[Haskell-cafe] Proposal: Non-recursive let

[Edward Kmett]

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 <andreas.abel at ifi.lmu.de> wrote:

> On 22.07.2013 10:50, MigMit wrote:
>> 
>> On Jul 22, 2013, at 12:27 PM, Andreas Abel <andreas.abel at ifi.lmu.de>
>> 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 at ifi.lmu.de
> http://www2.tcs.ifi.lmu.de/~abel/
> 
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