- Haskell-Cafe - Haskell.org

RE: [Haskell] is $ a no-op?
by Graham Klyne 13 Oct '04

13 Oct '04

[Switched to haskell-cafe] At 13:24 13/10/04 -0400, Jacques Carette wrote: > > > -- It's kind of like an converse map. > > > > I have attempted, unsuccessfully, to write flist above in a point-free > > manner. Is it possible? > > > Of course it is, but why? > > flist = flip (map . flip ($)) > >Some functions are simpler point-free, others are simpler with points. I >was curious about this one (I like the pointwise version better). > >Also, the statement "It's kind of like a converse map" becomes quite clear >from the point-free way to write it, while still not so obvious in the >pointwise version. The point-free form can be deduced thus, I think: flist fs a = map ($ a) fs (flip flist) a fs = map ($ a) fs -- swap flist args (flip flist) a = map ($ a) -- redundant arg (flip flist) a = map ((flip ($)) a) -- expand section (flip flist) a = (map . (flip ($))) a -- definition of . (flip flist) = map . (flip ($)) -- redundant arg flist = flip (map . (flip ($))) -- move flip to RHS #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Re: [Haskell] lazy constructors
by Serge D. Mechveliani 13 Oct '04

13 Oct '04

On Wed, Oct 13, 2004 at 10:06:44AM +0200, Sander Evers wrote: > > >Question 2 > >---------- > >How to arrange the above `lazy' output? > > > > > Your function addToSPair > > addToSPair :: Char -> (String, String) -> (String, String) > addToSPair c (xs, ys ) = (c:xs, ys ) > > does a strict pattern match on the pair constructor. It needs to know > that its second argument is a pair before it produces anything of its > result. If you use a lazy pattern match > > addToSPair :: Char -> (String, String) -> (String, String) > addToSPair c ~(xs, ys ) = (c:xs, ys ) > > instead, it doesn't need to evaluate this argument immediately. !! This is great, thank you. This improves a lot. I never recalled of the tilda for several years. Excuse me for the stupidity, but I have a next naive question. As the types are resolved before the computation, as the above program shows, how can addToSPair expect anything besides a string pair? Why tilda is not a default? --------------------------------------------- repeating: bottomStr :: String bottomStr = let xs = xs in xs bottomSPair :: (String, String) bottomSPair = let p = p in p longToReach = 10^8 :: Int f1, f2, g1, g2 :: Char f1 = head ('a' : (if last [1 .. longToReach] > 0 then "b" else "c")) f2 = head ('a' : bottomStr) addToSPair :: Char -> (String, String) -> (String, String) addToSPair c (xs, ys ) = (c:xs, ys ) g1 = head $ fst $ addToSPair 'a' (if last [1 .. longToReach] > 0 then ("b", "") else ("c", "")) g2 = head $ fst $ addToSPair 'a' bottomSPair ----------------------------------------------------------------- The only doubt may be the matching of (xs, ys) against bottom :: (String, String) Is not this natural to have a result as (bottom :: String, bottom :: String) -- = xs = ys and compute further? What will occur if all the Haskell programmers set `~' before each data constructor in each pattern in their existing programs? As the types are recognized before the computation, the programs will remain safe and will become considerably faster. For example, the above program of g1 becomes a billion times faster. I wonder. Copy, please, the answer to mechvel(a)botik.ru ----------------- Serge Mechveliani mechvel(a)botik.ru

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Re: [Haskell-cafe] Re: [Haskell] lazy constructors
by MR K P SCHUPKE 13 Oct '04

13 Oct '04

>addToSPair :: Char -> (String, String) -> (String, String) What about: addToSPair :: Char -> String -> String -> (String,String) so that the pattern match is: addToSPair c xs ys = (c:xs,ys) This is irrefutable? Keean.

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Re: [Haskell] lazy constructors
by Serge D. Mechveliani 13 Oct '04

13 Oct '04

On Wed, Oct 13, 2004 at 09:23:43AM +0100, MR K P SCHUPKE wrote: > You can only return a list of pair's lazily, not > a pair of lists. If the two strings are independant, then > generate each in a separate function, and pair off the > results lazily. No, I have several labeled fields in a record, which are very dependent. As people teach, the costruct of ~(..,..) helps for the Pair. Maybe, the labeled record can also be handled ... ----------------- Serge Mechveliani mechvel(a)botik.ru

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Re: [Haskell-cafe] RE: [Haskell] lazy constructors
by MR K P SCHUPKE 13 Oct '04

13 Oct '04

>addToSPair c ~(xs, ys ) =3D (c:xs, ys ) I thought pattern bindings had an implicit "~"? Keean.

