[Haskell-cafe] local loops
I often use local loops in monadic code, e.g. main = do ... let loop = do ... if cond then loop else return () loop It seems that I can encode this idiom slightly more concise with the 'fix' operator (from Control.Monad.Fix), i.e. main = do ... fix (\loop - do ... if ...) But is it really semantically equivalent? Is it as efficient? Per ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell] Implicit parameters
When using implicit parameters I have noticed (at least for me) a rather
puzzling behaviour with GHC and Hugs.
Given the declarations
data Env = Env {numlines :: Int, numcols :: Int}
initEnv = Env {numlines = 0, numcols = 1}
withEnv :: ((?env :: Env) = IO a) - IO a
withEnv io = let ?env = initEnv in io
I can write code like:
main = withEnv (do
let lines = numlines ?env
putStrLn (Initial number of lines is ++ (show lines)))
which works as expected, but the version below
main = withEnv $ do
let lines = numlines ?env
putStrLn (Initial number of lines is ++ (show lines))
is not accepted by either GHC or Hugs! Is this a bug or have stumbled into a
context where it actually matters if you use '$' or explicit grouping with
parentheses?
Per Larsson
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[Haskell-cafe] GHC and libc
When I compile my haskell program (on a linux machine) with GHC and send it to a colleague he can't run it because he has a somewhat older version of libc. Is there a GHC switch that I have missed that enables you to statically link the parts of libc that is used by the haskell program? Alternatively have someone other tips for solving this problem? (I'm reluctant to force all my colleagues to install GHC, because of the size and long installation time, and I can't let them use Hugs because I've used some GHC-only features in my program.) Per Larsson ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC and libc
Thanks Duncan and Sven for your helpful answers. Per Larsson P.S Now everything seems to work, except that I get the compiler message: /usr/local/lib/ghc-6.2/libHSunix.a(User__17.o)(.text+0x160): In function 'SystemziPosixziUser_getUserEntryForName_entry': : Using 'getpwnam_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking This seems to indicate that there are a few functions (probably in the Posix package) which can't be can't be statically linked? ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
In my humble opionon explicit module prefixes are a feature, which enhance code clarity, and not something you want get rid of using rather complex namespace extensions. However, as Alastair Reid's mail in this thread indicates there are weaknesses in haskell's export mechanism. But these would be far more easy to fix than introducing the suggested per type namespace extension. The present export list is also rather ugly from an aesthetic point of view, clottering the module header. I would love to see an, repeatable, explicit 'export' directive with similar keywords as the import directive. Allowing for example: 'export [all] hiding (...)' with obvious semantics. Per Larsson ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] regular expression syntax - perl ain't got nothin on haskell
On Tuesday 24 February 2004 03.07, John Meacham wrote:
Inspired by an idea by Andrew Pang and an old project of mine, I decided
to fill out a reusable regular expression library which is similar to
Perl's, but much more expressive.
...
Hi,
Thanks! I am grateful of your efforts because I have since long missed some
typical text processing functionality in haskell. (Besides a more complete
regular expression library I think many haskellers miss string constructors
like an 'official' version of printf/format and maybe also some sort of
'here documents'.) Below follows some of my thoughts regarding a complete
regex library, in the hope that this will be of any inspiration.
1. Replacement
A regex library must contain functions for replacement with regular
expressions. One could think this is trivial to implement given a match
function, but there are some tricky choices to be made regarding empty
matches (this also applies to splitting a string into fields with a regexp).
Also there is questions about the interface of these functions.
