Re: [Haskell-cafe] record update

13 Sep 2010


      Am Samstag, den 11.09.2010, 11:21 -0600 schrieb Jonathan Geddes:
...
I know that record updates is a topic that has become a bit of a dead
horse, but here I go anyway:
I find that most of the record updates I read and write take the form
...
someUpdate :: MyRecord -> MyRecord
someUpdate myRecord = myRecord
    { field1 = f $ field1 myRecord
    , field2 = g $ field2 myRecord
    , field3 = h $ filed3 myRecord
    }
I find myself wishing I could write something more like
...
someUpdate :: MyRecord -> MyRecord
someUpdate myRecord = myRecord
    { field1 => f
    , field2 => g
    , field3 => h
    }
with equivalent semantics. Here => reads "is transformed by". Operator
= could still be used for assignment as in current record updates.
The best part about such an extension, in my opinion, is that it would
open the door for anonymous lambda record updates. Something like:
...
someUpdate :: MyRecord -> MyRecord
someUpdate = \{field1 => f, field2 => g, field3 => h}
again, with the same semantics. This becomes possible because you no
longer need to refer to the record within the {} part of the update.
This would be useful, for example, in the State monad. We could write:
...
someStateTransform :: State MyRecord ()
someStateTransform = do
    modify $ \{field1 => (++"!")}
    ...
where currently we see code like
...
someStateTransform :: State MyRecord ()
someStateTransform = do
    modify $ \record->record{field1 = (++"!") $ field1 record}
    ...
which repeats the record name 3 times and the field name twice. The
repetition just feels out of place next to all the other terse,
readable Haskell code in the program.
So what do my fellow haskellers think? Is this idea worth writing up a
proposal for?
Alternatively, can you offer me some advice on writing code in Haskell
2010 that avoids the ugly, repetitive style of record update?
--Jonathan
You might want to have a look at the records package:

    http://hackage.haskell.org/package/records

Here is a code example:
...
import Data.Kind
import Data.TypeFun
import Data.Record
import Data.Record.Combinators
data Surname = Surname deriving (Show)
data Age     = Age     deriving (Show)
data Room    = Room    deriving (Show)
instance Name Surname where name = Surname
instance Name Age     where name = Age
instance Name Room    where name = Room
oldData = X :& Surname := "Jeltsch"
            :& Age     := 31
            :& Room    := "EH/202"
            `withStyle` Id KindStar
newData = modify (X :& Age := (+2) :& Room := const "HG/2.39") oldData
Evaluating newData gives you:
...
X :& Surname := "Jeltsch" :& Age := 33 :& Room := "HG/2.39"
If you have any question regarding the records package, please ask,
since I’m its author. :-) 

Best wishes,
Wolfgang

Re: [Haskell-cafe] record update

Wolfgang Jeltsch