Add hasUpperBound and hasLowerBound

[snoyberg]

This is useful for performing PVP bounds checking, especially for
tooling that wants to automate such dependency bounds. A demonstration
of this being used in practice is the --pvp-bounds flag for stack:

commercialhaskell/stack@c774f99

[23Skidoo]

I don't think that this is correct. Consider the intersection of >=1.0 with <=1.1.

[snoyberg]

You're right, good catch, thanks.

[23Skidoo]

This may be easier to implement by converting VersionRange to VersionIntervals first. Also, it looks like other code in that file assumes that if the lower bound is not specified, then it is >=0 by default, so in a sense a lower bound always exists.

[snoyberg]

Thanks for the pointer on VersionIntervals. It looks like it will work for upper bounds. However, with lower bounds, it's not going to work (at least for the way I'd want to implement this, which has to do with your second point). For PVP bounds purposes, the presence of an implicit lower bound (i.e., that 0 is the minimum version possible) is not my goal: it's to detect if the user explicitly stated a lower bound that should be respected (as opposed to omitting a lower bound, where tooling can add it in for the user).

[23Skidoo]

LGTM now, so merging.

[hvr]

c.f. #2774 which also contains similar predicates and or similar needs

[snoyberg]

Interesting, that looks like a very similar feature to what I was adding to stack