[Haskell-cafe] record update

[Chris Eidhof]

For completeness, using fclabels (yet another record package) you can write it like this:


> {-# LANGUAGE TemplateHaskell #-}
> module Records where
> 
> import Data.Record.Label
> 
> data MyRecord = MyRecord { _field1 :: String, _field2 :: Int, _field3 :: Bool }
> 
> $(mkLabels [''MyRecord])
> 
> modifyThree f g h = modL field1 f
>                   . modL field2 g
>                   . modL field3 h

-chris

On 11 sep 2010, at 19:21, Jonathan Geddes wrote:

> 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
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