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Today's Topics:
1. Parse error in pattern (Zhi-Qiang Lei)
2. Re: Parse error in pattern (Brent Yorgey)
3. Re: Parse error in pattern (Zhi-Qiang Lei)
4. Imperfect Graham Scan (Zhi-Qiang Lei)
5. Guard in class def (Henry Lockyer)
----------------------------------------------------------------------
Message: 1
Date: Sun, 8 Jan 2012 00:11:29 +0800
From: Zhi-Qiang Lei <[email protected]>
Subject: [Haskell-beginners] Parse error in pattern
To: Haskell Beginer <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hi,
I am writing a Graham Scan function. What puzzles me is it cannot be compiled.
Does anyone know what is wrong with "scan"? Thanks.
==== compile ====
bogon% ghc GrahamScan.hs
-- NORMAL --
[1 of 1] Compiling Main ( GrahamScan.hs, GrahamScan.o )
GrahamScan.hs:30:1: Parse error in pattern: scan
==== compile ====
==== code ====
data Point a = Point {
x :: a,
y :: b
}
instance Ord (Point a) where
compare (Point x1 y1) (Point x2 y2) = compare (y1, x1) (y2, x2)
data Vector a = Vector {
start :: Point a,
end :: Point a
}
cosine :: Vector a -> Ratio a
cosine (Vector (Point x1 y1) (Point x2 y2)) = (x2 - x1) / ((x2 - x1) ^ 2 + (y2
- y1) ^ 2)
instance Ord (Vector a) where
compare a b = compare (f a) (f b) where
f = negate . cosine
sort' :: [Point a] -> [Point a]
sort' xs = minPoint : fmap end sortedVectors where
sortedVectors = sort $ map (Vector minPoint) $ delete minPoint xs
minPoint = minimum xs
ccw :: Point a -> Point a -> Point a -> Bool
ccw (Point x1 y1) (Point x2 y2) (Point x3 y3) = (x2 - x1) * (y3 - y1) - (y2 -
y1) * (x3 - x1) > 0
scan :: [Point a] -> [Point a]
scan p1 : p2 : p3 : xs
| ccw p1 p2 p3 = p1 : scan (p2 : p3 : xs)
| otherwise = scan (p1 : p3 : xs)
scan xs = xs
grahamScan :: [Point a] -> [Point a]
grahamScan = scan . sort'
==== code ====
Best regards,
Zhi-Qiang Lei
[email protected]
------------------------------
Message: 2
Date: Sat, 7 Jan 2012 11:49:51 -0500
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Parse error in pattern
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Sun, Jan 08, 2012 at 12:11:29AM +0800, Zhi-Qiang Lei wrote:
>
> scan :: [Point a] -> [Point a]
> scan p1 : p2 : p3 : xs
Patterns with constructors and multiple components must be surrounded
by parentheses. So this should be
scan (p1 : p2 : p3 : xs)
-Brent
------------------------------
Message: 3
Date: Sun, 8 Jan 2012 01:10:31 +0800
From: Zhi-Qiang Lei <[email protected]>
Subject: Re: [Haskell-beginners] Parse error in pattern
To: Brent Yorgey <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
What an idiot am I! Thank you very much.
On Jan 8, 2012, at 12:49 AM, Brent Yorgey wrote:
> On Sun, Jan 08, 2012 at 12:11:29AM +0800, Zhi-Qiang Lei wrote:
>>
>> scan :: [Point a] -> [Point a]
>> scan p1 : p2 : p3 : xs
>
> Patterns with constructors and multiple components must be surrounded
> by parentheses. So this should be
>
> scan (p1 : p2 : p3 : xs)
>
> -Brent
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
Best regards,
Zhi-Qiang Lei
[email protected]
------------------------------
Message: 4
Date: Sun, 8 Jan 2012 15:31:44 +0800
From: Zhi-Qiang Lei <[email protected]>
Subject: [Haskell-beginners] Imperfect Graham Scan
To: Haskell Beginer <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hi,
The Graham Scan function I wrote, looks like running well. But when I put it in
QuickCheck, it just failed in some case. Anyone can show me some clues about
the problem? Thanks.
