[Haskell-cafe] Polymorphic functions over string libraries

[John Lato]

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 <
kyle.marek.spartz at gmail.com> wrote:

> 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:
>
> > 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
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