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July 2021

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Re: [Hugs-users] [Haskell-cafe] Prolog-style list syntax?
by Anthony Clayden 07 Jul '21

07 Jul '21

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) On Wed, 30 Jun 2021 at 19:00, Anthony Clayden <anthony.d.clayden(a)gmail.com> wrote: > 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. > > >

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