In my own Text.Regex wrapper I have the functions.
data Match = Match {before :: String, after :: String, groups :: [String]}
...
substWithPat :: Rexex - String - (Int - Bool) - String - (String,Int)
substWithFun :: Regex - (Match-String) - (Int-Bool) -
String - (String,Int)
substWithFunM :: (Monad m) = Regex - (Match - m String) -
(Int-Bool) - String - m (String,Int)
Where a call to 'substWithPat pat rpat mode str' replaces matches
of 'pat' in 'str' by 'rpat' and returns the resulting string and the number of
replacements done. The 'rpat' replace pattern can contain backreferences on
the form \m where \m refers to the mth subgroup in the corresponding match
(\0 refers to the entire match). The call replaces only 'replaceable
matches'. A match m is replaceable if its the nth match and (mode n) is true
and, m is either the first match, a proper match or an empty match succeding
an empty match. This schema gives results which are conformant with replace
functionality in several other regex libraries, e.g. in Tcl, Python and Perl.
For example, replacing matches of _* by _ in awk gives _a_w_k_, and
replacing matches of _* by _ in sed_and_awk gives _s_e_d_a_n_d_a_w_k.
(Compare the discussion in 'Mastering Regular Expressions', O'Reilly, pages
187-188.) The functions substWithFun and substWithFunM are obvious variations
on the substWithPat function.
2. Constructing regular expressions.
There is the well known problem that the backslash is used both as a string
escape character and a regexp operator. I know of three approaches to the
problem:
a) Bite the bullet and, e.g. write regexps like in order to match a
single backslash (e.g. as in emacs lisp).
b) Use a language extensions for 'raw' strings where the backslash is not
interpreted (e.g. /regex/ in awk, rregex in python and {regex} in Tcl).
c) Use a different operator than the backslash in regular expressions, this
has the benefit of not demanding a language extension, but is nonstandard on
the negative side.
There is also the problem with inserting string values in regular expressions.
Appending with ++ is not particular convenient with complicated regexps
because the result can be rather unreadable. I suppose we have to wait for a
standard implementation of printf in template haskell for this problem.
Cheers
Per
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Re: [Haskell] regular expression syntax - perl ain't got nothin on haskell
On Tuesday 24 February 2004 15.25, Johannes Waldmann wrote: Per Larsson wrote: .. I have since long missed some typical text processing functionality in haskell. it is often the case that people process text only because they have no better (structured and typed) way of representing their data... I agree with your point in the general case. For serious parsing and pretty printing tasks there are great haskell libraries like Parsec and PrettyPrint. But I can't see why the haskell user shouldn't also have access to concise text processing notations, e.g. regular expressions and printf, to be used in, e.g. short script-like programs? Cheers Per ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
[Haskell-cafe] Template Haskell question?
I have written a small TH application (module THRecord) which creates update
functions for records. It is intended to be used like this:
import THRecord
data Record = R {...
...
$(THRecord.generateRecordModifiers (reifyDecl Record))
It works great, but only if the splice is placed last in the importing module!
If I place it in the middle of the module it seems to hide subsequent
declarations and I get 'Variable not in scope ...' compiler errors. If I
place it at the top of the module I get:
ghc-6.2: panic! (the `impossible' happened, GHC version 6.2):
nameModule Record {- tc 01D -}
Has anybody a clue of what is happening here?
