Yes that looks right. A class instance can’t dispatch on a type family application. In haskell we don’t allow
reverse (a ++ b) = reverse a ++ reverse b and it’s the same for type families and class instances. You should do the type-family reduction yourself, as you do below. Simon From: ghc-devs [mailto:[email protected]] On Behalf Of Alan & Kim Zimmerman Sent: 18 May 2017 08:15 To: [email protected] Subject: Re: Type families and classes 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 <[email protected]<mailto:[email protected]>> wrote: 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<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Farxiv.org%2Fabs%2F1610.04799&data=02%7C01%7Csimonpj%40microsoft.com%7Cec7c24b0b3aa4d0b0af608d49dbdc4c5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636306885764359008&sdata=ojtHcyFU%2BooW%2FdxDMaPpsDSUH8kfEF6y0NF4Encv1WE%3D&reserved=0> [2] https://github.com/alanz/ghc/blob/df1c3b3d42284dd121086e6c571793f19e758977/compiler/hsSyn/HsExtension.hs#L73<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Falanz%2Fghc%2Fblob%2Fdf1c3b3d42284dd121086e6c571793f19e758977%2Fcompiler%2FhsSyn%2FHsExtension.hs%23L73&data=02%7C01%7Csimonpj%40microsoft.com%7Cec7c24b0b3aa4d0b0af608d49dbdc4c5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636306885764359008&sdata=SSvg%2FOyBUAJoZ3HMAEpaUYuYdXROVNGeyo7bXTnUe2k%3D&reserved=0>
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