April 2001 - Haskell - Haskell.org

toRational (0.9). Reply
by S.D.Mechveliani 18 Apr '01

18 Apr '01

People wrote about toRational (0.9) == 9%10 = False ... Probably, the source of a `bug' is a language agreement that the input is in decimal representation (`0.9') and its meaning is a floating approximation in _binary_ representation. For example, 1%5 yields a finite mantissa in decimal representation and an infinite (periodic) mantissa for the binary representation of toBinary(1%5) = 1B % 1001B. Therefore applying toRational to any finite float approximation (Double, or other) of 1%5 does not return 1%5. - ? ----------------- Serge Mechveliani mechvel(a)botik.ru

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Help about programming a module in haskell ?
by Mickaël GAUTIER 18 Apr '01

18 Apr '01

Hi, i am a french beginner in programming in haskell language. My first module was to prove that a boolean proposition is always right. So it was about logical mathematic. Now i am trying to program a modul which function not with mathematic rules but with logical rules as in Prolog. I've a game named nani (it's a boy who wants to find his nani which is in one of the three rooms of the house) wrote in prolog and i'm trying to convert it in haskell. If someone is interest by my subject can he send me his e-mail adress ? Thanks Mickaël GAUTIER

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enquiry
by imed sabri 17 Apr '01

17 Apr '01

plz i want some informations about haskell yours faithfully ___________________________________________________________ Do You Yahoo!? -- Pour dialoguer en direct avec vos amis, Yahoo! Messenger : http://fr.messenger.yahoo.com

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Re: toRational problem
by Ashley Yakeley 17 Apr '01

17 Apr '01

At 2001-04-16 18:35, pablocardenal(a)laplatavive.com wrote: >> Why (toRational 0.9) == (9 % 10) is False? I tried this in Hugs. It gives (toRational 0.9) as 7549747 % 8388608 (i.e. 37748735/41943040 instead of 37748736/41943040 = 9/10). This is probably because Hugs attempts a floating-point approximation of the string "0.9" before passing it to a 'from' function (fromSingle, fromDouble) to cast it to 'Fractional a => a'. This is a bug in my opinion. The string "0.9" unambiguously represents 9/10, and Hugs has types for representing such values. Instead, it uses floating point. On a more general note, floating point representations are overused in software. It seems like about 90% of the time the programmer should have used integer arithmetic over a small 'quantum', and 5% of the time should have used rational arithmetic. Floating point is only really appropriate when proportional accuracy is needed over a wide range of scales. -- Ashley Yakeley, Seattle WA

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toRational problem
by pablocardenal＠laplatavive.com 17 Apr '01

17 Apr '01

Hi, > Why (toRational 0.9) == (9 % 10) is False? Saludos, Pablo "Yeti" Cardenal from La Plata, Buenos Aires, Argentina

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Concurrent Haskell
by Andre W B Furtado 16 Apr '01

16 Apr '01

I am trying to create a program that prints interleaved sequences of 'a's and 'z's, using Haskell Direct (under GHC). I've tryied the following: import Concurrent main :: IO() main = let loop ch = putStr(ch) >> loop ch in forkIO (loop "a") >> loop "z" But this program prints only 'z's, and no 'a's. I was surprised by this, since GHC uses preemptive multitasking, not cooperative multitasking. Does anyone knows what's going wrong? Thanks, Andre W b Furtado

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RE: help wanted with type error message
by Simon Peyton-Jones 15 Apr '01

15 Apr '01

| A row has a formula and a weight. | | > data Formula f => Row a = Row (f a, Weight) | | Line 14 is the definition of Row. Can anyone explain the | proper use of a context in a data definition? All the type variables in the constructor arguments must be introduced as type constructor args. Thus, for example: data Formula f => Row a f = Row (f a, Weight) woudl be ok. But I bet you wanted to say "f a is a formula, that has exactly the operations of class Formula available". You probably don't want the 'f' to appear in the type (Row a f). In that case you need an exisitential type (not Haskell 98), which GHC and Hugs support thus: data Row a = forall f. Formula f => Row (f a, Weight) The existential is written 'forall' because the type of the Row constructor really is Row :: forall a f. Formula f => (f a, Weight) -> Row a Now you can pattern-match thus: f :: Row a -> Environment a -> Bool f (Row (x,w)) = eval x env A context on the LHS of a data type declaration is (in my opinion) one of Haskell's few blunders. It makes the constructor into an overloaded function, thus: Row :: (Formula f) => (f a, Weight) -> Row a That forces (Formula f) to hold at the call site of Row, but when you pattern match it isn't. So it simply makes fewer programs typecheck without increasing expressiveness. Indeed, as you found, it's positively misleading. Simon

