[Haskell-beginners] Re: Beginners Digest, Vol 19, Issue 30

29 Jan 2010

      I copy pasted Daniel's code. Changed the code to use Mersenne
genrerator and got 4200% (!) improvement:

import System.Random.Mersenne
import System( getArgs )

inCirc :: Double -> Double -> Int
inCirc x y
    | dx*dx + dy*dy < 0.25  = 1
    | otherwise             = 0
      where
        dx = x - 0.5
        dy = y - 0.5

-- transform a list of coordinates into a list of indicators
-- whether the point is inside the circle (sorry for the
-- stupid name)
inCircles :: [Double] -> [Int]
inCircles (x:y:zs) = inCirc x y : inCircles zs
inCircles _ = []

-- given a count of experiments and an infinite list of coordinates,
-- calculate an approximation to pi
calcPi :: Int -> [Double] -> Double
calcPi n ds = fromIntegral ct / fromIntegral n * 4
      where
        ct = sum . take n $ inCircles ds

-- now the IO part is only
-- * getting the number of experiments and
-- * getting the StdGen
main :: IO ()
main = do
    args <- getArgs
    sg <- getStdGen
    let n = case args of
                (a:_) -> read a
                _ -> 10000
    rands  <- randoms sg :: IO [Double]
    print $ calcPi n rands

time ./slow-pi +RTS -K1G -RTS 1000000
3.14222

real	0m6.886s
user	0m6.680s
sys	0m0.200s

time ./improved-pi 1000000
3.14364

real	0m0.163s
user	0m0.160s
sys	0m0.000s

Thanks all for the help !

