Re: [Haskell-cafe] desactivate my Show instance implementations temporarily
On 23 Apr 2012, at 17:49, Evan Laforge wrote: I use a custom Pretty class along with HughesPJ, ala ghc's Outputable. It means I can omit data or print it out in a more readable form (even just rounding floats to %.03f can help a lot), and also get nice layout and wrapping. I think I do something similar, although not hooking it up with a proper pretty printing library (that manages indentation etc), but which just stupid builds strings à la Show. http://hackage.haskell.org/packages/archive/GenI/latest/doc/html/NLP-GenI-Pretty.html I wish I knew about a Commonly Agreed Idiom (or library) for just recursively creating human-friendly text, but I suspect I only say this because I don't have a clear picture what I'm really wishing for. Does everybody just use HughesPJ? I'll also mention that it took me a while to untangle all the various notions of showing things in my code. Now it's * Show: Haskell * GraphvizShow: dot * GeniShow: outputting structures in my custom text format * Pretty: for talking to humans Embarrassing to say that until recently my code was a random mismash of uses and unclear thinking. For example, it hadn't occurred to me that I shouldn't define GeniShow in terms of other things (but vice versa) because I don't want to accidentally change my file format just because I was trying to make something prettier. Oops. -- Eric Kow http://erickow.com signature.asc Description: Message signed with OpenPGP using GPGMail ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] desactivate my Show instance implementations temporarily
I use a custom Pretty class along with HughesPJ, ala ghc's Outputable. It means I can omit data or print it out in a more readable form (even just rounding floats to %.03f can help a lot), and also get nice layout and wrapping. The downside is a certain amount of boilerplate to write output routines for records. Actually, types with multiple constructors are the most annoying. At some point I should investigate the new generic deriving feature to see if some of that can be automated. As a user of libraries, I get annoyed when they implement Show to something nonstandard. It makes it harder to figure out how the library works. On Sun, Apr 22, 2012 at 4:20 AM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: On 22 April 2012 19:55, TP paratribulati...@free.fr wrote: On Sunday 22 April 2012 19:37:19 Ivan Lazar Miljenovic wrote: Is there any particular reason you're *not* using the defaults? This is a good question which I have asked myself. I have searched about the topic, and found that: http://blog.romanandreg.com/post/13545420287/haskells-show-and-pretty- printing-bad-practice So, according to this address, Show implementation should be used with Read so as to have show.read equal to identity: this is the only good practice requirement. In my case, I use Show to print mathematical expressions, but it is not strictly pretty printing (not over several lines as in classical Computer Algebra Sytems). Why not using my own Show implementation to do that? For exactly the same reason you're discovering: Show/Read exist for debugging. Show and Read are meant to produce/read valid Haskell code (modulo [qualified] imports) so that you can work out what code is going wrong (and coincidentally used as a quick`n`dirty serialisation method). The term pretty-printing is meant in regards to producing _human-readable_ versions of your data (though the pretty-printing libraries can also be used to produce code formatted for some other tool to parse, etc.). Show/Read happen to be auto-derivable classes that implement one such form of pretty-printing (i.e. printing values that look like the actual source code that represents them). Let me provide you with a personal anecdote: when I took over maintaining the graphviz library, it was still using the Show class for printing (but a proper parser library for parsing), and it was working with the limited functionality it had. However, whenever I tried to do something new, I found problems: * Existing Show instances meant that it was very difficult to extend what the library could represent and then print properly. * As you've found, it can then be a PITA to debug because you have no idea what the internal values actually are. In the end, I eventually wrote a custom pretty-printing class (the existing pretty-printing classes had instances that didn't suit, just like Show) and it's worked a lot better since. The only time it's valid to override the default Show/Read instances is when the constructors aren't exported (e.g. Data.Map), but even then it should be valid Haskell (if you ignore imports, etc.). So leave Show/Read as they are, and write a custom function[s] that does the actual pretty-printing you want. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com http://IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] desactivate my Show instance implementations temporarily
Hello, I have a module where I have made several types as instances of the Show typeclass. For debugging purpose, I would like to use the default implementation for Show (the one obtained when using deriving, which shows all the constructors). Is there some option to do that, or have I to comment all the Show instances of my code, and add Show in deriving (...) for each of my types? If this is the only possibility, is there some script around here to do that automatically? Thanks in advance, TP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] desactivate my Show instance implementations temporarily
