[Haskell-cafe] typeclasses considered unnecessary/the power of the @

[MarLinn]

On 2018-05-16 04:09, Anthony Clayden wrote:
> Haskell 98/2010 keeps a strict distinction between term-level syntax 
> and tokens vs type-level.
>
> With ExplicitTypeApplications that's being eased: there's a 
> lightweight way to sneak a type expression into a term. And ghc's 
> internal term language 'core' has had explicit type applications for 
> many years.
>
> Then consider a lightweight syntax using type applications in patterns 
> to define typeclass instances by (pattern) 'matching' on types:
>
> > (==) @Int x y = primEqInt x y         -- or eta-reduced
> > (==) @Int     = primEqInt             -- shorthand for
> >
> > instance Eq Int where
> >   (==) = primEqInt
>
> Given that many typeclasses have a single method (or one base method 
> with others defined in terms of it) [Note **], having to invent a 
> typeclass name is a bit of a pain. Here's a lighweight method decl:
>
> > meth :: @a @b. => a -> b -> Bool  -- typevar binding to left of => 
> (there might also be superclass constraints)
> >                                       -- shorthand for
> >
> > class Classfor_meth a b where         -- generated class name
> >   meth :: a -> b -> Bool

As Oleg Kiselyov pointed out over a decade ago, all typeclasses can be 
reduced to just one, namely [1]

	class C l t | l -> t where ac :: l -> t

Which in turn is basically an untagged version of the HasField typeclass 
that's being used to create overloaded records. [2]

class  HasField  (x  ::  k)r  a  |  x  r  ->  a  where  getField  ::  r  ->  a

(Not very surprising, because classes are basically compile-time records)

In other words

 1. You only have one class to replace (left as an exercise to the reader)
 2. I don't think you need any fancy new tricks or syntax for
    multi-function classes
 3. You might be able to also implement records with just type
    application and type families

Cheers,
MarLinn

[1] http://okmij.org/ftp/Haskell/Haskell1/Haskell1.txt
[2] 
https://github.com/adamgundry/ghc-proposals/blob/overloaded-record-fields/proposals/0000-overloaded-record-fields.rst#hasfield-class

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20180518/543cd975/attachment.html>