-- Per
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Re: FFI preprocessor for GHC
Hello, I would instead suggest greencard as a good preprocessor. It is portable in the sense that it outputs haskell code in accordance with the new FFI specification. Also, it has an input format which, in my opinion, is easier to learn and is better documented than hsc2hs. You can download the program and manual at: http://haskell.org/greencard/ Per Larsson On Thursday 15 January 2004 18.40, Jeremy Shaw wrote: Hello, One answer might be to not use a preprocessor at all. I found that I could not understand what the preprocessers where actually doing until I understood how to do it by hand. The nice thing about doing it by hand, is it is probably the most portable, since it does not require any preprocessor. On the otherhand, there is nothing wrong with using a preprocessor, and it can certainly save time. hsc2hs is included in the ghc source tree, so that would probably be my first choice. Jeremy Shaw. At Thu, 15 Jan 2004 18:18:11 +0100, Gour wrote: Hi! I'd like to create Haskell bindings for swisseph C library for calculating ephemeris. Which preprocessor would be a suitable for ghc compiler with the ability to run the code both on Linux Win32? Sincerely, Gour -- Gour [EMAIL PROTECTED] Registered Linux User #278493 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: getting the path to the executing program
On Thursday 08 January 2004 18.06, Hal Daume III wrote: is there a function, related to getProgName, which returns the (absolute) path to the current program? basically, i want to be able to read a file which i know will be in the same directory as the current program, but not necessarily in the same directory that we're running it from. I don't know how to do this in a portable way, but on unix/linux you can use the standard trick of reading the symbolic link /proc/process-id/exe . For example: import System.Posix.Files (readSymbolicLink) import System.Posix.Process (getProcessID) main = do pid - getProcessID path - readSymbolicLink (/proc/ ++ (show pid) ++ /exe) putStrLn (Absolute path of this program is ++ path) PS. This doesn't work with interpreted code (Hugs, GHCi) in which case you get the path to the interpreter itself. Cheers Per ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Haddock and man pages
Hi, When you have a good understanding of a programming library and only need to quickly refresh your memory regarding the type signature of a specific function, etc., I find man pages very convenient. Are there any plans of adding this as an alternative output format in Haddock? If not, maybe I will try to write a patch when I find some spare time, but it would be encouraging to know if this would interest anyone else but myself? Cheers Per ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: pretty newby
On Tuesday 23 September 2003 16.05, Luc Taesch wrote:
are there any facility to pretty print an haskell program ?
im aware of HPJ combinators library, but i was looking for a command line
utility, rather.. am i missing an entry in HPJ ?
thanks
Luc
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Hi,
In GHC (and HUGS?) you can use the 'haskell-src' package which contains
functions for parsing and pretty-printing haskell code. Using these, it only
takes a couple of lines to make your own command line, pretty-printer for
haskell code. If you want to, you can use my implementation which I attach to
this mail. The problem with the parser, however, is that it doesn't handle
comments at all.
Regards
Per-- FILE: HsIndent.hs
-- AUTH: Per Larsson
-- DATE: 03/10/2003
-- CODE: Haskell Code
module Main where
import System.Environment
import System.Exit
import System.IO
import Control.Monad
import Language.Haskell.Parser
import Language.Haskell.Pretty
import System.Console.GetOpt
header = hsindent [OPTION ...] FILE
version = hsindent 1.0
usage = usageInfo header options
data Config = Config {showHelp, showVersion :: Bool, pmode :: PPHsMode}
defaultConfig = Config False False defaultStyle
defaultStyle :: PPHsMode
defaultStyle = PPHsMode {
classIndent = 8,
doIndent = 3,
caseIndent = 5,
letIndent = 4,
whereIndent = 6,
onsideIndent = 2,
spacing = True,
layout = PPOffsideRule,
linePragmas = False,
comments = True
}
options :: [OptDescr (Config - Config)]
options =
[ opt 'h' help print this help information and exit
(\c - c {showHelp = True})
, opt 'v' version print version information and exit
(\c - c {showVersion = True})
, opt 'u' nospacing don't insert blank lines
(\c - c {pmode = (pmode c) {spacing = False}})
, opt 'p' pragmas insert source pragmas
(\c - c {pmode = (pmode c) {linePragmas = True}})
