Re: [Haskell-cafe] Proposal: Non-recursive let

let x = x +1 is perfectly cromulent when x is sufficiently lazy, e.g. in the one point compactification of the naturals: data Conat = S Conat | Z There it represents infinity with proper sharing. -Edward On Jul 22, 2013, at 10:24 AM, Andreas Abel wrote:

On 22.07.2013 10:50, MigMit wrote:

...

On Jul 22, 2013, at 12:27 PM, Andreas Abel wrote:

...

On 20.07.13 9:36 PM, Evan Laforge wrote:

...

However, I'm also not agitating for a non-recursive let, I think that ship has sailed. Besides, if it were added people would start wondering about non-recursive where, and it would introduce an exception to haskell's pretty consistently order-independent declaration style.

For functions, recursive-by-default let makes sense. But for *values*, intended recursion is rather the exception. It is useful for infinite lists and the like. For values of atomic type like Int or Bool, recursive let is a bug.

It seems hard to distinguish between them. What about values that contain functions, like data T = T Int (Int -> Int)? What about polymorphic values, that could be functions and could be not?

I agree. It cannot be implemented like that. A thing that could be implemented is that

let x = e

is an error if x appears strictly in e. In practice, this could catch some unintended cases of recursion like

let x = x +1

, but not all of them.

Cheers, Andreas

-- Andreas Abel <>< Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY

andreas.abel@ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe