[web-devel] Data and Typeable

Clint Moore clint at ivy.io
Fri Apr 8 20:42:34 CEST 2011

On Thu, Apr 7, 2011 at 12:01 PM, Michael Snoyman <michael at snoyman.com>wrote:
> First, what's the 'right way' to get a string representation of a Key?
>> I generate select form elements from subsets of data in a table.  Usually,
>> resulting in 'select name="name_column" value="id_column"'
>> I would probably use the Show instance, unless you have a compelling
> reason not to.

Show on the key gives me 'TableId 1' instead of simply 1.  I'm assuming the
expectation is not to split the table name off every time, though I don't
see a function to get just the id of a row.  ie tableId

> x <- myConnect $ selectList [TableNameNe "honk"] [] 0 0
> fst $ head x
TableId 1

> tableId $ snd x  <- convenient!

Second, for types that have relationships, is there a shortcut or something
>> I'm not seeing that will allow me to derive Data and Typeable?
>> example:
>> Species
>>  name String Eq
>>  deriving Data Typeable
>> Animal
>>  name String
>>  species SpeciesId
>>  deriving Data Typeable  <-- error!  There aren't instances generated, at
>> least that it can find, of SpeciesId for Data.Data
>> I wanted to enable this by default in Persistent, but it would require
> user code to have DeriveDataTypeable enabled. I can consider putting it back
> in, but I wouldn't mind web-devels input.

For my uses, the only place that this makes life much easier is when using
HStringTemplate.  If it were enabled, you could simply set a row as an
argument directly with 'setAttribute' instead of having to massage the data

Other than that, I can't think of a compelling reason to enable it by

As I write this, I thought perhaps you could add cabal flags to enable this
if you knew that you needed it, but then you start having to litter your
code with #ifdefs and that starts to make code look very ugly very quickly.

Maybe someone has another compelling use case for enabling this by default?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/web-devel/attachments/20110408/34176f4b/attachment.htm>

More information about the web-devel mailing list