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[Haskell-cafe] could we get a Data instance for Data.Text.Text?

Subject: [Haskell-cafe] could we get a Data instance for Data.Text.Text?
From: Jeremy Shaw
Date: Fri, 22 Jan 2010 16:24:19 -0600
Hello,

Would it be possible to get a Data instance for Data.Text.Text? This would allow us to create a Serialize instance of Text for use with happstack -- which would be extremely useful.

We (at seereason) are currently using this patch:


which basically adds:

+textType = mkStringType "Data.Text"
+
+instance Data Text where
+   toConstr x = mkStringConstr textType (unpack x)
+   gunfold _k z c = case constrRep c of
+                     (CharConstr x) -> z (pack [x])
+                     _ -> error "gunfold for Data.Text"
+   dataTypeOf _ = textType
+

This particular implementation avoids exposing the internals of the Data.Text type by casting it to a String in toConstr and gunfold. That is similar to how Data is implemented for some numeric types. However, the space usage of casting in Float to a Double is far less than casting a Text to a String, so maybe that is not a good idea?

Alternatively, Data.ByteString just does 'deriving Data'. However, bytestring also exports Data.ByteString.Internal, wheres Data.Text.Internal is not exported.

Any thoughts? I would like to get this handled upstream so that all happstack users can benefit from it.

- jeremy
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