If you're concerned about performance, you should take a look at bytestring-lexing[1]. The `readDecimal` function does the basic "parse as a pair of ints then divide" trick, whereas `readDecimalLimited` does some other tricks to take advantage of types with limited precision— but which can still give massive performance improvements for types with unlimited precision, in certain circumstances. If you want to read the exponential notation rather than just the decimal notation, then there's also `readExponential` and `readExponentialLimited`. And there are benchmarks[2] comparing them to other parsing/lexing libraries. [1] http://hackage.haskell.org/package/bytestring-lexing-0.5.0.2/docs/Data-ByteS... [2] http://community.haskell.org/~wren/bytestring-lexing/bench/html/ -- Live well, ~wren