Type families and classes

[Alan & Kim Zimmerman]

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 <alan.zimm at 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/1610.04799
> [2] https://github.com/alanz/ghc/blob/df1c3b3d42284dd121086e6c571793
> f19e758977/compiler/hsSyn/HsExtension.hs#L73
>
>
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