On Fri, Jan 29, 2010 at 12:31 PM,  <beginners-request@haskell.org> wrote:
...
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Today's Topics:
  1.  Could not deduce (Matrix m (Maybe a)) from the   context
     (Matrix m a) (Lyndon Maydwell)
  2. Re:  subset - a little add (Daniel Fischer)
  3. Re:  Could not deduce (Matrix m (Maybe a)) from   the context
     (Matrix m a) (Daniel Fischer)
  4. Re:  Could not deduce (Matrix m (Maybe a)) from   the context
     (Matrix m a) (Lyndon Maydwell)
  5.  subset - a little add (Luca Ciciriello)
  6. Re:  Could not deduce (Matrix m (Maybe a)) from   the context
     (Matrix m a) (Daniel Fischer)
  7.  PI calculation - Newbie question (Gabi)
----------------------------------------------------------------------
Message: 1
Date: Fri, 29 Jan 2010 16:52:37 +0800
From: Lyndon Maydwell <maydwell@gmail.com>
Subject: [Haskell-beginners] Could not deduce (Matrix m (Maybe a))
       from the        context (Matrix m a)
To: beginners@haskell.org
Message-ID:
       <da8fea9e1001290052x233f51ev8e4c04e2649277e7@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Hi Beginners.
I'm writing a matrix class for a game of life implementation. When I
try to compile it I get the error "Could not deduce (Matrix m (Maybe
a)) from the context (Matrix m a)" for the method vicinityMatrix.
However, when I query the type of an identical implementation to
vicinityMatrix in ghci it is successful:
:t \m x y -> fromRows $ vicinityRows m x y
\m x y -> fromRows $ vicinityRows m x y
 :: forall (m :: * -> *) (m1 :: * -> *) a.
    (Matrix m (Maybe a), Matrix m1 a) =>
    m1 a -> Integer -> Integer -> m (Maybe a)
What might be preventing the class from compiling?
Thanks guys.
---
My Matrix class definition follows below:
module Matrix (Matrix) where
import Data.Array
import Data.Maybe (catMaybes)
import Control.Monad (guard)
class Matrix m a
 where
   fromRows       :: [[a]] -> m a
   toList         :: m a   -> [a]
   rows           :: m a   -> Integer
   columns        :: m a   -> Integer
   row            :: m a   -> Integer -> [a]
   column         :: m a   -> Integer -> [a]
   at             :: m a   -> Integer -> Integer -> a
   (!!!)          :: m a   -> Integer -> Integer -> a
   vicinityRows   :: m a   -> Integer -> Integer -> [[Maybe a]]
   vicinityMatrix :: m a   -> Integer -> Integer -> m (Maybe a)
   neighbours     :: m a   -> Integer -> Integer -> [a]
   toList m = do
     x <- [0 .. columns m - 1]
     y <- [0 .. rows m - 1]
     return $ at m x y
   row    m n = [at m x n | x <- [0 .. columns m - 1]]
   column m n = [at m n y | y <- [0 .. rows    m - 1]]
   at    = (!!!)
   (!!!) = at
   vicinityRows m x y = do
     x' <- [x - 1 .. x + 1]
     return $ do
       y' <- [y - 1 .. y + 1]
       return cell where
         cell
           | x <  0         = Nothing
           | y <  0         = Nothing
           | x >= columns m = Nothing
           | y >= rows m    = Nothing
           | otherwise      = Just $ at m x y
   vicinityMatrix m x y = fromRows $ vicinityRows m x y
   -- neighbours = catMaybes . toListN . vicinityMatrix
toListN :: Matrix m a => m a -> [a]
toListN m = do
 x <- [0 .. columns m - 1]
 y <- [0 .. rows m - 1]
 guard $ x /= 1 && y /= 1
 return $ at m x y
------------------------------
Message: 2
Date: Fri, 29 Jan 2010 10:06:29 +0100
From: Daniel Fischer <daniel.is.fischer@web.de>
Subject: Re: [Haskell-beginners] subset - a little add
To: beginners@haskell.org
Message-ID: <201001291006.30011.daniel.is.fischer@web.de>
Content-Type: text/plain;  charset="utf-8"
Am Freitag 29 Januar 2010 08:36:35 schrieb Luca Ciciriello:
...
Just a little add to may previous mail.
The solution I've found from myself is:
subset :: [String] -> [String] -> Bool
subset xs ys = and [elem x ys | x <- xs]
Variant:
subset xs ys = all (`elem` ys) xs
but is that really what you want? That says subset [1,1,1,1] [1] ~> True.
If you regard your lists as representatives of sets (as the name suggests),
then that's correct, otherwise not.
However, this is O(length xs * length ys). If you need it only for types
belonging to Ord, a much better way is