On Sun, Apr 22, 2012 at 11:11:51AM +0200, TP wrote: Hello, I have a module where I have made several types as instances of the Show typeclass. For debugging purpose, I would like to use the default implementation for Show (the one obtained when using deriving, which shows all the constructors). Is there some option to do that, or have I to comment all the Show instances of my code, and add Show in deriving (...) for each of my types? If this is the only possibility, is there some script around here to do that automatically? Thanks in advance, TP You could use some preprocessor's #if/#else/#endif pragmas. See here: http://stackoverflow.com/questions/6556778/using-if-else-endif-in-haskell L. -- Lorenzo Bolla http://lbolla.info pgppqbachV6j0.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] desactivate my Show instance implementations temporarily
On 22 April 2012 19:11, TP paratribulati...@free.fr wrote: Hello, I have a module where I have made several types as instances of the Show typeclass. For debugging purpose, I would like to use the default implementation for Show (the one obtained when using deriving, which shows all the constructors). Is there some option to do that, or have I to comment all the Show instances of my code, and add Show in deriving (...) for each of my types? If this is the only possibility, is there some script around here to do that automatically? Is there any particular reason you're *not* using the defaults? Thanks in advance, TP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com http://IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] desactivate my Show instance implementations temporarily
On Sunday 22 April 2012 19:37:19 Ivan Lazar Miljenovic wrote: Is there any particular reason you're *not* using the defaults? This is a good question which I have asked myself. I have searched about the topic, and found that: http://blog.romanandreg.com/post/13545420287/haskells-show-and-pretty- printing-bad-practice So, according to this address, Show implementation should be used with Read so as to have show.read equal to identity: this is the only good practice requirement. In my case, I use Show to print mathematical expressions, but it is not strictly pretty printing (not over several lines as in classical Computer Algebra Sytems). Why not using my own Show implementation to do that? TP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] desactivate my Show instance implementations temporarily
On 22 April 2012 19:55, TP paratribulati...@free.fr wrote: On Sunday 22 April 2012 19:37:19 Ivan Lazar Miljenovic wrote: Is there any particular reason you're *not* using the defaults? This is a good question which I have asked myself. I have searched about the topic, and found that: http://blog.romanandreg.com/post/13545420287/haskells-show-and-pretty- printing-bad-practice So, according to this address, Show implementation should be used with Read so as to have show.read equal to identity: this is the only good practice requirement. In my case, I use Show to print mathematical expressions, but it is not strictly pretty printing (not over several lines as in classical Computer Algebra Sytems). Why not using my own Show implementation to do that? For exactly the same reason you're discovering: Show/Read exist for debugging. Show and Read are meant to produce/read valid Haskell code (modulo [qualified] imports) so that you can work out what code is going wrong (and coincidentally used as a quick`n`dirty serialisation method). The term pretty-printing is meant in regards to producing _human-readable_ versions of your data (though the pretty-printing libraries can also be used to produce code formatted for some other tool to parse, etc.). Show/Read happen to be auto-derivable classes that implement one such form of pretty-printing (i.e. printing values that look like the actual source code that represents them). Let me provide you with a personal anecdote: when I took over maintaining the graphviz library, it was still using the Show class for printing (but a proper parser library for parsing), and it was working with the limited functionality it had. However, whenever I tried to do something new, I found problems: * Existing Show instances meant that it was very difficult to extend what the library could represent and then print properly. * As you've found, it can then be a PITA to debug because you have no idea what the internal values actually are. In the end, I eventually wrote a custom pretty-printing class (the existing pretty-printing classes had instances that didn't suit, just like Show) and it's worked a lot better since. The only time it's valid to override the default Show/Read instances is when the constructors aren't exported (e.g. Data.Map), but even then it should be valid Haskell (if you ignore imports, etc.). So leave Show/Read as they are, and write a custom function[s] that does the actual pretty-printing you want. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com http://IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
