Re: [Haskell-cafe] -- comments
On Sat, Dec 12, 2009 at 5:34 AM, Brandon S. Allbery KF8NH allb...@ece.cmu.edu wrote: On Dec 11, 2009, at 23:30 , michael rice wrote: I'm just noticing that -- comments don't seem to work properly when the first character following them is a '*'. I believe the spec only treats -- as a comment leader; this is why Haddock markup has a space between the -- and the markup. The spec [1] treats anything that starts with two or more dashes not followed by a non-symbol to be the start of a comment. [1] http://www.haskell.org/onlinereport/syntax-iso.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] accessible layout proposal?
On Tue, Sep 22, 2009 at 10:01 AM, Jimmy Hartzell j...@shareyourgifts.net wrote: I am in love with this proposal: http://www.haskell.org/haskellwiki/Accessible_layout_proposal I'm not sure whether I like the idea in general or not. It looks a bit odd. The suggestion on the talk page ( http://www.haskell.org/haskellwiki/Talk:Accessible_layout_proposal ) might be preferable, although I wonder about the implications. For example, what should (#) be parsed as? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Mapping over multiple values of a list at once?
Just wondering, what should be the expected output be of something like mavg 4 [1..3]? [3%2] or []? Patai's and Eugene's solutions assume the former. On Thu, Aug 27, 2009 at 10:19 AM, hask...@kudling.de wrote: Hi, Imagine you have a list with n-values. You are asked to iterate over the list and calculate the average value of each 3 neighbouring values. For example, starting from [4,3,2,6,7] you need to find the averages of 4,3,2 and 3,2,6 and 2,6,7 resulting in [3,4,5] What is the most elegant way to do that? The naive ansatz to use (!!) excessively sounds pretty inefficient. Bye, Lenny ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A mistake in haskellwiki
On 8/6/09, Don Stewart d...@galois.com wrote: leaveye.guo: Hi haskellers: There is a mistake in http://www.haskell.org/haskellwiki/State_Monad It post two functions like this : evalState :: State s a - s - a evalState act = fst $ runState act execState :: State s a - s - s execState act = snd $ runState act Both the '$' operators should be '.'. Anyone would correct it ? Fixed :) Well, it's a wiki ... :-) -- Don That might be true, but not everyone has an account :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: excercise - a completely lazy sorting algorithm
On 7/9/09, Heinrich Apfelmus apfel...@quantentunnel.de wrote: Of course, some part of algorithm has to be recursive, but this can be outsourced to a general recursion scheme, like the hylomorphism hylo :: Functor f = (a - f a) - (f b - b) - (a - b) hylo f g = g . fmap (hylo f g) . f Is that definition of hylo actually usuable? A few on IRC tried to use that definition for a few examples, but the examples failed to terminate or blew up the stack. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Hylomorphisms (was: [Haskell-cafe] excercise - a completely lazy sorting algorithm)
On 7/12/09, Heinrich Apfelmus apfel...@quantentunnel.de wrote: Raynor Vliegendhart wrote: On 7/9/09, Heinrich Apfelmus apfel...@quantentunnel.de wrote: Of course, some part of algorithm has to be recursive, but this can be outsourced to a general recursion scheme, like the hylomorphism hylo :: Functor f = (a - f a) - (f b - b) - (a - b) hylo f g = g . fmap (hylo f g) . f Is that definition of hylo actually usable? A few on IRC tried to use that definition for a few examples, but the examples failed to terminate or blew up the stack. The implementation of quicksort with hylo works fine for me, given medium sized inputs like for example quicksort (reverse [1..1000]) . What were the examples you tried? One of the examples I tried was: hylo (unfoldr (\a - Just (a,a))) head $ 42 This expression fails to determinate. Here are two examples copumpkin tried on IRC: copumpkin let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) length 5 lambdabot 5 copumpkin let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) sum 5 lambdabot * Exception: stack overflow ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monoid wants a (++) equivalent
On Tue, Jun 30, 2009 at 6:45 PM, Bryan O'Sullivanb...@serpentine.com wrote:
I've thought for a while that it would be very nice indeed if the Monoid
class had a more concise operator for infix appending than a `mappend` b.
I wonder if other people are of a similar opinion, and if so, whether this
is worth submitting a libraries@ proposal over.
We could use (Control.Category..) as an operator, but this would
require an additional wrapping layer if we wish to use the existing
Monoid instances:
import Prelude hiding (id, (.))
import Control.Category
import Data.Monoid
-- Category wrapper for existing Monoid instances
newtype MonoidC m a b = MonoidC {unwrapMC :: m} deriving (Show)
instance Monoid m = Category (MonoidC m) where
id = MonoidC mempty
MonoidC m . MonoidC n = MonoidC $ m `mappend` n
Furthermore, writing Category instances for monoids require dummy type
parameters:
-- Example instance
newtype SumC m a b = SumC {getSumC :: m} deriving (Show, Eq)
instance Num a = Category (SumC a) where
id = SumC (fromIntegral 0)
SumC x . SumC y = SumC $ x + y
Another disadvantage of this approach is that we cannot have a default
monoid instance for lists (kind mismatch).