import qualified Data.Set as Set
import Data.Set (fromList, isSubsetOf, ...)
subset xs ys = fromList xs `isSubsetOf` fromList ys
or, if you don't want to depend on Data.Set,
subset xs ys = sort xs `isOrderedSublistOf` sort ys
xxs@(x:xs) `isOrderedSublistOf` (y:ys)
   | x < y     = False
   | x == y    = xs `isOrderedSublistOf` ys
   | otherwise = xxs `isOrderedSublistOf` ys
[] `isOrderedSublistOf` _ = True
_ `isOrderedSublistOf` [] = False
...
My question is if exists a more elegant way to do that.
Luca.
------------------------------
Message: 3
Date: Fri, 29 Jan 2010 10:17:10 +0100
From: Daniel Fischer <daniel.is.fischer@web.de>
Subject: Re: [Haskell-beginners] Could not deduce (Matrix m (Maybe a))
       from    the context (Matrix m a)
To: beginners@haskell.org
Message-ID: <201001291017.10744.daniel.is.fischer@web.de>
Content-Type: text/plain;  charset="utf-8"
Am Freitag 29 Januar 2010 09:52:37 schrieb Lyndon Maydwell:
...
Hi Beginners.
I'm writing a matrix class for a game of life implementation. When I
try to compile it I get the error "Could not deduce (Matrix m (Maybe
a)) from the context (Matrix m a)" for the method vicinityMatrix.
However, when I query the type of an identical implementation to
vicinityMatrix in ghci it is successful:
:t \m x y -> fromRows $ vicinityRows m x y
\m x y -> fromRows $ vicinityRows m x y
  :: forall (m :: * -> *) (m1 :: * -> *) a.
     (Matrix m (Maybe a), Matrix m1 a) =>
     m1 a -> Integer -> Integer -> m (Maybe a)
What might be preventing the class from compiling?
Well, the error says the compiler (the type checker) can't deduce the
context (Matrix m (Maybe a)) from the givens. If you supply that
information,
vicinityMatrix :: Matrix m (Maybe a) =>
              m a -> Integer -> Integer -> m (Maybe a)
it'll work.
...
Thanks guys.
---
My Matrix class definition follows below:
module Matrix (Matrix) where
import Data.Array
import Data.Maybe (catMaybes)
import Control.Monad (guard)
class Matrix m a
  where
    fromRows       :: [[a]] -> m a
    toList         :: m a   -> [a]
    rows           :: m a   -> Integer
    columns        :: m a   -> Integer
    row            :: m a   -> Integer -> [a]
    column         :: m a   -> Integer -> [a]
    at             :: m a   -> Integer -> Integer -> a
    (!!!)          :: m a   -> Integer -> Integer -> a
    vicinityRows   :: m a   -> Integer -> Integer -> [[Maybe a]]
    vicinityMatrix :: m a   -> Integer -> Integer -> m (Maybe a)
    neighbours     :: m a   -> Integer -> Integer -> [a]
    toList m = do
      x <- [0 .. columns m - 1]
      y <- [0 .. rows m - 1]
      return $ at m x y
    row    m n = [at m x n | x <- [0 .. columns m - 1]]
    column m n = [at m n y | y <- [0 .. rows    m - 1]]
    at    = (!!!)
    (!!!) = at
    vicinityRows m x y = do
      x' <- [x - 1 .. x + 1]
      return $ do
        y' <- [y - 1 .. y + 1]
        return cell where
          cell
            | x <  0         = Nothing
            | y <  0         = Nothing
            | x >= columns m = Nothing
            | y >= rows m    = Nothing
            | otherwise      = Just $ at m x y
    vicinityMatrix m x y = fromRows $ vicinityRows m x y
    -- neighbours = catMaybes . toListN . vicinityMatrix
toListN :: Matrix m a => m a -> [a]
toListN m = do
  x <- [0 .. columns m - 1]
  y <- [0 .. rows m - 1]
  guard $ x /= 1 && y /= 1
  return $ at m x y
------------------------------
Message: 4
Date: Fri, 29 Jan 2010 17:45:32 +0800
From: Lyndon Maydwell <maydwell@gmail.com>
Subject: Re: [Haskell-beginners] Could not deduce (Matrix m (Maybe a))
       from    the context (Matrix m a)
To: Daniel Fischer <daniel.is.fischer@web.de>
Cc: beginners@haskell.org
Message-ID:
       <da8fea9e1001290145n4597e3abk51f6bbc87ef78f89@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Thanks Daniel.
It works, but I'm a bit confused as to why the extra type information is needed.
On Fri, Jan 29, 2010 at 5:17 PM, Daniel Fischer
<daniel.is.fischer@web.de> wrote:
...
Am Freitag 29 Januar 2010 09:52:37 schrieb Lyndon Maydwell:
...
Hi Beginners.
I'm writing a matrix class for a game of life implementation. When I
