[Hs-Generics] pervasiveness of Rec in TH-derived instant-generics representation types

[José Pedro Magalhães]

Hi,

On Wed, Feb 16, 2011 at 23:03, Nicolas Frisby <nicolas.frisby at gmail.com>wrote:

> In instant-generics-0.2.1, the TH functions for deriving the Rep type
> instances insert the "Rec" type at every field. Is there a consequence
> to this?
>
> This behavior seems at odds with the paper; isn't Rec only meant as an
> indicator of recursive occurrences within the original data type?
>

I think that is the current behavior, yes. I've given some thought to this
in the generic deriving paper [1]; in fact, tagging a recursive occurrence
with Var or Rec doesn't make much difference, since in the end it is just a
tag. You can't, for instance, define an fmap function by mapping over the
things tagged with Var, because you know nothing about the types of those
things.

This doesn't mean they are entirely useless, though. In fact,
instant-generics makes use of this in the empty function [2]: on a sum, it
looks ahead to see if there are Rec's on the left or right, and proceeds to
the side that has no Rec's. This guarantees that a finite element will be
generated (if there is one). So, tagging only the occurrences of the
original type with Rec wouldn't be very helpful, since datatypes can be
mutually recursive.

I think the best approach is to tag everything as Rec, except for parameters
and base types. That is what we do in UHC [1], if I recall correctly.

And, as the comment says, the TH code for instant-generics should be updated
to generate Var tags too. I haven't gotten around to do that yet, but if
it's important for you I can have a look. Of course, patches are welcome too
[3].


Cheers,
Pedro

[1] http://www.dreixel.net/research/pdf/gdmh.pdf
[2]
http://hackage.haskell.org/packages/archive/instant-generics/0.2.1/doc/html/Generics-Instant-Functions-Empty.html
[3] https://subversion.cs.uu.nl/repos/project.dgp-haskell.libraries/Instant/


>
> Thanks.
>
> ---
>
> snippet from http://j.mp/gK8MOk ; repField is invoked for every field
> of every constructor by all of the relevant TH entry-points.
>
>  repField :: (Name, [Name]) -> Type -> Q Type
>  --repField d t | t == dataDeclToType d = conT ''I
>  repField d t = conT ''Rec `appT` return t
>
>  repField' :: (Name, [Name]) -> Name -> (Name, Strict, Type) -> Q Type
>  --repField' d ns (_, _, t) | t == dataDeclToType d = conT ''I
>  repField' (dt, vs) ns (f, _, t) = conT ''Rec `appT` return t
>  -- Note: we should generate Var too, at some point
>
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