Café,

SYB-style libraries (and especially Uniplate) make it very easy to run generic

traversals (queries/transformations) on ADTs.

> data Expr = ...

> x :: Expr

> f :: Expr -> Expr

> transform :: (Expr -> Expr) -> Expr -> Expr

> transform f x :: Expr -- applies f to x (and its children) in a bottom-up manner

Uniplate is very powerful and doesn't require any hand written type-class

instances to get going reasonably efficiently.

I can simply re-implement the functionality of Uniplate myself, modulo

requiring hand-written instances. (I didn't want to go into that road, since I

don't need it for this argument.)

I did this exercise, because I wanted to understand why we don't have similar

generic traversal libraries for GADTs. And, it didn't take long :)

I can post my trials but they are really baby steps and they wouldn't be

helpful.

My question is:

Considering how useful generic traversal libraries and GADTs are, has anybody

tried implementing a generic traversal library (like Uniplate or similar) for

GADTs? Is it a hard (or impossible, or not efficient, or problematic in some

other way) thing to implement, or is it just not a requirement for anyone

else?

Best,

Ozgur