Re: [Haskell-cafe] Comparing on multiple criteria
David Menendez wrote: This is a great example of why it's a bad idea to introduce new functionality with a Monoid instance. Even if you know the instance exists, mappend is so general that it's difficult or impossible to predict what it will do at a given type. There should be an explicit function for combining Ordering values lexicographically, with a note in the documentation saying that it's the basis of the Monoid instance. Agreed. Supposing such an explicit function exists (with proper documentation), would the original example be worth mentioning in or around comparing's documentation? If it saved the next user a couple of minutes of coming up with something similar, I'd be happy. Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing on multiple criteria
David Menendez schrieb: On Sun, Dec 21, 2008 at 11:20 AM, Jan-Willem Maessen jmaes...@alum.mit.edu wrote: On Dec 21, 2008, at 8:52 AM, Martijn van Steenbergen wrote: Hello all, Data.Ord has a handy function called comparing, and its documentation shows an example of its use. But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this: compareTuple = mconcat [comparing fst, comparing snd] The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!) This is a great example of why it's a bad idea to introduce new functionality with a Monoid instance. Even if you know the instance exists, mappend is so general that it's difficult or impossible to predict what it will do at a given type. There should be an explicit function for combining Ordering values lexicographically, with a note in the documentation saying that it's the basis of the Monoid instance. +1 http://haskell.org/haskellwiki/Slim_instance_declaration ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing on multiple criteria
On Dec 21, 2008, at 8:52 AM, Martijn van Steenbergen wrote: Hello all, Data.Ord has a handy function called comparing, and its documentation shows an example of its use. But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this: compareTuple = mconcat [comparing fst, comparing snd] The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!) Indeed, this is great to know. I can't help but notice that there is no documentation of any kind at all for the Monoid instance of Ordering; how were we supposed to know this behavior existed in the first place, except by hunting down the source code for the instance declaration? -Jan-Willem Maessen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing on multiple criteria
On Sun, Dec 21, 2008 at 9:20 AM, Jan-Willem Maessen jmaes...@alum.mit.eduwrote: Indeed, this is great to know. I can't help but notice that there is no documentation of any kind at all for the Monoid instance of Ordering; how were we supposed to know this behavior existed in the first place, except by hunting down the source code for the instance declaration? It seems we have no good way to associate documentation with instances. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing on multiple criteria
On Sun, Dec 21, 2008 at 11:20 AM, Jan-Willem Maessen jmaes...@alum.mit.edu wrote: On Dec 21, 2008, at 8:52 AM, Martijn van Steenbergen wrote: Hello all, Data.Ord has a handy function called comparing, and its documentation shows an example of its use. But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this: compareTuple = mconcat [comparing fst, comparing snd] The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!) Indeed, this is great to know. I can't help but notice that there is no documentation of any kind at all for the Monoid instance of Ordering; how were we supposed to know this behavior existed in the first place, except by hunting down the source code for the instance declaration? This is a great example of why it's a bad idea to introduce new functionality with a Monoid instance. Even if you know the instance exists, mappend is so general that it's difficult or impossible to predict what it will do at a given type. There should be an explicit function for combining Ordering values lexicographically, with a note in the documentation saying that it's the basis of the Monoid instance. -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
