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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