the part I would really like to avoid is writing the
New.Foo { a=a, b=b, ... z=1 } part, where the field
names are many, long, and varied.
OK, here is another hack-ish trick, since I notice your data is stored
on disk as text, using "show". I assume you are using something like
Read to retrieve it. Well, how about using a real parser instead?
The parser during conversion can be slightly more lax, automatically
adding in the extra field.
For instance, using polyparse's Text.Parse, and DrIFT to derive the
appropriate Parse instance for your datatype:
module Foo where
data Foo = Foo { a :: Int
, b :: Bool
, c :: Maybe Foo }
{-! derive : Parse !-}
DrIFT gives you this instance:
{-* Generated by DrIFT : Look, but Don't Touch. *-}
instance Parse Foo where
parse = constructors
[ ( "Foo"
, return Foo `discard` isWord "{" `apply` field "a"
`discard` isWord "," `apply` field "b"
`discard` isWord "," `apply` field "c"
`discard` isWord "}"
)
]
Let's say the field 'b' is new, and your existing data does not have
it. So just take the parser generated by DrIFT and make a small
modification:
{-* Generated by DrIFT but modified by hand for conversion
purposes *-}
instance Parse Foo where
parse = constructors
[ ( "Foo"
, return Foo `discard` isWord "{" `apply` field "a"
`apply` return True -- this field does not yet exist in
data
`discard` isWord "," `apply` field "c"
`discard` isWord "}"
)
]
Then do the obvious thing: parse the old data, immediately write it
out again, and then throw away the modified parser in favour of the
pure generated one.
Regards,
Malcolm
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