We allow a user-specified type to represent the foreign type of haskell
types encountered in the API. This lets users map Integer, Date etc. to
representations other than Text, and have those representations
available in the returned list of Req.
For example, we might want to map a type which has an instance of
Generic to both a foreign type name and a class declaration for that
foreign type such that it can encode/decode itself to JSON. The previous
limitation to a single Text output prevented this case.
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