makeFields fails for ExistentialQuantification and GADTs syntax

[emchristiansen]

makeFields in Control.Lens.TH fails for record types declared using ExistentialQuantification and GADTs syntax.
Tested with GHC 7.6.3 and current lens (I believe 4.0.5).
For example, the following fails with "verboseLenses: Expected the name of a record type".

data Foo a = (Num a) => Foo 
  { _fooBarL :: a
  }
makeFields ''Foo

This fails with the same error when using GADTs syntax.
I've also noticed silent failures when using makeLenses and makeClassy.

We should either fix the code or update the docs to reflect the limits of the TH methods.

[glguy]

makeFields doesn't support a context before the constructor, which is usually related to existential quantification. In this case I expect you meant to write:

data Foo = forall a. Num a => Foo 
  { _fooBarL :: a
  }

and makeFields has needed documentation for a while. There's already a ticket for that if anyone wants to write the documentation.

makeLenses makes a best effort which ranges from Iso though Lens, Traversal, Fold, Getter, and nothing for each constructor depending on what makes sense. Silent "failure" is key since users will want the generated accessors for the constructors where it makes sense. The best that could be hoped for is this setter, which might be useful.

fooBarL :: Num a => Setter Foo Foo () a
fooBarL = setting (\f _ -> Foo (f ()))

[ekmett]

I'm rather against the non-law abiding Setter solution.

over fooBarL id /= id

It also closes off the "more correct but still broken" solution:

foo :: (Functor f, Num b) => (forall a. Num a => a -> f b) -> Foo -> f Foo

Unfortunately, you can't write combinators that can accept that version, which are reasonably parametric in Haskell, so all code working with it has to be written in direct style, and you can't compose them with (.), so I don't want to generate those either.

Also that version still isn't right as it makes the choice of b independent of the choice of a and it should be possible to pick b = a, but due to the ordering of the quantifiers it has to make its choice too early!

I think the right solution is to document that we can't provide lenses for existentially quantified fields, and to fix it so we supply lenses, traversals, getters and folds for the remaining fields if we can if we're skipping them just because of the context.

[glguy]

Support for fields of constructors under a forall is added in 111341a