[Haskell-cafe] Proposal: Non-recursive let
Andreas Abel
andreas.abel at ifi.lmu.de
Wed Jul 24 10:14:56 CEST 2013
Sure. I have not looked a concrete strictness analyses, but I expect
they would treat Conat differently than Integer. In particular,
x does *not* appear strictly in S x
if S is a lazy constructor.
On 22.07.13 4:54 PM, Edward Kmett wrote:
> 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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--
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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