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Re: [Haskell-cafe] Obscure weirdness
On 6/20/09, Andrew Coppin andrewcop...@btinternet.com wrote: Is this a known bug in GHC 6.10.1? Will upgrading fix it? (Obviously, it's quite a lot of work to change GHC.) Suffice it to say that my program is quite big and complicated; it worked fine when it was still small and simple. ;-) There is a bug in ghci 6.10.1 that seems to be fixed in 6.10.3 (not sure whether it's fixed in 6.10.3). Certain non-terminating expressions causes ghci to crash immediately and go back to the prompt: C:\ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude let loop = loop in loop C:\ Compiling a module with a non-terminating expression first with ghc and then loading it in ghci does not seem to result in a crash. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining laziness: another example
This might be slightly related. When I was assisting a Haskell lab
course, I encountered solutions like the following:
removeRoot :: BSTree a - BSTree a
removeRoot (Node x Empty Empty) = Empty
removeRoot (Node x left Empty) = left
removeRoot (Node x Empty right) = right
removeRoot (Node x left right) = undefined {- not needed for discussion -}
The first removeRoot case is unnecessary. Students new to Haskell (or
maybe new to recursion in general?) seem to consider more base cases
than needed.
-Raynor
On 6/8/09, Martijn van Steenbergen mart...@van.steenbergen.nl wrote:
Hello,
While grading a Haskell student's work I ran into an example of a program
not being lazy enough. Since it's such a basic and nice example I thought
I'd share it with you:
One part of the assignment was to define append :: [a] - [a] - [a],
another to define cycle2 :: [a] - [a]. This was her (the majority of the
students in this course is female!) solution:
append :: [a] - [a] - [a]
append [] ys = ys
append xs [] = xs
append (x:xs) ys = x : (append xs ys)
cycle2 :: [a] - [a]
cycle2 [] = error empty list
cycle2 xs = append xs (cycle2 xs)
This definition of append works fine on any non-bottom input (empty, finite,
infinite), but due to the unnecessary second append case, cycle2 never
produces any result.
Martijn.
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Re: [Haskell-cafe] beginners question about fromMaybe
If you're absolutely certain that the lookup always succeeds, then you can use pattern matching as follows: where jr = joinTuples sc x val key = getPartialTuple is x Just val = Map.lookup key m On 6/3/09, Nico Rolle nro...@web.de wrote: hi there heres a code snipped, don't care about the parameters. the thing is i make a lookup on my map m and then branch on that return value probePhase is sc [] m = [] probePhase is sc (x:xs) m | val == Nothing = probePhase is sc xs m | otherwise = jr ++ probePhase is sc xs m where jr = joinTuples sc x (fromMaybe [] val) key = getPartialTuple is x val = Map.lookup key m the line jr = joinTuples sc x (fromMaybe [] val) is kind of ugly because i know that it is not Nothing. is there a better way to solve this? regards ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] beginners question about fromMaybe
I just noticed that my suggestion doesn't work. You're testing whether val is Nothing and in my code snipped val has a different type. On 6/3/09, Raynor Vliegendhart shinnon...@gmail.com wrote: If you're absolutely certain that the lookup always succeeds, then you can use pattern matching as follows: where jr = joinTuples sc x val key = getPartialTuple is x Just val = Map.lookup key m On 6/3/09, Nico Rolle nro...@web.de wrote: hi there heres a code snipped, don't care about the parameters. the thing is i make a lookup on my map m and then branch on that return value probePhase is sc [] m = [] probePhase is sc (x:xs) m | val == Nothing = probePhase is sc xs m | otherwise = jr ++ probePhase is sc xs m where jr = joinTuples sc x (fromMaybe [] val) key = getPartialTuple is x val = Map.lookup key m the line jr = joinTuples sc x (fromMaybe [] val) is kind of ugly because i know that it is not Nothing. is there a better way to solve this? regards ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] iota
The iota function you're looking for can be a whole lot simpler if you know about monads (list monad in particular) and sequence. For lists, sequence has the following behaviour: sequence [xs1,xs2, ... xsn] = [[x1,x2, ... , xn] | x1 - xs1, x2 - xs2, ... , xn - xsn] Using this, you can reduce your iota function to a powerful one-liner: iota = sequence . map (enumFromTo 0 . pred) Kind regards, Raynor Vliegendhart On 6/1/09, Paul Keir pk...@dcs.gla.ac.uk wrote: Hi all, I was looking for an APL-style “iota” function for array indices. I noticed “range” from Data.Ix which, with a zero for the lower bound (here (0,0)), gives the values I need: let (a,b) = (2,3) index ((0,0),(a-1,b-1)) [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2)] However, I need the results as a list of lists rather than a list of tuples; and my input is a list of integral values. I ended up writing the following function instead. The function isn’t long, but longer than I first expected. Did I miss a simpler approach? iota :: (Integral a) = [a] - [[a]] iota is = let count = product is tups = zip (tail $ scanr (*) 1 is) is buildRepList (r,i) = genericTake count $ cycle $ [0..i-1] = genericReplicate r lists = map buildRepList tups in transpose lists length $ iota [2,3,4] 24 Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
