[Haskell-cafe] Polymorphic Type Safe URL building -- I need advice on my project
Hello, So I started working on a project involving polymorphic type safe urls. I know there are other type-safe url's out there, but I want some advice before I start reorganizing it. It is located here: https://github.com/dracule/web-scatter One might ask why you would want polymorphic types in your URL. Current implementation of type-safe URLs use a single type and then have it pattern match on the constructors. However, It is a pretty common occurrence that you might want to use a type from other parts of code. Where this becomes prevalent is in IxSet and AcidState. For IxSet your Index keys all have to have separate types . So with this, you can convert your url into your index key's type and then just make your handler like this: myHandler :: BlogId -> SomeWebHandler I know my code probably has some terrible inefficiencies or something, so I am looking for some feedback on some jarring problems. Thank You ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Polymorphic type
Thanks all. I think for my function, I only need to throw an error message for the out of range index. But through this, I know more some ways to deal with the polymorphic type. On 6/22/06, Brian Hulley <[EMAIL PROTECTED]> wrote: Sara Kenedy wrote: > Hello all, > > Now I am trying with the function of polymorphic type: This function > returns the Nth element of list with type a. I try it as below. > > getNthElem :: Int -> [a] -> Maybe a > getNthElemt _ [] = Nothing > getNthElem 0 _ = Nothing > getNthElem n s >> n > length s = Nothing >> otherwise = Just ((drop (n-1) (take n s))!!0) > >> getNthElem 2 ["a","b","c"] > Just "b" > > However, I do not satisfy with this function because I want to return > the Nth element of type a, not (Maybe a). For example, I want this > function: > getNthElem :: Int -> [a] -> a > > But, I do not know how to define the empty element of type a. > > getNthElemt _ [] = > getNthElem 0 _ = > > If you have some ideas about this, please give me some clues. Thanks > a lot. You might find it's always a lot easier to start counting from zero rather than 1, so that "a" is the 0th element, "b" is the 1st element etc. Just like a building with 2 floors has a ground floor and a first floor, and if you want to find what day of the week it is in 46 days from today you just use (today + 46) `mod` 7 instead of (((today - 1) + 46) `mod` 7) + 1 That aside, why not just throw an error when the function is called with an index that's out of range? getNthElemt _ [] = error "getNthElemt" Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Polymorphic type
Sara Kenedy wrote: Hello all, Now I am trying with the function of polymorphic type: This function returns the Nth element of list with type a. I try it as below. getNthElem :: Int -> [a] -> Maybe a getNthElemt _ [] = Nothing getNthElem 0 _ = Nothing getNthElem n s n > length s = Nothing otherwise = Just ((drop (n-1) (take n s))!!0) getNthElem 2 ["a","b","c"] Just "b" However, I do not satisfy with this function because I want to return the Nth element of type a, not (Maybe a). For example, I want this function: getNthElem :: Int -> [a] -> a But, I do not know how to define the empty element of type a. getNthElemt _ [] = getNthElem 0 _ = If you have some ideas about this, please give me some clues. Thanks a lot. You might find it's always a lot easier to start counting from zero rather than 1, so that "a" is the 0th element, "b" is the 1st element etc. Just like a building with 2 floors has a ground floor and a first floor, and if you want to find what day of the week it is in 46 days from today you just use (today + 46) `mod` 7 instead of (((today - 1) + 46) `mod` 7) + 1 That aside, why not just throw an error when the function is called with an index that's out of range? getNthElemt _ [] = error "getNthElemt" Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Polymorphic type
2006/6/22, Sara Kenedy <[EMAIL PROTECTED]>: Hello all, Now I am trying with the function of polymorphic type: This function returns the Nth element of list with type a. I try it as below. getNthElem :: Int -> [a] -> Maybe a getNthElemt _ []= Nothing getNthElem 0 _ = Nothing getNthElem n s | n > length s = Nothing | otherwise = Just ((drop (n-1) (take n s))!!0) >getNthElem 2 ["a","b","c"] Just "b" However, I do not satisfy with this function because I want to return the Nth element of type a, not (Maybe a). For example, I want this function: getNthElem :: Int -> [a] -> a But, I do not know how to define the empty element of type a. getNthElemt _ []= getNthElem 0 _ = If you have some ideas about this, please give me some clues. Thanks a lot. hi, precisely, you want to return an "a" only when there is one accordingly to your above code. the only way to handle this without resorting to [] or Nothing to say there is no such value is to use error or default value. infact, ask yourself, what do you want ? getNthElem 5 ["a","b","c"] or getNthElem 0 ["a","b","c"] or getNthElem (-1) ["a","b","c"] do you want "a" [] Nothing "wrong" or raise an exception (i.e. you use the "error" function) ? once you know what you want, you can code it. note, i think your 'take" is unnecessary here | otherwise = Just ((drop (n-1) (take n s))!!0) also you can use | otherwise = Just (n!!some_value) -- :) this is where you see that the function you're trying to write is really close of (!!) cheers, mt ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Polymorphic type
