an answer that way. Regards, Balazs [1] http://hackage.haskell.org/package/stb-image Hello, > > I am using bitmap-0.2 [1] to load a Bitmap file [2]. However, the > following sample GHCi session fails for me. > > > Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO > (Bitmap Word8) > > *** Exception: Data/Bitmap/Internal.hs:(63,17)-(67,15): Non-exhaustive > patterns in case > > I downloaded the sources of bitmap-0.2 from Hackage and modified the > relevant function by appending the last line. > > > decodeCType :: CInt -> PixelComponentType > > decodeCType k = case k of > > 1 -> PctWord8 > > 2 -> PctWord16 > > 3 -> PctWord32 > > 4 -> PctFloat > > _ -> error $ "decodeCType: unexpected integer (" ++ show k ++ ")" > > With this modification the same GHCi session given above results in > the following error. > > > Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO > (Bitmap Word8) > > *** Exception: decodeCType: unexpected integer (7077888) > > I don't have any idea where 7077888 comes from. Am I doing something > wrong? Is my bitmap corrupted in some way? The number 7077888 has no > apparent significance except that it is a perfect cube [3]. So I guess > this is not a problem with endianness. > > In fact as far as I can tell `decodeCType` is only called on > `(PixelComponent t => c_type t)` and `c_type t` only yields integer > values in the (inclusive) range of 1 to 4. That is if I am not missing > some orphaned instance. > > Could anyone please enlighten me what is happening and if I am doing > something wrong? > > In case this is relevant: I am using bitmap-0.2 from Hackage with GHC > 7.4.1 on a 3.2.20 kernel i686 architecture Linux. > > Regards, > Alexander Foremny > > [1] http://hackage.haskell.org/package/bitmap > [2] https://www.dropbox.com/s/pyutvni9vx6f6mo/b.bmp > [3] http://www.wolframalpha.com/input/?i=7077888 >

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