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:ghc-devs-bounces@haskell.org] On Behalf Of Alan & Kim Zimmerman
Sent: 18 May 2017 08:15
To: ghc-devs@haskell.org
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 mailto:alan.zimm@gmail.com> 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/1610.04799https://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/c...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
