Hi all

I am experimenting with Trees that Grow [1] in the context of the GHC HsSyn AST, and wanting to express that a given extension point needs to have certain properties.

The specific case is to be able to contain a SourceText, in the context of HsLit

So I have (stripped down)

data GHCX

type family XHsString x

type instance XHsString GHCX = SourceText

class HasSourceText a where
-- Provide setters to mimic existing constructors
noSourceText :: a
sourceText    :: String -> a

getSourceText :: a -> SourceText

instance HasSourceText (XHsString GHCX) where
noSourceText    = NoSourceText
sourceText s    = SourceText s
   getSourceText a = a

But this gives an error

compiler/hsSyn/HsExtension.hs:80:10: error:
    • Illegal type synonym family application in instance:
        XHsString GHCX
    • In the instance declaration for ‘HasSourceText (XHsString GHCX)’

Is there some way to achieve this, or is it simply impossible?

The full work-in-progress source is here[2]

Regards

Alan

[1] https://arxiv.org/abs/1610.04799
[2] https://github.com/alanz/ghc/blob/df1c3b3d42284dd121086e6c571793f19e758977/compiler/hsSyn/HsExtension.hs#L73