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Today's Topics:
1. do not understand Semantics of Case Expressions in report
(Michael Mekhanoshin)
2. Weird compilation error while doing Euler problems? (Ian Duncan)
3. Re: Weird compilation error while doing Euler problems?
(Brandon S. Allbery KF8NH)
4. Re: Weird compilation error while doing Euler problems?
(Daniel Fischer)
5. Re: do not understand Semantics of Case Expressions in
report (John Dorsey)
6. Re: do not understand Semantics of Case Expressions in
report (Michael Mekhanoshin)
7. what type is 'Val 9' when 'Val Int' a ctor for 'Expr e'?
(Larry Evans)
----------------------------------------------------------------------
Message: 1
Date: Tue, 21 Oct 2008 00:17:56 +0600
From: "Michael Mekhanoshin" <[EMAIL PROTECTED]>
Subject: [Haskell-beginners] do not understand Semantics of Case
Expressions in report
To: Beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset=UTF-8
at http://www.haskell.org/onlinereport/exps.html
on figure Figure 4 (Semantics of Case Expressions, Part 2)
one can see:
(m) case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -
> e' }
= case e' of {
y ->
case v of {
K { f1 = p1 } ->
case v of { K { f2 = p2 , ... } -> e ; _ ->
y };
_ -> y }}
where f1, f2, ... are fields of constructor K; y is a new variable
(In Figures 3.1--3.2: e, e' and ei are expressions; g and gi are
boolean-valued expressions; p and pi are patterns; v, x, and xi are
variables; K and K' are algebraic datatype (data) constructors
(including tuple constructors); and N is a newtype constructor.)
I'm totally confused with this identity. Especialy with " = case e'".
Can anyone explain a little what does this rule stand, please?
It is better to see some example.
Respect,
Michael
------------------------------
Message: 2
Date: Mon, 20 Oct 2008 22:36:03 -0500
From: "Ian Duncan" <[EMAIL PROTECTED]>
Subject: [Haskell-beginners] Weird compilation error while doing Euler
problems?
To: beginners@haskell.org
Message-ID:
<[EMAIL PROTECTED]>
Content-Type: text/plain; charset="iso-8859-1"
'm trying to compile my file that has my Euler problems in it to output the
solution to problem four, but I'm getting a compile error. Here's my .hs
file:
-------------------------
module Euler1 where
import Data.List
import Data.Ord
main = mapM_ putStrLn problem4
problem1 = foldl1' (+) $ nub $ (takeWhile (< 1000) [3,6..] ++ takeWhile (<
1000) [5,10..])
problem2 = sum $ takeWhile (<= 4000000) [x | x <- fibs, even x]
where fibs = unfoldr (\(a,b) -> Just (a,(b,a+b))) (0,1)
problem3 z = maximumBy compare (filter (\x -> z `mod` x == 0) (takeWhile (<=
ceiling (sqrt (fromIntegral z))) primes))
problem4 = nub [ show $ y * z | y <- [100..999], z <- [100..999], show (y*z)
== reverse (show $ y*z)]
prime p = p `elem` primes
primes = small ++ large
where
1:p:candidates = roll $ mkWheel small
small = [2,3,5,7]
large = p : filter isPrime candidates
isPrime n = all (not . divides n) $ takeWhile (\p -> p*p <= n) large
divides n p = n `mod` p == 0
mkWheel ds = foldl nextSize w0 ds
nextSize (Wheel n rs) p =
Wheel (p*n) [r' | k <- [0..(p-1)], r <- rs, let r' = n*k+r, r' `mod` p /=
0]
w0 = Wheel 1 [1]
roll (Wheel n rs) = [n*k+r | k <- [0..], r <- rs]
data Wheel = Wheel Integer [Integer]
------------------------
Here's my output when I try to compile:
ian$ ghc ~/Documents/eulerProblem1.hs -o test
Undefined symbols:
"___stginit_ZCMain", referenced from:
___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o)
"_ZCMain_main_closure", referenced from:
_ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o)
ld: symbol(s) not found
collect2: ld returned 1 exit status
What's going on? I'm running GHC 6.8.3, if that helps.
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Message: 3
Date: Mon, 20 Oct 2008 23:46:50 -0400
From: "Brandon S. Allbery KF8NH" <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] Weird compilation error while doing
Euler problems?
To: "Ian Duncan" <[EMAIL PROTECTED]>
Cc: beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset="us-ascii"
On 2008 Oct 20, at 23:36, Ian Duncan wrote:
> 'm trying to compile my file that has my Euler problems in it to
> output the solution to problem four, but I'm getting a compile
> error. Here's my .hs file:
> -------------------------
> module Euler1 where
If you're not using the standard module Main, you need to tell ghc
which module you put your main function in:
ghc --main-is Euler1 ...
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon university KF8NH
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------------------------------
Message: 4
Date: Tue, 21 Oct 2008 05:55:41 +0200
From: Daniel Fischer <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] Weird compilation error while doing
Euler problems?
To: "Ian Duncan" <[EMAIL PROTECTED]>, beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset="iso-8859-1"
Am Dienstag, 21. Oktober 2008 05:36 schrieb Ian Duncan:
> 'm trying to compile my file that has my Euler problems in it to output the
> solution to problem four, but I'm getting a compile error. Here's my .hs
> file:
> -------------------------
> module Euler1 where
> import Data.List
> import Data.Ord
>
> main = mapM_ putStrLn problem4
>
> problem1 = foldl1' (+) $ nub $ (takeWhile (< 1000) [3,6..] ++ takeWhile (<
> 1000) [5,10..])