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RE: [Haskell] lazy constructors
by Simon Peyton-Jones 13 Oct '04

13 Oct '04

[I think this thread would be more appropriate on haskell-café, so I'm redirecting it] | addToSPair :: Char -> (String, String) -> (String, String) | addToSPair c (xs, ys ) = (c:xs, ys ) | | --------------------------------------------------------------- | | g1 is similar to f1, g2 similar to f2. | | f1 and f2 are evaluated immediately to 'a'; | | g1 evaluates to 'a' after a long computation; | g2 evaluates to Fail: <<loop>> | | As g2 is similar to f2, should it have a value 'a' ? | In | head $ fst $ addToSPair 'a' bottomSPair Try using lazy pattern matching, thus addToSPair :: Char -> (String, String) -> (String, String) addToSPair c ~(xs, ys ) = (c:xs, ys ) The only change is the "~" in the defn of addToSPair. Now g2 will have value 'a'. Simon

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strictness and the simple continued fraction
by William Lee Irwin III 13 Oct '04

13 Oct '04

So, I discovered that simple continued fractions are supposed to be spiffy lazy lists and thought I'd bang out some continued fraction code. But then I discovered ContFrac.hs and couldn't really better it. Of course, I went about trying to actually do things relying on their laziness, and discovered they weren't as lazy as I hoped they'd be. Simple uses of approximations at the ghci command line such as: instance Ord ContFrac where (ContFrac (x:xs)) `compare` (ContFrac (y:ys)) = case x `compare` y of LT -> LT GT -> GT EQ -> (ContFrac ys) `compare` (ContFrac xs) (ContFrac []) `compare` cf = case cf of ContFrac (x:_) -> 0 `compare` x ContFrac [] -> EQ cf `compare` (ContFrac []) = case cf of ContFrac (x:_) -> x `compare` 0 ContFrac [] -> EQ x = expCF (1/2) where expCF x | x < 0 = recip . expCF $ negate x | x == 0 = 1 | x == 1 = let ContFrac es = ecf in ContFrac (take 100 es) | otherwise = case x of ContFrac [y] -> (expCF 1)^y ContFrac (y:ys) -> if y /= 0 then ((expCF 1)^y) *(expCF (ContFrac (0:ys))) else (1+x+x^2/2+x^3/6+x^4/24+x^5/120) ContFrac [] -> expCF 0 where the instance was added to ContFrac.hs seems to fail to terminate, where manually reducing things a bit appears to restore termination. So, what hit me? -- wli

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Tutorial about low-level I/O in Haskell
by Peter Simons 12 Oct '04

12 Oct '04

Hi, while polishing a small library of mine, I ended up writing a tutorial about the use of hGetBufNonBlocking and the related Foreign.* functions you need to do fast, block-oriented I/O in Haskell. There is something to be said for literate programming. Using recent topics for inspiration, the text describes the implementation of a fast word-counting program. :-) Here is the URL: http://cryp.to/blockio/docs/tutorial.html Peter

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Strict evaluation not working?
by Christian Hofer 12 Oct '04

12 Oct '04

Hi, having found a bit of time to play with Haskell, I am trying to figure out how to enforce strict evaluation. I wrote the following program: main = let x = zipWith (+) [5..] [6..] in putStrLn $ show $ x `seq` head x I expected this program not to terminate - because of the seq-Operator, but it happily returns 11 in ghc as well as in ghci. What do I make wrong? Chris

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Re: Language extension idea (was Re: [Haskell-cafe] Re: OCaml list sees...)
by J.Garrett.Morris＠Dartmouth.EDU 12 Oct '04

12 Oct '04

--- Tom Pledger wrote: It could get more convenient, though, if we have data-constructors-as-class-members and move the familiar list constructors into the List class. (The following might need to be built into the compiler, because of the special [] syntax.) --- end of quote --- Changing the subject very slightly, this brings up two questions: 1) Wouldn't implementing the Views proposal allow this? and, 2) Why haven't views been implemented (and why are they a "never" in the HaskellTwo discussion in HaWiki)? /gXm

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