Am Dienstag, 7. Oktober 2008 04:21 schrieb Jason Dagit: > On Mon, Oct 6, 2008 at 7:06 PM, Mike Coleman <[EMAIL PROTECTED]> wrote: > > Hi, > > > > I could use a little help. I was looking through the Real World > > Haskell book and came across a trivial program for summing numbers in > > a file. They mentioned that that implementation was very slow, as > > it's based on String's, so I thought I'd try my hand at converting it > > to use lazy ByteString's. I've made some progress, but now I'm a > > little stuck because that module doesn't seem to have a 'read' method. > > > > There's a readInt method, which I guess I could use, but it returns a > > Maybe, and I don't see how I can easily strip that off. > > > > So: > > > > 1. Is there an easy way to strip off the Maybe that would allow an > > equivalently concise definition for sumFile? I can probably figure > > out how to do it with pattern matching and a separate function--I'm > > just wondering if there's a more concise way. > > I'm not a ByteString expert, but there should be an easy way to solve this > issue of Maybe. > > If you go to hoogle (http://www.haskell.org/hoogle/) and type this: > [Maybe a] -> [a] > it says: > Data.Maybe<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Mayb >e.html> > .catMaybes<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Mayb >e.html#v:catMaybes>:: [Maybe > a] -> > [a]<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Maybe.html# >v:catMaybes> > > As the top search result. > > This means that you can convert any list of maybes into a list of what you > want. It just tosses out the Nothings.
Since readInt returns a Maybe (Int,ByteString), Data.List.unfoldr would be a better fit for his needs. The bytestring-lexing package (http://hackage.haskell.org/packages/archive/bytestring-lexing/0.1.2/doc/html/Data-ByteString-Lex-Double.html) provides readDouble, which is also pretty fast, I think. > > > 2. Why doesn't ByteString implement 'read'? Is it just that this > > function (like 'input' in Python) isn't really very useful for real > > programs? > > I think probably for things more complex than parsing ints it's best to > make your own parser? I seem to recall that someone was working on a > library of parsing functions based on bytestring? Maybe someone else can > comment? At least parsec 3.0.0 has ByteString parsing modules (I've never used it, so I don't know how well they work). IIRC, there's a plan to expand the bytestring-lexing package, too. > > 3. Why doesn't ByteString implement 'readDouble', etc.? That is, why > > > are Int and Integer treated specially? Do I not need readDouble? > > I think readInt was mostly implemented because integer reading was needed a > lot for benchmarks and programming challenge sites and people noticed it > was way slower than needed so someone put in the effort it optimize it. > Once it was optimized, that must have satisfied the need for fast number > reading. More's underway, readDouble already delivered. > > I would agree that at least for Prelude types it would be nice to have > efficient bytestring based parsers. Do we have Read/Show classes > specifically for working in bytestrings? Maybe that's what the world needs > in the bytestring api. Then again, I'm not really qualified to comment :) > For all I know it already exists. partially. > > Jason _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
