Broken parser error messages

[agrafix]

Currently, decode returns FromRecord a => Either String a. This is suboptimal, because some error messages include the row that failed - and if that row is utf-8 encoded and contains japanese for example, the error message becomes an unreadable mess of characters. This is the bad code:

https://github.com/hvr/cassava/blob/545b86d60276c51ec29681f1917f1e5fb9b67c54/Data/Csv/Encoding.hs#L340-L343 (Specifically BL8.unpack)

Resolution options:

• As we already assume that the input is utf-8 (seeText FromField instance), instead of BL8.unpack we could try to decode utf8 first and if that fails fall back to BL8.unpack
• The left Either part should be a byte string

What do you think?

[ivan-m]

I think we should instead have Either (String, ByteString) a: return both the error message and the corresponding raw Field/Row that caused the issue.