The actual parser is a bit more complicated than I let on. First, it's important that not all of a file be read at the same time as the files can be huge. Second, it keeps track of column row position information as an IORef, which makes sense because the ref is bundled in a structure with the handle and manipulated only in the IO Monad in conjunction with operations on the handle.
John On Thu, Feb 28, 2013 at 9:00 AM, Erik Hesselink <[email protected]> wrote: > Is your parser impure? I would expect a function from > String/Text/ByteString to Maybe (SExpr Pos). > Then you have no need for a Handle. > > Regards, > > Erik > > On Thu, Feb 28, 2013 at 2:32 PM, John D. Ramsdell <[email protected]> wrote: >> I think I wasn't clear about my question. I want something that >> creates a value of type System.IO.Handle. You see, I have a high >> performance S-expression parser that I'd like to use in GHCi reading >> strings while at the command loop. >> >> Here is more details on my module SExpr that exports the SExpr data >> type and the load function. The desired function is called >> stringHandle. >> >> -- An S-expression >> data SExpr >> = S String -- A symbol >> | Q String -- A quoted string >> | N Int -- An integer >> | L [SExpr a] -- A proper list >> >> -- Read one S-expression or return Nothing on EOF >> load :: Handle -> IO (Maybe (SExpr Pos)) >> >> In GHCi, I want to type something like: >> >> SExpr> let h = stringHandle "()" >> SExpr> load h >> Just (L []) >> SExpr> load h >> Nothing >> SExpr> >> >> It seems to me right now that I have to implement a duplicate parser >> that implements Read. At least S-expression parsing is easy. >> >> John >> >> On Thu, Feb 28, 2013 at 3:02 AM, Ganesh Sittampalam <[email protected]> wrote: >>> Hi, >>> >>> On 27/02/2013 20:38, John D. Ramsdell wrote: >>>> How does one create a value of type System.IO.Handle for reading that >>>> takes its input from a string instead of a file? I'm looking for the >>>> equivalent of java.io.StringReader in Java. Thanks in advance. >>> >>> http://hackage.haskell.org/package/bytestring-handle can make handles >>> that read and write to ByteStrings. >>> >>> Cheers, >>> >>> Ganesh >>> >>> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> [email protected] >> http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
