attoparsec incorrect parsing of doubles

Question

I am using attoparsec's built-in parsers 'double' and 'number' to parse floating point values and I get different results from different parsers.

>parse number "8.918605790440055e-2"

Done "" 8.918605790440054e-2

> parse double "8.918605790440055e-2"

Done "" 8.918605790440055e-2

Using the 'number' parser seems to lose some precision whilst the 'double' parser does not. As it's possible to represent 8.918605790440055e-2 as a double since the 'double' parser manages to do it why does the 'number' parser return a different result? Is this a bug?

I am using attoparsec 0.10.4.0.

Answer 1

It is intentional:

Note: This function is almost ten times faster than rational. On integral inputs, it gives perfectly accurate answers, and on floating point inputs, it is slightly less accurate than rational.

attoparsec traded accuracy for speed by default, if you need perfectly accurate parsing, you have to use the slower rationalparser.

The difference in the results is so small that for most purposes, it doesn't matter, so the faster default parser is on the whole probably a bigger win.