> problem2 = sum $ takeWhile (<= 4000000) [x | x <- fibs, even x]
> where fibs = unfoldr (\(a,b) -> Just (a,(b,a+b))) (0,1)
>
> problem3 z = maximumBy compare (filter (\x -> z `mod` x == 0) (takeWhile
> (<= ceiling (sqrt (fromIntegral z))) primes))
>
> problem4 = nub [ show $ y * z | y <- [100..999], z <- [100..999], show
> (y*z) == reverse (show $ y*z)]
>
> prime p = p `elem` primes
>
> primes = small ++ large
> where
> 1:p:candidates = roll $ mkWheel small
> small = [2,3,5,7]
> large = p : filter isPrime candidates
> isPrime n = all (not . divides n) $ takeWhile (\p -> p*p <= n) large
> divides n p = n `mod` p == 0
> mkWheel ds = foldl nextSize w0 ds
> nextSize (Wheel n rs) p =
> Wheel (p*n) [r' | k <- [0..(p-1)], r <- rs, let r' = n*k+r, r' `mod` p /=
> 0]
> w0 = Wheel 1 [1]
> roll (Wheel n rs) = [n*k+r | k <- [0..], r <- rs]
> data Wheel = Wheel Integer [Integer]
> ------------------------
>
> Here's my output when I try to compile:
>
> ian$ ghc ~/Documents/eulerProblem1.hs -o test
> Undefined symbols:
> "___stginit_ZCMain", referenced from:
> ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o)
> "_ZCMain_main_closure", referenced from:
> _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o)
> ld: symbol(s) not found
> collect2: ld returned 1 exit status
>
> What's going on? I'm running GHC 6.8.3, if that helps.
Your module isn't named Main, so to produce an executable you must pass the
flag
-main-is ModuleName
on the command line.
Another thing, it is recommendable to develop the habit of passing the --make
flag to ghc.
------------------------------
Message: 5
Date: Tue, 21 Oct 2008 12:30:47 -0400
From: John Dorsey <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] do not understand Semantics of Case
Expressions in report
To: Beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset=us-ascii
Michael,
(Sorry for duplicates; I sent this from the wrong address initially.)
> (m) case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ ->
> e' }
> = case e' of {
> y ->
> case v of {
> K { f1 = p1 } ->
> case v of { K { f2 = p2 , ... } -> e ; _ -> y };
> _ -> y }}
> where f1, f2, ... are fields of constructor K; y is a new variable
[...]
> I'm totally confused with this identity. Especialy with " = case e'".
> Can anyone explain a little what does this rule stand, please?
> It is better to see some example.
You got me curious, so I looked over this. The table you reference
lists identities that should hold for all implementations of case
expressions.
(m) is about how case matches against constructors with named fields,
and it shows that a case expression that matches against multiple
fields (f1, f2, etc.) can be broken into nested case expressions which
each match a single field (eg. f1). It looks like
"case e' of { y -> ..."
was introduced solely to give e' a name that can be used in two places,
since the nested form may fail to match at each level. It seems to me
that without this, you could get code explosion from duplicating the
expression e' below; with this you get sharing.
Here it is with a interim step added:
-- Match v against a constructor with multiple fields
case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> e' }
-- Same, but introduce y to represent e' without loss of sharing
case e' of { y ->
case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> y }}
-- Same, but match first field of K independent of the rest
case e' of { y ->
case v of { K { f1 = p1 } ->
case v of { K { f2 = p2 , ... } -> e ; _ -> y };
_ -> y }}
I hope this helps.
Regards,
John
------------------------------
Message: 6
Date: Tue, 21 Oct 2008 23:35:46 +0600
From: "Michael Mekhanoshin" <[EMAIL PROTECTED]>
Subject: Re: [Haskell-beginners] do not understand Semantics of Case
Expressions in report
To: Beginners@haskell.org
Message-ID:
<[EMAIL PROTECTED]>
Content-Type: text/plain; charset=UTF-8
2008/10/21 John Dorsey <[EMAIL PROTECTED]>:
> It looks like
>
> "case e' of { y -> ..."
>
> was introduced solely to give e' a name that can be used in two places,
> since the nested form may fail to match at each level. It seems to me
> that without this, you could get code explosion from duplicating the
> expression e' below; with this you get sharing.
...
>
> -- Match v against a constructor with multiple fields
> case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> e' }
>
> -- Same, but introduce y to represent e' without loss of sharing
> case e' of { y ->
> case v of { K { f1 = p1 , f2 = p2 , ... } -> e ; _ -> y }}
>
> I hope this helps.
>
Surely it does.
Most tricky part here (for me) is that e' goes in outer scope. It
works here like in rule (c) . I got it. Thank you for your help.
Regards,
Michael
------------------------------
Message: 7
Date: Tue, 21 Oct 2008 13:25:53 -0500
From: Larry Evans <[EMAIL PROTECTED]>
Subject: [Haskell-beginners] what type is 'Val 9' when 'Val Int' a
ctor for 'Expr e'?
To: Beginners@haskell.org
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset="iso-8859-1"
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