[Haskell] Proposal: Allow "\=" for field update in record update syntax

[Keean Schupke]

Yes, I have unreleased (yet) TH code for generating globally unique
labels, and lifting records.... you can do:

$(ttypelift [| data Record = Record {
    field1 :: Int,
    field2 :: String } |] )

and it lifts this to an HList style record with labels

field1
field2

Labels are assigned unique type level values by converting the
string of the label name to a numeric value which is encoded by
type level natural numbers.

    Keean.


Benjamin Franksen wrote:

>I think that the best solution is to define record labels as types, or rather 
>type proxies, like for instance in the HList library. This fixes the most 
>important deficiencies of Haskell98 records:
>
>- labels are now first class values
>- labels no longer need to be globally unique, but only unique per record
>- operations to get or set a field are normal (overloaded) functions (and
>  can be given operator aliases, if desired)
>
>This library class defines the operations on a record:
>
>	class RecordField r l t | r l -> t where
>	  getField :: l -> r -> t
>	  putField :: l -> t -> r -> r
>
>	updateField :: (Field r l t) => l -> (t -> t) -> r -> r
>	updateField lbl fun rec = putField lbl (fun $ getField lbl rec) rec
>
>The record declaration
>
>	data R = R {
>	    field1 :: T1,
>	    field2 :: T2
>	  }
>
>would be syntactic sugar for
>
>	data R = R T1 T2
>
>	data Label_field1
>
>	field1 :: Label_field1
>	field1 = undefined
>
>	instance RecordField R Label_field1 T1 where
>	  getField (Rec x _) _ = x
>	  putField (Rec _ y) _ v = Rec v x
>
>	-- analogous definitions for field2 left out
>
>Note that the compiler would leave out the definition of Label_field1 and 
>field1 if these are already in scope.
>
>Alexanders example
>
>  
>
>>    fun rec = rec // $(u field1 fn) . $(a field2 val)
>>    
>>
>
>resp.
>
>  
>
>>    fun rec = rec // u_field1 fn . a_field2 val
>>    
>>
>
>could now be written thus
>
>	fun = updateField field1 fn . putField field2 val
>
>without any need for additional syntax or infix operator splices.
>
>I wonder if something similar could be done with TH. The labels would need to 
>have a different name (e.g. l_field1, l_field2), so they don't collide with 
>their Haskell98 definitions, but otherwise everything should be as above. I 
>am thinking of something like
>
>	$(generateLabels R)
>
>(I am not very familiar with TH, so this could be wrong syntax or otherwise 
>impossible to do.)
>
>Ben
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