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Today's Topics:
1. Logging in Haskell (Sergey Mikhanov)
2. Re: Logging in Haskell (Brent Yorgey)
3. What is the Haskell idiom for "if then else if then"
(Peter Hickman)
4. Re: What is the Haskell idiom for "if then else if then"
(Daniel Fischer)
5. can run in ghci but can't compile. (Anand Mitra)
6. Re: can run in ghci but can't compile. (Brandon S. Allbery KF8NH)
7. Re: can run in ghci but can't compile. (Anand Mitra)
----------------------------------------------------------------------
Message: 1
Date: Mon, 30 Mar 2009 14:01:55 +0200
From: Sergey Mikhanov <[email protected]>
Subject: [Haskell-beginners] Logging in Haskell
To: "[email protected]" <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii; format=flowed;
delsp=yes
Hi community,
As my Haskell programs grow above 100 lines of code, I feel the
necessity to trace the execution of them more closely. In imperative
languages one may use logging facilities for checking intermediary
values of variables, the execution flow, etc. How this usually handled
in Haskell (I bet that despite the fact that in Haskell it is
relatively easier to write bug-free and checked code, the industrial
systems still need some logging being done)?
Regards,
Sergey
------------------------------
Message: 2
Date: Mon, 30 Mar 2009 08:52:19 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Logging in Haskell
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Mon, Mar 30, 2009 at 02:01:55PM +0200, Sergey Mikhanov wrote:
> Hi community,
>
> As my Haskell programs grow above 100 lines of code, I feel the necessity
> to trace the execution of them more closely. In imperative languages one
> may use logging facilities for checking intermediary values of variables,
> the execution flow, etc. How this usually handled in Haskell (I bet that
> despite the fact that in Haskell it is relatively easier to write bug-free
> and checked code, the industrial systems still need some logging being
> done)?
It depends what you want to do. If you just want to print out some
intermediate values to do a little debugging, try the 'trace' function
from Debug.Trace. If you actually want an industrial-strength logging
system, there's the hslogger library:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslogger
Generally collecting some information while a computation progresses
can be done with the Writer monad, from Control.Monad.Writer.
-Brent
------------------------------
Message: 3
Date: Mon, 30 Mar 2009 16:33:02 +0100
From: Peter Hickman <[email protected]>
Subject: [Haskell-beginners] What is the Haskell idiom for "if then
else if then"
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
I've just written this piece of code
if r < 0
then LEFT
else
if r > 0
then RIGHT
else STRAIGHT
which works just fine but it does look rather clunky (as it would in
any language I suppose). Is there a Haskell idiom for this type of code?
------------------------------
Message: 4
Date: Mon, 30 Mar 2009 17:50:20 +0200
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] What is the Haskell idiom for "if
then else if then"
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Am Montag 30 März 2009 17:33:02 schrieb Peter Hickman:
> I've just written this piece of code
>
> if r < 0
> then LEFT
> else
> if r > 0
> then RIGHT
> else STRAIGHT
>
> which works just fine but it does look rather clunky (as it would in
> any language I suppose). Is there a Haskell idiom for this type of code?
Usually, people use guards:
function r
| r < 0 = LEFT
| r > 0 = RIGHT
| otherwise = STRAIGHT
Another option would be a case:
case compare r 0 of
LT -> LEFT
GT -> RIGHT
EQ -> STRAIGHT
You can also have a case with guards:
case expression of
pattern1 | condition1.1 -> result1.1
| condition1.2 -> result1.2
pattern2 | condition2.1 -> result2.1
pattern3 -> result3
_ -> defaultresult
------------------------------
Message: 5
Date: Mon, 30 Mar 2009 22:42:21 +0530
From: Anand Mitra <[email protected]>
Subject: [Haskell-beginners] can run in ghci but can't compile.
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hi,
I can run the following program in the interpreter ghci but can't compile
it.
The program I am trying to compile is a simple copy using the lazy
bytestrings. I have been working out exercises from Real World haskell and I
was benchmarking this version which is supposed to be optimum.
,----
| import qualified System as S
| import qualified Data.ByteString.Lazy as L
|
| main = do
| [from, to ] <- S.getArgs
| file <- L.readFile from
| L.writeFile to file
`----
While trying to compile this i get the following errors.
