Re: [Haskell-cafe] Polymorphic functions over string libraries

Hi Gonzaw, Among other alternatives you could try using http://hackage.haskell.org/package/ListLike package. It provides similar interface to strings and text, but all functions operate on a typeclass so they may be used with either Text or Strings. E.g. import Data.Char import Data.Text (Text) import qualified Data.Text as T import Data.ListLike (ListLike, ListLikeIO) import qualified Data.ListLike as LL transform :: String -> String transform = map toUpper transform' :: Text -> Text transform' = T.map toUpper onFile :: (ListLikeIO s c) => FilePath -> FilePath -> (s -> s) -> IO () onFile fileIn fileOut f = LL.readFile fileIn >>= LL.writeFile fileOut . f main :: IO () main = do writeFile "foo" "hello world\n" onFile "foo" "bar" transform onFile "foo" "bar" transform' On Mon, Oct 27, 2014 at 7:50 PM, Adam Bergmark wrote:

I If you want to accept String and Text and ByteString your implicitly state that you do not make any assumptions about the encoding

I think the most pragmatic (and perhaps even the most appropriate) solution is to simply to expose one set of functions working on one type.

If you assume UTF-8 you should not accept a ByteString since it's not encoding aware. If you did you'd have to add documentation saying "this assumes that your bytestring contains valid UTF-8". Use Text instead since it's the best unicode-aware library available. If you don't make any assumptions about encoding, pick ByteString.

On Sun, Oct 26, 2014 at 8:43 PM, John Lato wrote:

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This could be solved by SML-style modules, but as you mention, that's an active area of research. Class-based solutions, such as ListLike or the newer mono-traversable, have been available for years.

On Sun, Oct 26, 2014 at 9:18 AM, Kyle Marek-Spartz wrote:

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This is one of the problems that an improved module language would solve such as the one in SML (Signatures and functors provide for this functionality). In Haskell-land, this is an active area of research. You may be interested in Backpack:

http://plv.mpi-sws.org/backpack/

gonzaw writes:

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Hi.

I was wondering what would be the best way to create a polymorphic function over any possible string library (Text, String, Bytestring, etc).

For instance, imagine I have to read a file with text, transform this text in some way and output it to another file, or to the console. If I wanted to use String, I'd just do this: / transform :: String -> String main = readFile "input.txt" >>= writeFile "output.txt" . transform / But if I wanted to use Text instead, I'd have to use this: / import qualified Data.Text.IO as T

transform :: Text -> Text main = T.readFile "input.txt" >>= T.writeFile "output.txt" . transform / Idem for ByteString.

I was wondering if there was a way to create these computations in a generic way, for any kind of string library, something like this: / class StringLibrary s where: sReadFile :: FilePath -> IO s sWriteFile :: FilePath -> s -> IO () ... / So then I'd just have this: / transform :: StringLibrary s => s -> s main = sReadFile "input.txt" >>= sWriteFile "output.txt" . transform / Now I can perform the computation I want without being tied down to a specific library. At times when I create some quick scripts, I find myself using one library (for example using String to get it finished more quickly, since I have less experience with the other ones), but find that it's too slow or has some problem that is solved by using one of the other libraries. Yet swapping from one to the other is more cumbersome than expected at times. In the example above, I could easily swap between them, just by forcing the compiler to typecheck to a specific one (for instance by changing the type of "transform"). Or if I wanted to, I could leave it as it is and export it as a library of my own.

Is there a way to do something like this in Haskell, with existing libraries? In terms of using the string datatype as some sort of container of characters, I think there are libraries like Lens and mono-traversable that allow you to do stuff like this. But I'm not too familiar with them (at least using them in this way).

-- Kyle Marek-Spartz _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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