Hello, I have a recursive type
data Foo = A | B [Foo] | C [Foo]
that I would like to restrict so that a C can only contain Bs, and a B can only contain As. If I use a GADT as follows the bar function, understandably, will not type check.
data AType data BType data CType
data Foo a where A :: Foo AType B :: [Foo AType] -> Foo BType C :: [Foo BType] -> Foo CType
--get the successor of the Foo bar c@(C [] ) = c bar (C (b:_)) = b bar b@(B [] ) = b bar (B (a:_)) = a bar a@A = a
How do I achieve this restriction? Do I need to have some sort of successor class with dependent types? Ta Daniel
My thanks for the responses received off-list. Dimitrios pointed out that a 'successor' class solution would require the typechecking to depend somehow on whether the lists were empty or not. Probably a clue that I was on the wrong track. Both Cale and Dimitrios suggested better solutions involving different data types: data FooA = A data FooB = B [FooA] data FooC = C [FooB] data Foo = FA FooA | FB FooB | FC FooC or data Node a = N [a] deriving Show data Tree a = Zero a | Succ (Tree (Node a)) deriving Show or data EXFoo where EXFoo :: Foo a -> EXFoo Embarrassingly simple! :) Thanks Daniel On Friday 17 February 2006 19:18, Daniel McAllansmith wrote:
Hello,
I have a recursive type
data Foo = A | B [Foo] | C [Foo]
that I would like to restrict so that a C can only contain Bs, and a B can only contain As. If I use a GADT as follows the bar function, understandably, will not type check.
data AType data BType data CType
data Foo a where A :: Foo AType B :: [Foo AType] -> Foo BType C :: [Foo BType] -> Foo CType
--get the successor of the Foo bar c@(C [] ) = c bar (C (b:_)) = b bar b@(B [] ) = b bar (B (a:_)) = a bar a@A = a
How do I achieve this restriction? Do I need to have some sort of successor class with dependent types?
Ta Daniel _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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Daniel McAllansmith