[Haskell-cafe] Joy Combinators (Occurs check: infinite type)
k.schupke at imperial.ac.uk
Sun Mar 6 04:31:28 EST 2005
Daniel Fischer wrote:
>I don't know Joy, but probably there the stack is (roughly) a heterogenous
>list, which is hard to model in Haskell, think of
>data Element = Bool Bool
> | Char Char
> | Int Int
> . . .
> | IntToBool (Int -> Bool)
> . . .
>type Stack = [Element]
>and how to define functions for that, urgh.
Not saying it isn't tricky - but thats what the HList library
(http://www.cwi.nl/~ralf/HList) implements, a heterogenous list.
It works slightly differently using classes, but does abstract the
workings enough to be useful, to construct an HList you can do:
hlist = "a" .*. (3:Int) .*. False .*. 'v' .*. HNil
And provides head/tail functions and other useful list and set
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