Any SML coders able to translate this to Python?
Chris Angelico
rosuav at gmail.com
Thu Sep 6 04:50:12 EDT 2018
On Thu, Sep 6, 2018 at 6:44 PM, Marko Rauhamaa <marko at pacujo.net> wrote:
> Chris Angelico <rosuav at gmail.com>:
>
>> On Thu, Sep 6, 2018 at 2:29 PM, Marko Rauhamaa <marko at pacujo.net> wrote:
>>> Marko Rauhamaa <marko at pacujo.net> (Marko Rauhamaa):
>>>> Steven D'Aprano <steve+comp.lang.python at pearwood.info>:
>>>>> I have this snippet of SML code which I'm trying to translate to Python:
>>>>>
>>>>> fun isqrt n = if n=0 then 0
>>>>> else let val r = isqrt (n/4)
>>>>> in
>>>>> if n < (2*r+1)^2 then 2*r
>>>>> else 2*r+1
>>>>> end
>>>> [...]
>>>> You must make sure "r" doesn't leak outside its syntactic context so:
>>>>
>>>> def isqrt(n):
>>>> if n == 0:
>>>> return 0
>>>> else:
>>>> def f2398478957():
>>>> r = isqrt(n//4)
>>>> if n < (2*r+1)**2:
>>>> return 2*r
>>>> else:
>>>> return 2*r+1
>>>> return f2398478957()
>>>
>>> Actually, this is a more direct translation:
>>>
>>> def isqrt(n):
>>> if n == 0:
>>> return 0
>>> else:
>>> def f2398478957(r):
>>> if n < (2*r+1)**2:
>>> return 2*r
>>> else:
>>> return 2*r+1
>>> return f2398478957(isqrt(n//4))
>>>
>>
>> I don't understand why you created that nested function instead of
>> something simple like renaming the variable. Is there a difference
>> here?
>
> Yes, in understanding the semantics of "let."
>
> "let" is used to introduce local bindings in some functional programming
> languages. I must admit I'm not fully versed in ML but it looks like the
> analogue in Lisp variants. This is how the above function would be
> written in Scheme:
>
> (define (isqrt n)
> (if (= n 0)
> 0
> (let ((r (isqrt (quotient n 4))))
> (if (< n (expt (1+ (* 2 r)) 2))
> (* 2 r)
> (1+ (* 2 r))))))
>
> Now, Lisp's "let" can be implemented/defined using "lambda":
>
> (let ((X A) (Y B) ...) . BODY)
>
> =>
>
> ((lambda (X Y ...) . BODY) A B ...)
>
> which gives us:
>
> (define (isqrt n)
> (if (= n 0)
> 0
> ((lambda (r)
> (if (< n (expt (1+ (* 2 r)) 2))
> (* 2 r)
> (1+ (* 2 r))))
> (isqrt (quotient n 4)))))
>
> Python does have a limited form of "lambda" and even a conditional
> expression so--as others have mentioned--this particular function could
> be translated pretty directly into Python using its lambda.
>
> More generally and idiomatically, though, Python's functions are named.
> So that explains the version I give above.
And even more idiomatically, Python doesn't require a new scope just
for a new variable. So a much more idiomatic translation would be to
simply ensure that the inner variable can't collide, and then ignore
the function boundary. And I'm not sure if there even is a name
collision. What's the issue with scoping at all here? What's the inner
function actually achieving?
ChrisA
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