Type families and classes

[Alan & Kim Zimmerman]

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
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