- Haskell-Cafe - Haskell.org

layout monads
by Thomas Conway 04 Oct '06

04 Oct '06

Hi All, Next monad query.... [*] In the 1995 paper "Composing Haggis", layout is done using a monad to compose individual elements. To modernize the syntax consider (forgive the operator, but it avoids parentheses): infixl 1 <| f <| x = f x mylayout = do hbox <| do button "ok" button "cancel" It was nice, because you didn't need to worry about temporary structures, variable names, and suchlike. In the 1996 version of the paper, this silently changed to mylayout = do o <- button "ok" c <- button "cancel" hbox [o, c] Does anyone know why the change occurred? IMO, the former was much more elegant. Also, assuming you want to use a different underlying monad (e.g. IO), how would you implement a layout monad. FWIW, I'm not actually interested in widgets per se, but in building html pages in such a way that you can specify the style elements/scripts with the code that creates the nodes of the document tree, but serialize them separately. Tom * Maybe we need a haskell-monads(a)haskell.org list. On second thoughts, no, because it would kill off the other mailingl lists. ;-)

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Either as a Monad instance
by Thomas Conway 04 Oct '06

04 Oct '06

Hi All, I've been [trying to] grapple with the various monads and transformers, and it occurs to me that the standard instance for Either as a monadic type is unnecessarily restrictive. Is there a compelling reason that it is not just instance Monad (Either e) where return = Right (Left e) >>= f = Left e (Right x) >>= f = f x abort = Left Tom

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Mis-understanding something in Haskell interpretation
by Edward Ing 03 Oct '06

03 Oct '06

Hi, I am new to Haskell and am learning Haskell on my own with "The Haskell School of Expression". Unfortunately there is no teacher that comes along with the book. I am having a problem with loading an excerise. I get this message from ghci on a :l Shapes.hs Shapes.hs:40:40: Couldn't match `Side' against `Int' Expected type: Side Inferred type: Int In the first argument of `sin', namely `angle' In the second argument of `(*)', namely `(sin angle)' Failed, modules loaded: none. The source is below. Side is types as Float. My assumption was that Haskell would know how to convert the Int to a float and all would be well. I am I mistaken somewhere? The problem is with the last line. Tips would be appreciated. Source Shapes.hs: module Shapes where data Shape = Rectangle Side Side | Ellipse Radius Radius | RtTriangle Side Side | Polygon [Vertex] deriving Show type Radius = Float type Side = Float type Vertex = (Float, Float) type Angle = Float rectangle :: Shape -> Shape rectangle (Rectangle width height )= Polygon [(0, 0),(0, height), (width, height), (width, 0)] rtTriangle :: Shape -> Shape rtTriangle (RtTriangle width height) = Polygon [(0,0),(0,height), (width, height)] regularPolygon :: Int -> Side -> Shape regularPolygon totalSides sideLength = let initial = (0.0,0.0) in Polygon (initial : vertices initial 1 totalSides sideLength ) vertices :: Vertex -> Int -> Int -> Side -> [Vertex] vertices _ 0 _ _ = [] vertices lastVertex currentSide totalSides length = let currentVertex = vertex lastVertex currentSide totalSides length in currentVertex: vertices currentVertex (totalSides - (currentSide + 1)) totalSides length vertex :: Vertex -> Int -> Int -> Side -> Vertex vertex (a ,b) currentSide totalSides length = let angle = 1.0 * (360 / totalSides) * currentSide in ( a + ( length * (sin angle)), b + ( (*) (cos angle) length ) )

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Haskell compiler from a USB Stick?
by David House 03 Oct '06

03 Oct '06

Hi all. Does anyone know of a Haskell compiler that will run from a USB stick? I think something like Hugs + minimal libraries + WinHugs should be small enough to install onto the stick, but I haven't tested this. That'd be the ideal scenario as I'm teaching Haskell (so won't need complicated libraries) on Windows (so user-friendliness is appreciated; terminals are something to avoid if at all possible). Has this/could this be done? Thanks in advance. -- -David House, dmhouse(a)gmail.com

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collection monads
by Matthias Fischmann 03 Oct '06

03 Oct '06

another beginners question about monads: given the type | data (Ix x) => Permutation x = Permutation [x] i wanted to define | instance Monad Permutation where | return xs = Permutation xs but of course nothing about the monad class guarantees xs to be of type list. the monad class seems unsuitable for holding collections, and i am inclined to not instantiate permutations. just to be sure: is there any easy way to do it that i missed? thanks, matthias

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DiffTime in Data.Time
by Chad Scherrer 03 Oct '06

03 Oct '06

I'm trying to use Data.Time, and I'm totally confused. DiffTime is abstract, and I don't see anything that maps into it. How do I construct one? I would like to then use the result to create a value of type UTCTime, but it seems (currently) like this might be easier. Thanks, -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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oddity with profiling
by Matthias Fischmann 02 Oct '06

