[Haskell-cafe] Playing with OverloadedLabels in GHC 8 RC2, how to do this?

Adam Gundry adam at well-typed.com
Fri Feb 26 22:08:45 UTC 2016

On 23/02/16 22:07, Daniel Díaz wrote:
> I was wondering: if I define a bunch of records in a module, how to make
> this the behaviour for all records in the module, without much
> boilerplate and without affecting any records elsewhere?
> One possible solution would be to define a empty type class that will
> not be exported:
>     class Marker r
> and the following instance:
>     instance (Marker r,IsLabel symbol (r -> String)) => IsLabel symbol
>     (r -> Text) where
>         fromLabel _ =  Text . fromLabel (proxy# :: (Proxy# symbol))

Unfortunately this will overlap with any fields defined elsewhere that
return Text, which is perhaps not ideal.

> And make every record in the module an instance of Marker:
>     instance Marker Person
> I'm not sure if there's a simpler way.

I think ultimately we want to pick a single IsLabel instance for the
function space, to be defined in base. That will create a standard way
to use overloaded labels with records. This is discussed a bit on the
wiki [1]. Probably the instance should delegate to another class that
captures which fields belong to which records. Unfortunately there are
some design trade-offs, so it's not entirely clear what this instance
should look like. The plan is to experiment with the options in 8.0 and
try to commit to something in a future GHC release.

> Even if we don't export the fields directly, another way to employ
> OverloadedLabels (OverloadedRecordFields, once it arrives) is for giving
> default implementations of public interfaces, in combination with
> DefaultSignatures. A not very useful example:
>     class Named r where
>         name :: r -> String      
>         default name :: IsLabel "name" (r -> String) => r -> String
>         name = #name
>            instance Named Person

Thanks, this is an interesting use case that hadn't occurred to me.

All the best,



Adam Gundry, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/

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