Ok so I implemented both ideas:
> elem2 _ [] = False
> elem2 x [y,: ys] = x == y || elem2 x ys
> let [x,y,z,w,: ws] = "hello there" in ws -- yields "o there"
In fact it was easier to implement both ideas than make a special case of `(:)`. So these work
> let [x,:+ xs] = [1,2,3,:+ Nily] in xs
> let [x,`ConsSet` xs] = [1,2,3,`ConsSet` NilSet] in xs
with decls
> infixr 5 :+
> data Listy a = Nily | a :+ (Listy a) deriving (Eq, Show, Read)
> infixr 5 `ConsSet`
> data Set a = NilSet | ConsSet a (Set a) deriving (Eq, Show, Read)
Ok thank you for the feedback, I get the message not to re-purpose currently valid syntax.
> [x, y ,: ys ] -- ? not currently valid
We could make list syntax work harder
> [x, y ,:+ ys ]
Means desugar the commas by applying constructor `:+` instead of `:`. That could be in general any constructor starting `:`.
Or indeed could be a pattern synonym starting `:`, which is a 'smart constructor' to build the list maintaining some invariant.