Re: [Haskell-cafe] How to make Prelude.read: no parse more verbose ...
g_sauthoff:
> Hi,
>
> I try to debug some existing Haskell-Code. Out of the blue I get a
> 'progname: Prelude.read: no parse'
> error message from GHC.
>
> Great.
>
> Well, the code includes
>
> # grep '\' *| wc -l
> 23 (sic!)
>
> calls to the read fn.
>
> Well, how do I compile a Haskell program in such a way, that I
> get a useful error message from read? I mean, like the
> filename/linenumber of the calling expression for starters.
>
> Really crazy would be the possibility to print out a backtrace,
> if some (library) function calls error ...
ghci now has a debugger that can backtrace like this, see this
blog post for more details.
http://cgi.cse.unsw.edu.au/~dons/blog/2007/11/14#no-exceptions
using reads or even read lifted to a monad, like this, can be
very useful, if it is code you control:
readM :: (Monad m, Read a) => String -> m a
readM s = case [x | (x,t) <- reads s, ("","") <- lex t] of
[x] -> return x
[] -> fail "readM: no parse"
_ -> fail "readM: ambiguous parse"
since you can handle failure without throwing an async exception.
-- Don
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Re: [Haskell-cafe] How to make Prelude.read: no parse more verbose ...
Hello, You can also just use reads which returns a list of (partial) parses. -Jeff [EMAIL PROTECTED] wrote on 12/19/2007 03:17:39 PM: > Hi > > > > Well, how do I compile a Haskell program in such a way, that I > > > get a useful error message from read? I mean, like the > > > filename/linenumber of the calling expression for starters. > > I use the Safe library to do this sort of stuff: > > http://www-users.cs.york.ac.uk/~ndm/safe/ > > You can call readMay to get a maybe result, or readNote which gives an > augmented error message on a crash. You can of course combine this > with the CPP trick: > > #define read readNote (__FILE__++":"++show __LINE__) > > Thanks > > Neil > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to make Prelude.read: no parse more verbose ...
Hi > > Well, how do I compile a Haskell program in such a way, that I > > get a useful error message from read? I mean, like the > > filename/linenumber of the calling expression for starters. I use the Safe library to do this sort of stuff: http://www-users.cs.york.ac.uk/~ndm/safe/ You can call readMay to get a maybe result, or readNote which gives an augmented error message on a crash. You can of course combine this with the CPP trick: #define read readNote (__FILE__++":"++show __LINE__) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to make Prelude.read: no parse more verbose ...
Georg Sauthoff <[EMAIL PROTECTED]> writes:
> Well, how do I compile a Haskell program in such a way, that I
> get a useful error message from read? I mean, like the
> filename/linenumber of the calling expression for starters.
It's dirty, it's mean, but you can use CPP. (On one line, and with
ghc -cpp):
#define read (\s -> case [ x | (x,t) <- reads s, ("","") <- lex t] of
{ [x] -> x ;
[] -> error("read: no parse at "++__FILE__++":"++show __LINE__);
_ -> error("read: no parse at "++__FILE__++":"++show __LINE__)})
-k
--
If I haven't seen further, it is by standing in the footprints of giants
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Re: [Haskell-cafe] How to make Prelude.read: no parse more verbose ...
On Dec 19, 2007, at 11:53 , Georg Sauthoff wrote: I try to debug some existing Haskell-Code. Out of the blue I get a 'progname: Prelude.read: no parse' error message from GHC. If you can install GHC 6.8.x, you can use ghci's interactive debugger. See http://cgi.cse.unsw.edu.au/~dons/blog/2007/11/14 for an example. -- 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 universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to make Prelude.read: no parse more verbose ...
Hi, I try to debug some existing Haskell-Code. Out of the blue I get a 'progname: Prelude.read: no parse' error message from GHC. Great. Well, the code includes # grep '\' *| wc -l 23 (sic!) calls to the read fn. Well, how do I compile a Haskell program in such a way, that I get a useful error message from read? I mean, like the filename/linenumber of the calling expression for starters. Really crazy would be the possibility to print out a backtrace, if some (library) function calls error ... Best regards Georg -- Fortune : 'Real programmers don't comment their code. It was hard to write, it should be hard to understand.' ;) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