When I test it in ghci with some example, it returns the right result.
*Main> let xs = [Point {x = 1.0, y = 1.0},Point {x = 0.0, y = 4.0},Point {x =
0.0, y = 6.0},Point {x = 3.0, y = 5.0},Point {x = 4.0, y = 4.0},Point {x = 4.0,
y = 1.0},Point {x = 3.0, y = 3.0},Point {x = 2.0, y = 2.0},Point {x = 5.0, y =
5.0}]
*Main> grahamScan xs
[Point {x = 1.0, y = 1.0},Point {x = 4.0, y = 1.0},Point {x = 5.0, y =
5.0},Point {x = 0.0, y = 6.0},Point {x = 0.0, y = 4.0}]
*Main> grahamScan it
[Point {x = 1.0, y = 1.0},Point {x = 4.0, y = 1.0},Point {x = 5.0, y =
5.0},Point {x = 0.0, y = 6.0},Point {x = 0.0, y = 4.0}]
However, QuickCheck find some points which can fail it. Could it be a data type
overflow problem?
prop_scan_idempotent xs = not (null xs) ==> (grahamScan . grahamScan) xs ==
grahamScan xs
*Main> quickCheck prop_scan_idempotent
*** Failed! Falsifiable (after 13 tests and 4 shrinks):
[Point {x = -6.29996952110807, y = -91.37172300100718},Point {x =
9.353314917365527, y = 64.35532141764591},Point {x = -23.826685687218355, y =
60.32049750442556},Point {x = -1.4281411275074123, y = 31.54197550020998},Point
{x = -2.911218918860731, y = 15.564623822256719}]
=== code ===
module GrahamScan (grahamScan, Point(..))
where
import Data.List
import Data.Ratio
data Point = Point {
x :: Double,
y :: Double
} deriving (Eq, Show)
instance Ord Point where
compare (Point x1 y1) (Point x2 y2) = compare (y1, x1) (y2, x2)
data Vector = Vector {
start :: Point,
end :: Point
} deriving (Eq)
cosine :: Vector -> Double
cosine (Vector (Point x1 y1) (Point x2 y2)) = (x2 - x1) / ((x2 - x1) ^ 2 + (y2
- y1) ^ 2)
instance Ord Vector where
compare a b = compare (f a) (f b) where
f = negate . cosine
sort' :: [Point] -> [Point]
sort' xs = pivot : fmap end sortedVectors where
sortedVectors = sort . fmap (Vector pivot) . delete pivot $ xs
pivot = minimum xs
counterClockwise :: Point -> Point -> Point -> Bool
counterClockwise (Point x1 y1) (Point x2 y2) (Point x3 y3) = (x2 - x1) * (y3 -
y1) > (y2 - y1) * (x3 - x1)
scan :: [Point] -> [Point]
scan (p1 : p2 : p3 : xs)
| counterClockwise p1 p2 p3 = p1 : scan (p2 : p3 : xs)
| otherwise = scan (p1 : p3 : xs)
scan xs = xs
grahamScan :: [Point] -> [Point]
grahamScan = scan . sort' . nub
=== code ===
Best regards,
Zhi-Qiang Lei
[email protected]
------------------------------
Message: 5
Date: Sun, 8 Jan 2012 09:29:51 +0000
From: Henry Lockyer <[email protected]>
Subject: [Haskell-beginners] Guard in class def
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hi,
I was just looking through the 'Monad Transformers' chapter in Real World
Haskell. They are using the "reader" monad to illustrate the transformer
structure but I fell off at the first bend when I saw the following:
class (Monad m) => MonadReader r m | m -> r where
ask :: m r
local :: (r -> r) -> m a -> m a
Could someone explain the use of a guard here / how to read this with the "| m
-> r" ?
I haven't come across this usage before (as far as I have noticed :-) and the
meaning hasn't jumped out at me yet...
Thanks in advance.
Henry
------------------------------
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