The worst piece of syntax in Haskell

[kahl at cas.mcmaster.ca]

Ashley Yakeley <ashley at semantic.org> wrote:
 > 
 > Josef Svenningsson wrote:
 > 
 > > This is one of the things that the Clean people got right. In Clean, my 
 > > examples from above would look like:
 > > 
 > >  > class MonadPlus m | Monad m where ...
 > >  >
 > >  > class Ix a | Ord a where ..
 > >  >
 > >  > instance Eq (Ratio a) | Integral a where ...
 > 
 > Not quite the same complaint, but I've always been bothered by the 
 > inconsistent use of "=>". I would prefer "A => B" to mean "if A, then 
 > B". Accordingly:
 > 
 >    class Monad m <= MonadPlus m
 >    class Ord a <= Ix a
 >    instance Integral a => Eq (Ratio a)
 >    foo :: (Monad m) => [m a] -> m [a]

It may be useful to keep in mind how this would translate into the module
language of OCaml
(remember that in *ML, type application is written argument-first):

module type MonadPlus =
   functor (M: Monad) ->
     sig
       type m 'a = 'a M.m
       val mzero : 'a m
       val mplus : 'a m -> 'a m -> 'a m
     end;;

module type Eq =
  sig
    type t
    val (==) : t -> t -> bool
    val (/=) : t -> t -> bool
  end;;

module IntegralRatioEq :: Eq
   functor (I: Integral) ->
     struct
       type t = I.t ratio
       let (==) = ...   
     end;

I am not claiming that all aspects of the syntax are more elegant,
but I think it makes the semantics clearer.
(Of course, OCaml does not have implicit module arguments,
 so ``foo :: (Monad m) => [m a] -> m [a]'' has no direct translation.)

<plug>See also ``Named Instances for Haskell Type Classes'':
http://www.cas.mcmaster.ca/~kahl/Publications/Conf/Kahl-Scheffczyk-2001.html
</plug>



 > >  > instance Eq (Ratio a) | Integral a

Those who think of type classes as a medium for logic programming
might indeed prefer

instance Eq (Ratio a) :- Integral a

;-)


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