On 6/22/06, Sara Kenedy <[EMAIL PROTECTED]> wrote: Hello all, Now I am trying with the function of polymorphic type: This function returns the Nth element of list with type a. I try it as below. getNthElem :: Int -> [a] -> Maybe a getNthElemt _ []= Nothing getNthElem 0 _ = Nothing getNthElem n s | n > length s = Nothing | otherwise = Just ((drop (n-1) (take n s))!!0) >getNthElem 2 ["a","b","c"] Just "b" However, I do not satisfy with this function because I want to return the Nth element of type a, not (Maybe a). For example, I want this function: getNthElem :: Int -> [a] -> a But, I do not know how to define the empty element of type a. Not all types (especially numbers) have an empty element (what does that even mean?). Suppose you have a list [0, 1, -2, -1, 2] and you try getNthElemt 4 and your program assumes that the empty element for integers is 0. How can you tell that 0 from the 0 at the beginning of the list [0, 1, 2]? Think really hard about what you are asking and you will see why Maybe a takes the type a and extends it, in a way, with an empty element, Nothing. To convert it from Maybe a to a, try, e.g. fromJust (Just 4) > 4 (it will give exceptions when Nothing shows up). getNthElemt _ []= getNthElem 0 _ = One possiblity is to make a class called empty with a single member: class Empty a where empty :: a instance Empty [a] where -- this also makes "" = empty for String empty = [] instance Empty Maybe a where -- is this desirable? empty = Nothing instance Integer where -- or this? empty = 0 ... and then add the constraint to your function: getNthElem :: Empty a => Int -> [a] -> a getNthElem :: Int -> [a] -> Maybe a getNthElemt _ []= empty getNthElem 0 _ = empty getNthElem n s | n > length s = empty | otherwise = ((drop (n-1) (take n s))!!0) but you need overlapping instances to instantiate [a]. Or you could use MonadPlus and mzero instead of Empty and empty, but that would only work for List, Maybe and other monads and not for Integer, etc. Note that in a dynamic language the same thing happens. In python 4 + None raises an exception. I don't think it's possible to get away from this whole "failure" concept (except silently ignore it---in perl 4+null yields 4 but is that always the right behavior in all situations? It makes bugs really hard to find.) Jared. -- http://www.updike.org/~jared/ reverse ")-:" ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Polymorphic type
Hello all, Now I am trying with the function of polymorphic type: This function returns the Nth element of list with type a. I try it as below. getNthElem :: Int -> [a] -> Maybe a getNthElemt _ []= Nothing getNthElem 0 _ = Nothing getNthElem n s | n > length s = Nothing | otherwise = Just ((drop (n-1) (take n s))!!0) getNthElem 2 ["a","b","c"] Just "b" However, I do not satisfy with this function because I want to return the Nth element of type a, not (Maybe a). For example, I want this function: getNthElem :: Int -> [a] -> a But, I do not know how to define the empty element of type a. getNthElemt _ []= getNthElem 0 _ = If you have some ideas about this, please give me some clues. Thanks a lot. S. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] polymorphic type
On Wed, 6 Jul 2005, wenduan wrote: > Dear all, > > Suppose we have defined two functions as below: > > case :: (a -> c,b -> c) -> Either a b -> c > case (f, g) (Left x) = f x > case (f, g) (Right x) = g x It seems to be case == uncurry either Prelude> :info either -- either is a variable either :: forall a c b. (a -> c) -> (b -> c) -> Either a b -> c > plus :: (a -> b, c -> d) -> Either a b -> Either c d > plus (f, g) = case(Left.f, Right.g) > of plus should be: >plus :: (a -> c, b -> d) -> Either a b -> Either c d That signature looks correct and it is accepted Prelude> let plus = (\(f,g) -> uncurry either (Left . f, Right . g)) :: (a->c,b->d) -> Either a b -> Either c d ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] polymorphic type
Wenduan, What I thought at first the signature of plus should be: plus :: (a -> c, b -> d) -> Either a b -> Either c d?Anyone know where I was wrong? Your initial thought was right: it should (a -> c, b -> d) -> Either a b -> Either c d Why didn't you just test it by feeding in to a compiler or interpreter; these are really useful, you know. ;) Besides... case :: (a -> c,b -> c) -> Either a b -> c case (f, g) (Left x) = f x case (f, g) (Right x) = g x You can't name your function case: it's a reserved word. You might want to check out the standard libraries: case' = uncurry either HTH, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] polymorphic type
Dear all, Suppose we have defined two functions as below: case :: (a -> c,b -> c) -> Either a b -> c case (f, g) (Left x) = f x case (f, g) (Right x) = g x plus :: (a -> b, c -> d) -> Either a b -> Either c d plus (f, g) = case(Left.f, Right.g) My question is regarding to the function signature of 'plus' , in its signature, does the 'a' in 'a -> b' and in 'Either a b', must be instantiated to the same object when the function is applied?E.g.,Either a b is instantiated to 'Either Char b', will the 'a' in 'a -> b' be instantiated to 'Either Char b'?Furthermore, are the two bs in 'a -> b' and 'Either a b' not conflicting?What I thought at first the signature of plus should be: plus :: (a -> c, b -> d) -> Either a b -> Either c d?Anyone know where I was wrong? -- X.W.D ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
