Re: No variables available for parameterised types

[Chris Warburton]

Looking into this a bit more, I think the issue is that functions have
no Ord instance; since getArbGen checks for Ord and for Arbitrary, it's
not returning a generator.

There are a few approaches we could take:

 - We could declare an `Ord` instance for functions.
  - Since the Ord instances are what actually determine equality in
    quickspec, we would have to do it "properly" or else risk breaking
    the output
  - There's no general way in Haskell to compare functions; they're
    black boxes, so this seems like a no go
 - We could use some intermediate datatype, such as lookup tables, which
   we can easily make an Ord instance for (e.g. convert to an assoc
   list, sort and compare lexicographically)
  - This requires some sort of conversion layer, since the functions in
    our signature are expecting normal functions (would be nice if ->
    were more polymorphic/overloadable in Haskell!)
  - Too invasive, probably requires too much work to integrate (there
    are off-the-shelf datatypes we can use, but it's the
    wrapping/unwrapping which would be a pain)
 - We could see if the Ord instance is still needed
  - It looks like it's needed for using the `fun0`, `fun1`, etc.
    functions, but not the `gvars` functions.
  - Even in the case of `funN`, only the last type needs to be Ord,
    e.g. it's fine to have `map :: (a -> b) -> [a] -> [b]`, as long as
    `[b]` is an instance of Ord; `[a]` and `(a -> b)` don't have to be.

I'll try dumping the Ord constraint in RuntimeArbitrary's getArbGen, to
see if that will work.