[Template-haskell] splicing types

[Claus Reinke]

>> data Bar {
>>         val1 :: String,
>>         val2 :: Int -- omit this in serialization
>>     }
..
> which could be generalized to a sort of 'annotation type', e.g.
> 
>> type Ann a b = a
>> type Omitted = () -- a particular annotation
> ...
>>        val2 ::  Ann Int Omitted
> 
> you could have multiple annotations, e.g.
>>        val2 :: Ann Int (Omitted,Ann1,Ann2) -- an Int with three annotations

try googling for "phantom types"?

..
>> type Omitted a = ()
> ...
>>        val2 :: Ann Int (Omitted 1000)

if you bake the default into the type, you can have only one
per type per program, so how about using a class to map
from type to default value?

    class Default a where default :: a
    
    valn :: Default a => a

    instance Default Bar where default = Bar undefined 1000
or
    instance Default Int where default = 1000

then you could omit types that have defaults, or use/infer an
omit annotation for fields that the default for the enclosing
type can fill in. Parsing a slightly extended data syntax might
be easier than overloading the type system, but maybe that
is just me?-)

Claus

ps if you absolutely want to go for more type-level
    programming, Oleg has some notes on number-
    parameterized types
    http://okmij.org/ftp/Haskell/number-parameterized-types.html