02 Oct '06

hi, i have been staring at a strange profile for a while now, and was wondering whether anybody could give me a hint. (using ghc.6.4. i know i should upgrade, but has the profiler changed after that in particular?) almost all time / memory costs occurs in a CAF cost center. as far as i understand, this means that the evaluation only takes place once each time i run the program, and doesn't depend on the input. Q1. why should one distinguish time spent in top-level constants from time spent in functions? (my program is basically a top-level constant that calls a function i am trying to understand with constant toy arguments.) curiously, the costs inherited by the CAF cost center and caused elsewhere do not show anywhere else, although i am pretty sure the call stack doesn't leave my code for another few levels. Q2. how can i learn which parts of my code causes the CAF cost centers to bloat? and what does it mean if the 100% inherited costs do not show in any other item in the cost center stack in my profile? user manual yields further confusion: | The cost of evaluating any expression in your program is attributed to | a cost-centre stack using the following rules: | | * If the expression is part of the one-off costs of evaluating the | enclosing top-level definition, then costs are attributed to the stack | of lexically enclosing SCC annotations on top of the special CAF | cost-centre. how can an expression be part of the one-off cost of anything? does the second part mean that the SCC annotations are on top of the special CAF cost-center? which is the special CAF cost-center? what does being on top of that mean? sorry, maybe i am just tired... :-/ | * Otherwise, costs are attributed to the stack of | lexically-enclosing SCC annotations, appended to the cost-centre stack | in effect at the call site of the current top-level | definition[7]. Notice that this is a recursive definition. | | * Time spent in foreign code (see Chapter 8, Foreign function | interface (FFI) ) is always attributed to the cost centre in force at | the Haskell call-site of the foreign function. thanks, matthias

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How can I redirect stdout?
by Matthew Bromberg 02 Oct '06

02 Oct '06

I'm reposting this in it's own new thread since I think it involves a general issue beyond exporting Haskell DLLs. I am having some problems with GHCs stdout when a Haskell program is called from a windows program. As I noted earlier I am calling some Haskell code from C as a bridge to being able to ultimately call Haskell from Matlab 6.5. The Haskell code is compiled into a .DLL file on a windows machine. Matlab calls some C code which then calls the Haskell code. As soon as it gets into the Haskell code I get this run time error in ghcDLL.dll <stdout>: hPutChars: invalid argument (Bad File Descriptor) The Haskell code seems to work correctly if called from a stand-alone C program. Moreover, if I scrub all print statements from the Haskell code, it appears to run correctly from Matlab. Matlab does not properly redirect stdout so any printf calls from the C code simply fail to print. However Haskell's behavior is to halt after generating a runtime error. The C code used to generate the Haskell .dll is of course mingw gcc, but the C code used to create my Matlab mex file is Microsoft VC++ 6.0. I tried to redirect stdout using freopen() in C, but that never seems to work with Microsoft. At any rate it certainly doesn't effect Haskells use of stdout. I think in general for windows programs there is no stdout defined, whereas it's always defined for console programs. My question is, is there some way I can redirect stdout from inside Haskell so that all output is sent to a file? Is there an equivalent of freopen() in Haskell that works?

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Problematic irrefutable pattern matching of existentials
by oleg＠pobox.com 02 Oct '06

02 Oct '06

I have come to realize that irrefutable pattern matching of existentials may indeed be problematic. Let us consider the following existential data type > data FE = forall a. Typeable a => Foo a > | forall a. Typeable a => Bar a The following tests type and run (the latter raising the expected exception). > test1 = case (Foo ()) of Foo x -> show $ typeOf x > test2 = case (Bar True) of Foo x -> show $ typeOf x Let us suppose that irrefutable pattern matching of existentials is allowed. What would be the value of the expression case (Bar True) of ~(Foo x) -> show $ typeOf x then? Hopefully not "Bool". One might say that the expression ought to bottom out. However, what reason one may give for that outcome? The only binding in the above expression is of 'x', whose value is never demanded. Indeed, "show $ typeOf (undefined :: ())" yields the non-bottom value "()". One may claim that the existential pattern match also binds an implicit dictionary argument, which is demanded above. That explanation is quite unsatisfactory, because it breaks the abstraction of type classes. The dictionary passing is merely an _implementation technique_ for type classes. Furthermore, it is not the only implementation technique: JHC and Chameleon, for example, use intensional type analysis to implement typeclasses. On a different note, Conor McBride wrote: > It isn't necessary to perform > constructor discrimination when it's statically known that exactly one > constructor is possible, so those patterns can always be made > irrefutable, with matching replaced by projection. that is, in essence, the idea behind `Tagless Staged Interpreters for Typed Languages' Pasalic, Taha, Sheard: ICFP02 It is well-written paper, with an excellent introduction and motivation. Incidentally, one can write their tagless staged interpreter in Haskell as it is. No dependent types are necessary.

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Re: [Haskell-cafe] cumulative sum
by Chad Scherrer 02 Oct '06

02 Oct '06

Tamas, try scanl (+) 0 for the cumulative sum >From there the zipWith idea you mentioned seems like the way to go. -Chad > Hi, > > I have two lists, p and lambda (both are finite). I would like to > calculate > > 1) the cumulative sum of lambda, ie if > > lambda = [lambda1,lambda2,lambda3,...] > > then > > csum lambda = [lambda1,lambda1+lambda2,lambda1+lambda2+lambda3,...] > > 2) the cumulative sum of p*lambda (multiplication elementwise) > > Once I know how to do the first, I know how to do the second I guess > (doing the multiplication using zipWith to get the p*lambda list, but > I would be interested in any other suggestions). Currently I take and > sum, but then I am calculating the same sums many times. > > Thanks, > > Tamas

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