, opt 'e' comments keep comments
(\c - c {pmode = (pmode c) {comments = True}})
, arg 's' class N indent class declarations N columns
(\s c - c {pmode = (pmode c) {classIndent = read s}})
, arg 'd' do N indent do expressions N columns
(\s c - c {pmode = (pmode c) {doIndent = read s}})
, arg 'w' where N indent where expressions N columns
(\s c - c {pmode = (pmode c) {whereIndent = read s}})
, arg 'l' let N indent let expressions N columns
(\s c - c {pmode = (pmode c) {letIndent = read s}})
, arg 'c' case N indent case expressions N columns
(\s c - c {pmode = (pmode c) {caseIndent = read s}})
, arg 'o' onside N indent at line continuations N columns
(\s c - c {pmode = (pmode c) {onsideIndent = read s}})
, arg 'y' layout ARG set layout style to ARG, one of\n\
\ 'OffsideRule', 'SemiColon', 'Inline' or 'NoLayout'
(\s c - c {pmode = (pmode c) {layout = toLayout s}})
]
where
opt short long msg update =
Option [short] [long] (NoArg update) msg
arg short long argdescr msg update =
Option [short] [long] (ReqArg update argdescr) msg
toLayout :: String - PPLayout
toLayout OffsideRule = PPOffsideRule
toLayout SemiColon = PPSemiColon
toLayout InLine = PPInLine
toLayout NoLayout= PPNoLayout
toLayout _ = error toLayout
--
main = do
args - getArgs
(conf,files) - case getOpt Permute options args of
(o,n,[]) - return (foldr ($) defaultConfig o, n)
(_,_,errs) - error (concat errs ++ usageInfo header options)
when (showHelp conf) (exitSuccess usage)
when (showVersion conf) (exitSuccess version)
unless (length files == 1) (exitFail usage)
file - return (head files)
h - openFile file ReadMode
s - hGetContents h
result - return (parseModuleWithMode (ParseMode file) s)
case result of
ParseOk hsModule -
exitSuccess (prettyPrintWithMode (pmode conf) hsModule)
ParseFailed pos msg -
exitFail (Parse Error at: ++ show pos ++ \n ++ show msg)
where
exitSuccess msg = (putStrLn msg exitWith ExitSuccess)
exitFail msg = (putStrLn msg exitFailure)
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Re: Library documentation
On Friday 05 September 2003 16.35, Konrad Hinsen wrote: Is there any place where I can find an introductory documentation for the Haskell libraries? To be more concrete, say I am interested in using Data.Array.ST.STUArray. I would need some explanation of the ST monad, and then an explanation of how I can use an STUArray in an ST monad. Plus ideally examples. But the same applies to most of the libraries. I can't find any documentation other than the very terse reference or the parts that happen to be covered by textbooks. Konrad. I have the same problem and as far as I know there is only haddock generated html-documentation from sparse source comments available for many parts of the library. But for some of the modules you can get exactly the information you are asking for, e.g. the Parsec module has its own readable documentation at http://www.cs.uu.nl/~daan/parsec.html and the Control.Monad module has recently been superbly documented by John Newbern at http://www.nomaware.com/monads/html/. Best Regards Per ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Alex 2.0
On Friday 22 August 2003 13.04, Immanuel Litzroth wrote:
Has anyone an example of reg{n,k} regular expression in Alex 2.0?
I can not get them to work, alex won't parse them (not the way I write
them anyway)
Immanuel
Hi,
Here comes a silly example which in any case
works as expected on my linux system with alex and ghc.
file foo.x contains:
{
module Main (main) where
}
%wrapper basic
words :-
$white+ ;
A{3,5} { \s - () }
{
main = do
s - getContents
print (length (alexScanTokens s))
}
---
% alex foo.x
% ghc --make foo.hs
% a.out testfile.txt
Regards Per
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Debugging
I need to learn to use a debugging tool for haskell. There seems to be a whole bunch of them (Hood, Freja, Hat, Buddha, ...). Can anyone give me a (subjective) recommendation. Thanks Per ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
source layout
Hi all, I'm looking for some kind of style-guide for layout and indentation of haskell source code, can anyone help me? Thanks, Per ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Happy Alex
I have successfully used the the excellent haskell tools Happy and Alex in a couple of parsing projects, but I have failed when trying to combine a monadic Happy grammar (using the %monad and %lexer directives) together with an Alex generated okenizer, nor are there any such examples in the Happy and Alex distributions. Have anyone tried this combination and can give me some advice or a simple example? -- Per ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
