Why does happstack-foundation-0.2.1 have mtl-2.1.* as dependency?

[Lambda Dusk]

Since the new update fixed some problems of mine, I switched to it, to then see that a whole bunch of other deps are broken. Pandoc, websockets and others have a restriction to mtl-2.0.*, while happstack-foundation changed its dependency from mtl-2.* to mtl-2.1.*, breaking a lot for me. Is there an important reason for this?

[Lambda Dusk]

Obviously this was not quite a problem of the deps, but of myself, consider it closed.