,----
| mi...@ravan:~/laptop/haskell/learn$ /opt/ghc-6.10.1/bin/ghc cp.hs
| cp.o: In function `sB1_info':
| (.text+0xab): undefined reference to
`bytestringzm0zi9zi1zi4_DataziByteStringziLazzy_writeFile_closure'
| cp.o: In function `sAW_info':
| (.text+0xe1): undefined reference to
`bytestringzm0zi9zi1zi4_DataziByteStringziLazzy_readFile_closure'
| cp.o: In function `sB5_info':
| (.text+0x30f): undefined reference to
`__stginit_bytestringzm0zi9zi1zi4_DataziByteStringziLazzy_'
| cp.o: In function `sB5_srt':
| (.data+0x4): undefined reference to
`bytestringzm0zi9zi1zi4_DataziByteStringziLazzy_readFile_closure'
| cp.o: In function `sB5_srt':
| (.data+0x8): undefined reference to
`bytestringzm0zi9zi1zi4_DataziByteStringziLazzy_writeFile_closure'
| collect2: ld returned 1 exit status
| mi...@ravan:~/laptop/haskell/learn$ rm cp.o
| mi...@ravan:~/laptop/haskell/learn$ /opt/ghc-6.10.1/bin/ghc cp.hs
| cp.o: In function `sB0_info':
| (.text+0xab): undefined reference to
`bytestringzm0zi9zi1zi4_DataziByteStringziLazzy_writeFile_closure'
| cp.o: In function `sAV_info':
`----
But this program is not syntactically or otherwise flawed since I can run it
under ghci.
,----
| mi...@ravan:~/laptop/haskell/learn$ /opt/ghc-6.10.1/bin/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> :l cp.hs
| Ok, modules loaded: Main.
| Prelude Main> :main /etc/passwd /tmp/xyz
| Loading package bytestring-0.9.1.4 ... linking ... done.
| Loading package unix-2.3.1.0 ... linking ... done.
| Loading package filepath-1.1.0.1 ... linking ... done.
| Loading package old-locale-1.0.0.1 ... linking ... done.
| Loading package old-time-1.0.0.1 ... linking ... done.
| Loading package directory-1.0.0.2 ... linking ... done.
| Loading package process-1.0.1.0 ... linking ... done.
| Loading package syb ... linking ... done.
| Loading package array-0.2.0.0 ... linking ... done.
| Loading package random-1.0.0.1 ... linking ... done.
| Loading package haskell98 ... linking ... done.
| Prelude Main>
`----
Since I suspect that this might be a problem with the way I have installed
gch-6.10.1 I will explain how I have installed it.
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Message: 6
Date: Mon, 30 Mar 2009 13:15:50 -0400
From: "Brandon S. Allbery KF8NH" <[email protected]>
Subject: Re: [Haskell-beginners] can run in ghci but can't compile.
To: Anand Mitra <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
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Message: 7
Date: Mon, 30 Mar 2009 23:24:28 +0530
From: Anand Mitra <[email protected]>
Subject: Re: [Haskell-beginners] can run in ghci but can't compile.
To: "Brandon S. Allbery KF8NH" <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
That does the job, Thanks !!
Just to understand what is happening under the hood I tried to resolve this
to the exact steps. The problem I faced seems to be equivalent of missing a
-L and -l in gcc. Looking at the man for ghc I did find it had the -l and -L
flags. Now I had to find the path and the name of the library. Did a find
over the location I have installed ghc I was able to short list the likely
candidates.
,----
| find /opt/ghc-6.10.1/ -type f |grep \\.a$
| ....
|
/opt/ghc-6.10.1/lib/ghc-6.10.1/bytestring-0.9.1.4/libHSbytestring-0.9.1.4.a
|
/opt/ghc-6.10.1/lib/ghc-6.10.1/bytestring-0.9.1.4/libHSbytestring-0.9.1.4_p.a
| ....
`----
and as expected adding the -l and -L in the ghc command line does the trick.
/opt/ghc-6.10.1/bin/ghc -L/opt/ghc-6.10.1/lib/ghc-6.10.1/bytestring-0.9.1.4/
-lHSbytestring-0.9.1.4 cp.hs
regards
--
Anand Mitra
On Mon, Mar 30, 2009 at 10:45 PM, Brandon S. Allbery KF8NH <
[email protected]> wrote:
> On 2009 Mar 30, at 13:12, Anand Mitra wrote:
>
> ,----
> | mi...@ravan:~/laptop/haskell/learn$ /opt/ghc-6.10.1/bin/ghc cp.hs
> | cp.o: In function `sB1_info':
> | (.text+0xab): undefined reference to
> `bytestringzm0zi9zi1zi4_DataziByteStringziLazzy_writeFile_closure'
> | cp.o: In function `sAW_info':
>
>
> Use "ghc --make". ghci does this automatically.
>
> --
> 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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