[Haskell-cafe] Filter by several predicates at once
Are the functions passall, passany :: [a - Bool] - a - Bool passall ps v = and $ map ($v) ps passany ps v = or $ map ($v) ps or something similar defined anywhere? Such that one can write filter (passany tests) [0..10] filter (passall tests) [0..10] where tests = [5, odd] Or is there a better way of filtering by several predicates for each value without using filter p3 . filter p2 . filter p1 or filter (\v - p1 v p2 v p3 v) vs Cheers, D -- Dougal Stanton [EMAIL PROTECTED] // http://www.dougalstanton.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Filter by several predicates at once
Hi passall, passany :: [a - Bool] - a - Bool passall ps v = and $ map ($v) ps passany ps v = or $ map ($v) ps or something similar defined anywhere? Such that one can write Don't think so. One thing I have often wanted is something like: or1 a b x = a x || b x or2 a b x y = a x y || b x y Then you can do: filter ((5) `or1` odd) [0..10] filter ((5) `and1` odd) [0..10] You can imagine that or1 could get a symbol such as ||#, and or2 could perhaps be ||## (if # wasn't already really overloaded) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Filter by several predicates at once
Neil Mitchell wrote: Hi passall, passany :: [a - Bool] - a - Bool passall ps v = and $ map ($v) ps passany ps v = or $ map ($v) ps or something similar defined anywhere? Such that one can write nearly; using Prelude: passall ps v = all ($v) ps passany ps v = any ($v) ps One thing I have often wanted is something like: or1 a b x = a x || b x or2 a b x y = a x y || b x y yep, there's the idea of putting Bools in a typeclass that allows you to (||) functions-returning-Bool-class-instance for example, which I haven't used much but seems like a good idea (though potentially confusing, especially if the Prelude-Bool-specific names are reused) ~Isaac ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Filter by several predicates at once
On 17/01/2008, Stuart Cook [EMAIL PROTECTED] wrote: On Jan 18, 2008 1:46 AM, Isaac Dupree [EMAIL PROTECTED] wrote: nearly; using Prelude: passall ps v = all ($v) ps passany ps v = any ($v) ps Yes, thanks Isaac. That should have been obvious, argh... passall = swing all passany = swing any Whether that's any better than the pointwise version is up to you. I think in this case I will use the explicit version, because I wouldn't remember how swing worked. What is the motivation for the name? ;-) (Do I want to hear the answer...?) D -- Dougal Stanton [EMAIL PROTECTED] // http://www.dougalstanton.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Filter by several predicates at once
On Jan 18, 2008 1:46 AM, Isaac Dupree [EMAIL PROTECTED] wrote: Neil Mitchell wrote: Hi passall, passany :: [a - Bool] - a - Bool passall ps v = and $ map ($v) ps passany ps v = or $ map ($v) ps or something similar defined anywhere? Such that one can write nearly; using Prelude: passall ps v = all ($v) ps passany ps v = any ($v) ps See also http://haskell.org/haskellwiki/Pointfree#Swing, which would let you define passall = swing all passany = swing any Whether that's any better than the pointwise version is up to you. Stuart ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Filter by several predicates at once
Hello Isaac, Thursday, January 17, 2008, 5:46:20 PM, you wrote: yep, there's the idea of putting Bools in a typeclass that allows you to (||) functions-returning-Bool-class-instance for example, which I haven't used much but seems like a good idea (though potentially confusing, especially if the Prelude-Bool-specific names are reused) -- Datatypes having default values classDefaults a where defaultValue :: a instance Defaults () where defaultValue = () instance Defaults Bool where defaultValue = False instance Defaults [a]where defaultValue = [] instance Defaults (a-a) where defaultValue = id instance Defaults (Maybe a)where defaultValue = Nothing instance Defaults (a-IO a)where defaultValue = return instance Defaults a = Defaults (IO a) where defaultValue = return defaultValue instance Num a = Defaults a where defaultValue = 0 -- Datatypes that can be checked for default value classTestDefaultValue awhere isDefaultValue :: a - Bool instance TestDefaultValue Bool where isDefaultValue = not instance TestDefaultValue [a] where isDefaultValue = null instance Num a = TestDefaultValue a where isDefaultValue = (==0) infixr 3 infixr 2 ||| a ||| b | isDefaultValue a = b | otherwise= a a b | isDefaultValue a = defaultValue | otherwise= b my code contains countless examples of using these funcs: 1. here it is used to conditionally include options in cmdline: [rar, x, arcname]++ (isAddDir [-ad])++ (arcdir files [-ap++arcdir])++... 2. here it is used to get list of files where current directory may be specified as : files - dirList (dirName ||| .) 3. here it is used to show file basename or full path if basename is empty: putStr (takeBaseName file ||| file) 4. here it's used for conditional code execution: do opt_debug command testMalloc ... 5. here it is used to additionally print amount of bad sectors if it's non-zero: putStrLn$ show recoverable_sectors++ recoverable errors ++ (bad_sectors and ++show bad_sectors++ bad sectors) 6. here it's used to create tempfile in current directory unless temporary directory was explicitly specified in --tempdir option let filename = (opt_tempdir command ||| .) / $$temp$$ 7. here it is use to apply additional reorder step to sorted list only if --reorder option was specified sorted_diskfiles - (opt_reorder command reorder) (sort_files command diskfiles) (reorder has type [String] - IO [String]) -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
