add tryError, withError, handleError, mapError (#60)

[ddssff]

Fixes #60

[chessai]

This LGTM. @RyanGlScott any thoughts?

[chessai]

Oh, needs a changelog entry!

[ddssff]

I see there is another PR with tryError.

[Lysxia]

I see there is another PR with tryError.

No worries about that. That PR I wrote is really meant to do something else, so I can rebase it on top of this later. I thought I needed tryError to write some property but actually I don't anymore (because the property is invalid).

[chessai]

@ddssff the builds failed because older versions of base's Prelude do not export <$>. i think on all GHCs before 7.10.3 you need to import Data.Functor ((<$>))

[ddssff]

MIN_VERSION_base(4,8,0) if I'm not mistaken

[chessai]

yeah, something like

#if !MIN_VERSION_base(4,8,0)
import Data.Functor ((<$>))
#endif

Alternatively, we could just use fmap, which has been exported by the Prelude for all supported versions of base. This would remove that bit of CPP. I don't have strong opinions about that, though I do prefer the least amount of CPP possible.

[ddssff]

Oh frabjous day.

[chessai]

Thanks!