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

Jeremy Shaw jeremy at n-heptane.com
Fri Jan 22 17:24:19 EST 2010


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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