Sorry, I think I misunderstood your question, if I understand correctly you want some function to convert nested Expr to Fix'ed Exprs. You can do that with a typeclass, but you have to provide the Fix'ed type at the bottom: -------------------------------------- {-# LANGUAGE FlexibleInstances #-} data E e = Lit Int | Add e e data Fix f = Fix {unFix :: f (Fix f)} type Expr = Fix E lit :: Int -> Expr lit = Fix . Lit add :: Expr -> Expr -> Expr add e1 e2 = Fix (Add e1 e2) term :: Expr term = add (lit 1) (add (lit 2) (lit 3)) class FixE e where fix :: e -> Expr instance FixE Expr where fix = id instance FixE e => FixE (E e) where fix (Lit i) = lit i fix (Add e1 e2) = add (fix e1) (fix e2) ------------------------------------ This is because your `term' works since you don't have any occurrence of `expr' at leaves of your Expr tree, and that works because the leaves are all literals. However, we can't guarantee this statically. We can, of course, write an unsafe instance based on the assumption that the the values at the leaves of the expression tree will be literals: ------------------------------------ instance FixE (E e) where fix (Lit i) = lit i fix _ = error "non-literal!" ------------------------------------ Francesco. On 06/05/12 13:59, Sebastien Zany wrote:
Hi,
Suppose I have the following types:
data Expr expr = Lit Nat | Add (expr, expr) newtype Fix f = Fix {unFix :: f (Fix f)}
I can construct a sample term:
term :: Expr (Expr (Expr expr)) term = Add (Lit 1, Add (Lit 2, Lit 3))
But isn't quite what I need. What I really need is:
term' :: Fix Expr term' = Fix . Add $ (Fix . Lit $ 1, Fix . Add $ (Fix . Lit $ 2, Fix . Lit $ 3))
I feel like there's a stupidly simple way to automatically produce term' from term, but I'm not seeing it.
Any ideas?
Best, Sebastien
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