try to compile it I get the error "Could not deduce (Matrix m (Maybe
a)) from the context (Matrix m a)" for the method vicinityMatrix.
However, when I query the type of an identical implementation to
vicinityMatrix in ghci it is successful:
:t \m x y -> fromRows $ vicinityRows m x y
\m x y -> fromRows $ vicinityRows m x y
  :: forall (m :: * -> *) (m1 :: * -> *) a.
     (Matrix m (Maybe a), Matrix m1 a) =>
     m1 a -> Integer -> Integer -> m (Maybe a)
What might be preventing the class from compiling?
Well, the error says the compiler (the type checker) can't deduce the
context (Matrix m (Maybe a)) from the givens. If you supply that
information,
vicinityMatrix :: Matrix m (Maybe a) =>
              m a -> Integer -> Integer -> m (Maybe a)
it'll work.
...
Thanks guys.
---
My Matrix class definition follows below:
module Matrix (Matrix) where
import Data.Array
import Data.Maybe (catMaybes)
import Control.Monad (guard)
class Matrix m a
  where
    fromRows       :: [[a]] -> m a
    toList         :: m a   -> [a]
    rows           :: m a   -> Integer
    columns        :: m a   -> Integer
    row            :: m a   -> Integer -> [a]
    column         :: m a   -> Integer -> [a]
    at             :: m a   -> Integer -> Integer -> a
    (!!!)          :: m a   -> Integer -> Integer -> a
    vicinityRows   :: m a   -> Integer -> Integer -> [[Maybe a]]
    vicinityMatrix :: m a   -> Integer -> Integer -> m (Maybe a)
    neighbours     :: m a   -> Integer -> Integer -> [a]
    toList m = do
      x <- [0 .. columns m - 1]
      y <- [0 .. rows m - 1]
      return $ at m x y
    row    m n = [at m x n | x <- [0 .. columns m - 1]]
    column m n = [at m n y | y <- [0 .. rows    m - 1]]
    at    = (!!!)
    (!!!) = at
    vicinityRows m x y = do
      x' <- [x - 1 .. x + 1]
      return $ do
        y' <- [y - 1 .. y + 1]
        return cell where
          cell
            | x <  0         = Nothing
            | y <  0         = Nothing
            | x >= columns m = Nothing
            | y >= rows m    = Nothing
            | otherwise      = Just $ at m x y
    vicinityMatrix m x y = fromRows $ vicinityRows m x y
    -- neighbours = catMaybes . toListN . vicinityMatrix
toListN :: Matrix m a => m a -> [a]
toListN m = do
  x <- [0 .. columns m - 1]
  y <- [0 .. rows m - 1]
  guard $ x /= 1 && y /= 1
  return $ at m x y
------------------------------
Message: 5
Date: Fri, 29 Jan 2010 10:01:36 +0000
From: Luca Ciciriello <luca_ciciriello@hotmail.com>
Subject: [Haskell-beginners] subset - a little add
To: <beginners@haskell.org>
Message-ID: <SNT128-W4078FB7A2593E1F547AAA29A5B0@phx.gbl>
Content-Type: text/plain; charset="iso-8859-1"
Thanks Daniel.
Yes my function operate only in a set-theory contest and your solution:
subset xs ys = all (`elem` ys) xs
is indeed more elegant than mine.
Thanks again for your help.
Luca.
...
From: daniel.is.fischer@web.de
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] subset - a little add
Date: Fri, 29 Jan 2010 10:06:29 +0100
CC: luca_ciciriello@hotmail.com
Am Freitag 29 Januar 2010 08:36:35 schrieb Luca Ciciriello:
...
Just a little add to may previous mail.
The solution I've found from myself is:
subset :: [String] -> [String] -> Bool
subset xs ys = and [elem x ys | x <- xs]
Variant:
subset xs ys = all (`elem` ys) xs
but is that really what you want? That says subset [1,1,1,1] [1] ~> True.
If you regard your lists as representatives of sets (as the name suggests),
then that's correct, otherwise not.
However, this is O(length xs * length ys). If you need it only for types
belonging to Ord, a much better way is
import qualified Data.Set as Set
import Data.Set (fromList, isSubsetOf, ...)
subset xs ys = fromList xs `isSubsetOf` fromList ys
or, if you don't want to depend on Data.Set,
subset xs ys = sort xs `isOrderedSublistOf` sort ys
xxs@(x:xs) `isOrderedSublistOf` (y:ys)
| x < y = False
| x == y = xs `isOrderedSublistOf` ys
| otherwise = xxs `isOrderedSublistOf` ys
[] `isOrderedSublistOf` _ = True
_ `isOrderedSublistOf` [] = False
...
My question is if exists a more elegant way to do that.
Luca.
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