Hi, In these cases is good to define smart constructors, e.g.:
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))
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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