Excellent bikeshedding opportunity! Frontend syntax for pattern synonym types

[Dr. ERDI Gergo]

On Sun, 22 Dec 2013, Dr. ERDI Gergo wrote:

>> If it's only A and B, perhaps abominations like these could be considered:
>> 
>>     -- implicit foralls
>>     pattern Show t => P t :: (Num t, Eq b) => b -> T t
>> 
>>     -- explicit foralls
>>     pattern forall t. Show t => P t :: forall b. (Num t, Eq b) => b -> T t
>
> I'm not 100% sure what that 't' in 'P t' is supposed to be in your example. 
> 'P' is not like a type constructor at all; it's a lot more like a data 
> constructor.

Thinking further about it, I think this could work, using a syntax similar 
to data constructor definitions instead of sticking to the function type 
syntax:

pattern (Num a, Eq b) => P a b :: (Show a) => T a

or with explicit foralls (using the fact that we can deduce which tyvars 
are universial vs existential simply by seeing if they occur in 'T a'):

pattern forall a b. (Num a, Eq b) => P a b :: (Show a) => T a

my only concern with this one is that the direction of the first double 
arrow doesn't "feel right".

Other examples with this syntax:

-- Number literal patterns
pattern Z :: (Num a, Eq a) => a 
pattern Z = 0

-- Monomorphic patterns
pattern TrueAnd Bool :: [Bool]
pattern TrueAnd b = [True, b]

-- Infix notation
pattern a :< Seq a :: Seq a
pattern x :< xs <- (Seq.viewl -> x Seq.:< xs)

I'm liking this so far.

Bye,
 	Gergo

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