Type families and classes

Simon Peyton Jones simonpj at microsoft.com
Thu May 18 07:56:06 UTC 2017

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.


From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Alan & Kim Zimmerman
Sent: 18 May 2017 08:15
To: ghc-devs at 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.

noSyntaxExpr :: (HasSourceText (XHsString x)) => SyntaxExpr x id

On 18 May 2017 at 09:04, Alan & Kim Zimmerman <alan.zimm at gmail.com<mailto: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]


[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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