[Hs-Generics] Request for code review: Generics for tuple/newtype introspection

[José Pedro Magalhães]

Hi AntC,


On Fri, Dec 13, 2013 at 12:04 AM, Anthony Clayden <
anthony_clayden at clear.net.nz> wrote:

>
>
> Questions:
> * These pile-ups of types (M1 D ... (M1 C ... (...))) seem
> hairy.
>   Is that the best way to control the depth of recursion?
>

I think I would define a generic function |typeOfRecord :: a -> TypeRep|,
which
expects a newtype |a| and returns the |TypeRep| of its argument. Then I'd
use that
function instead of your call to |from| expecting to match on |M1 (M1 (M1
(K1 x)))|.
I'd define that function as a regular generic consumer (like generic show),
and give
runtime errors for unexpected values (such as sums, or products, as we don't
expect those in newtypes). The advantage of the "hairy types" you have
right now
is that these would be (obscure) compile-time errors.


>
> * The suggestion from John Lato (see gist from his message
> http://www.haskell.org/pipermail/haskell-cafe/2013-October/111139.html
> )
>   uses a different style for the class types,
>   avoiding the mysterious unbound typevar
>
>        class GConName a where    getConName :: a -> String
> -- cp: class ...      f where    ...        :: f a -> String
>
>   Are there places where one or other is more suitable?
>   (I did try John's style at first, but the constraints got
> uglier.)
>

Perhaps this is more or less what I suggest above?


>
> * The explicit data constructors in the pattern matching
>   are a bit temperamental w.r.t. undefined values.
>   (Specifically, my instance for (:*:) was crashing.)
>   Should I use dummy `_` patterns throughout?
>   (Then the type annotations are monsters!)
>

I don't see why you need |undefined| at all. Unless you are
calling |tupToAttribs| with |undefined|; is that the case? Else,
your generic function can just pattern-match on |a :*: b| on
the product case, for example.


Cheers,
Pedro


>
>
> Thank you
> AntC
>
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