Redefine MonadFail Gen to call discard

[ocharles]

I find it very counterintuitive that we can't have failing pattern matches act like discard when generating values.

To provide an example of where this is useful, I'm trying to generate trees of well typed terms. Some types have a rich structure that lets me learn about more type class constrains. In my list of alternatives, I would like to write:

        union = do
          Just Dict <- return (isEqTable t)
          Union <$> genQuery t <*> genQuery t 

But I can't do this, because if isEqTable returns Nothing this whole generator just calls error, rather than not considering this alternative. Instead, I have to write

          Dict <- just $ pure $ isEqTable t
          Union <$> genQuery t <*> genQuery t 

which I find a little more clunky (and this doesn't scale to other patterns).

[ocharles]

Alternatively I would propose this instance be entirely removed, as calling error is never the right implementation imo

[ocharles]

Duplicate of #257