[web-devel] Automatically making every Cereal.Serialize instance of PersistField - bad idea?

[Michael Snoyman]

My guess is that you will eventually end up needing an
OverlappingInstances extension as well, and those two extensions
together can lead to some funny stuff. It *may* work, but I would
strongly recommend a newtype wrapper instead.

Michael

On Fri, Jan 28, 2011 at 8:28 AM, Max Cantor <mxcantor at gmail.com> wrote:
> I should have added, as long as the type-checker terminates and doesn't go into an infinite loop, is there any danger in doing this?
>
> On Jan 28, 2011, at 2:24 PM, Max Cantor wrote:
>
>> With a bit of code, you can save a lot of effort if you add new types to Persistent:
>>
>> instance Cereal.Serialize a => PersistField a where
>>  toPersistValue a = PersistByteString $ Cereal.encode a
>>  fromPersistValue (PersistByteString b) = Cereal.decode b
>>  fromPersistValue _ = Left "bad type for cereal encoded data"
>>  sqlType _ = SqlBlob
>>  isNullable _ = False
>>
>> BUT, this requires UndecidableInstances.  Is this a bad idea?
>>
>> mc
>
>
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