[Haskell-cafe] Generics (SYB) with parametrized types

JP Moresmau jpmoresmau at gmail.com
Thu Aug 11 17:56:53 CEST 2011

Hello, I'm banging my head against a wall there trying to use the syb
generics schemes against the GHC API. I'm looking at implementing a
search mechanism in the AST in the more direct way that what Scion
does (creating an instance of a TypeClass for each AST node type).
Since the GHC AST types derive from Data and Typeable, I thought it
would be possible.
So the problem is a follows: I have a start type, say
TypecheckedSource, that derives from Data. What I'd like to do is to
find all instances of the Located type inside that source that span a
particular range. The problem is that Located is a parametrized type
that wraps anything and just add location information. So the result
of the search could be anything. Ideally, I'd like to restrict my
search to Located instances that wrap an instance of Outputable,
pretty print that and output only the result.
So I'm looking to implement something that would have that signature:
TypecheckedSource -> (Line,Column) -> [String]

I have written code along these lines:
everything (++) ([] `mkQ` overlap) ts
        overlap :: forall b1 . (Outputable b1, Typeable b1) =>Located
b1  -> [String]
        overlap (a::Located b1)= ... trivial code here finding if the
location overlaps, and if it does, pretty print the object, otherwise
returns []

And GHC complains:
 Ambiguous type variable `b10' in the constraints:
   (Outputable b10) arising from a use of `overlap'
                    at ...
   (Typeable b10) arising from a use of `overlap'
                    at ...

So it doesn't like the fact that I don't know which types my Located
instances wrap. But that's the point, I don't care, I just want to
restrict my search to the ones I can pretty print. If I just try the
code against simple non parametrized types it of course works. If I
add some forall b1 . (Outputable ... in my main function signature it
complains that I never use b1 anywhere else in the signature, of

Is there a way to achieve what I want
(-XOhPleaseDoWhatIwantEvenIfIamNotSureItMakesSense or something)?

Thanks a million!

JP Moresmau

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