bytestring-lexing (hackage): The bytestring-lexing package offers extremely efficient ByteString parsers for some common lexemes: namely integral and fractional numbers. In addition, it provides efficient serializers for (some of) the formats it parses.  As of version 0.3.0, bytestring-lexing offers the best-in-show parsers for integral values. (According to the Warp web server's benchmark of parsing the Content-Length field of HTTP headers.) And as of version 0.5.0 it offers (to my knowledge) the best-in-show parser for fractional/floating numbers.  Some benchmarks for this package can be found at: https://github.com/wrengr/bytestring-lexing/tree/master/bench/html
Keywords: bsd3|data|library