And to answer my own question, it seems
instance HasSourceText SourceText where
noSourceText = NoSourceText
sourceText s = SourceText s
getSourceText a = a
And then applying the appropriate constraint at the use-site does the job.
So
noSyntaxExpr :: (HasSourceText (XHsString x)) => SyntaxExpr x id
Alan
On 18 May 2017 at 09:04, 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/df1c3b3d42284dd121086e6c571793 f19e758977/compiler/hsSyn/HsExtension.hs#L73