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ghc 5.00 released??
by Dylan Thurston 13 Apr '01

13 Apr '01

At http://haskel.org/ghc/, there seems to be an announcement for ghc 5.00, released April 9, 2001. Is this correct? I was surprised not to see an announcement posted here. (And this was just after I decided to compile the whole darn thing from CVS...) Best, Dylan Thurston

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RE: Question about hugs´s DEBUG_SHOWSC
by Chris Angus 13 Apr '01

13 Apr '01

is this not the dictionary passing (or whatever)to achieve overloading. > -----Original Message----- > From: dario.bahena(a)correo.unam.mx [mailto:dario.bahena@correo.unam.mx] > Sent: 13 April 2001 06:48 > To: hugs-users(a)haskell.org > Cc: hugs-bugs(a)haskell.org; haskell(a)haskell.org > Subject: Question about hugs´s DEBUG_SHOWSC > > > hi haskellers ... > > I´m trying to use the hugs´s flag DEBUG_SHOWSC. Everything > seems to work > fine, except for one(simple?) detail: > > When you use Int or Double literals, hugs always add extra variables, > related to the fromInt/fromDouble aplication. > > For example: > > f = 666 > > should be translated to: > > f = fromInt 666 > > but hugs returns : > > f = fromInt v35 666 > > What is the var. v35 for???, the function fromInt receives > just one argument. > > > If the function has parameters, the resulting function has > extra-parameters, > which seems to be out-of-place in both sides of the binding ... > here you are a couple of examples: > > the function: > > f x = 666 > > generates the output: > > f o2 o1 = fromInt o2 666 > > And the reported type for f is: Num a => b -> a , which is > inconsistent with > the previous result. > > the function: > > f x = 666 + x > > generates : > > f o2 o1 = (+) o2 (fromInt o2 666) o1 > > which has similar troubles to the previous example. > > What are these extra-variables for? Are they a debug-feature? > what´s the > meaning? > > And finally, the obligated question: how can I avoid them? I > suppose that > I have to take a look at the source code, but: what´d be the > part which > is responsible for this(parser,static,type... no please,compiler)? > > Thanks in advance. > > saludos > dario estepario ... > > ------------------------------------------------- > Obtén tu correo en www.correo.unam.mx > UNAMonos Comunicándonos > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Question about hugs´s DEBUG_SHOWSC
by dario.bahena＠correo.unam.mx 13 Apr '01

13 Apr '01

hi haskellers ... I´m trying to use the hugs´s flag DEBUG_SHOWSC. Everything seems to work fine, except for one(simple?) detail: When you use Int or Double literals, hugs always add extra variables, related to the fromInt/fromDouble aplication. For example: f = 666 should be translated to: f = fromInt 666 but hugs returns : f = fromInt v35 666 What is the var. v35 for???, the function fromInt receives just one argument. If the function has parameters, the resulting function has extra-parameters, which seems to be out-of-place in both sides of the binding ... here you are a couple of examples: the function: f x = 666 generates the output: f o2 o1 = fromInt o2 666 And the reported type for f is: Num a => b -> a , which is inconsistent with the previous result. the function: f x = 666 + x generates : f o2 o1 = (+) o2 (fromInt o2 666) o1 which has similar troubles to the previous example. What are these extra-variables for? Are they a debug-feature? what´s the meaning? And finally, the obligated question: how can I avoid them? I suppose that I have to take a look at the source code, but: what´d be the part which is responsible for this(parser,static,type... no please,compiler)? Thanks in advance. saludos dario estepario ... ------------------------------------------------- Obtén tu correo en www.correo.unam.mx UNAMonos Comunicándonos

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