[Haskell-cafe] Fixed point newtype confusion

[Francesco Mazzoli]

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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