Re: class instance with nested types
On Fri, 27 Oct 2000, Matthias Höchsmann wrote: Hello, I have the following problem: basic datatypes type Sequence a = [a] data Tree a = N a (Forest a) deriving (Ord,Eq,Show) type Forest a = Sequence (Tree a) i want to construct a class Xy class Xy s a where test :: s a - a and make an instance for list of characters instance Xy [] Char where test [a] = a this works, and an instance for a forest and tried something like this instance ([] Tree) Char where test x@(N a xs):txs = a Don't you mean test (N a xs:txs) = a ? /Lars L ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: class instance with nested types
Matthias Höchsmann wrote: type Sequence a = [a] data Tree a = N a (Forest a) deriving (Ord,Eq,Show) type Forest a = Sequence (Tree a) i want to construct a class Xy class Xy s a where test :: s a - a [...] instance ([] Tree) Char where test x@(N a xs):txs = a To make it syntactically correct this should at least be something like instance Xy ([] Tree) Char where test (N a xs:txs) = a But the real problem is in the expression ([] Tree), which is the same as writing [Tree]. This is not a legal type expression, since Tree is a type constructor, not a ground type, so you cannot apply it to the list constructor. What you are trying to say is probably something like this: instance Xy (\a . [Tree a]) Char -- not Haskell But unfortunately there are no lambdas on the type level - they would render the type system undecidable. For the same reason it is not allowed to use a type synonym in an instance declaration: instance Xy Forest Char -- illegal The only thing you can do is turning Forest into a data type: data Tree a = N a (Forest a) deriving (Ord,Eq,Show) data Forest a = Forest [Tree a] instance Xy Forest Char where test (Forest (N a xs:txs)) = a HTH, - Andreas -- Andreas Rossberg, [EMAIL PROTECTED] :: be declarative. be functional. just be. :: ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: class instance with nested types
I mumbled: This is not a legal type expression, since Tree is a type constructor, not a ground type, so you cannot apply it to the list constructor. The other way round, of course: you cannot apply the list constructor to it. - Andreas -- Andreas Rossberg, [EMAIL PROTECTED] :: be declarative. be functional. just be. :: ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: class instance with nested types
Yes, I wanted to type it like you do. But anyway, i fixed the problem following Andreas Rossbergs suggestion. Matthias Don't you mean test (N a xs:txs) = a ? /Lars L ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
