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On 06-09-18 10:50, Chris Angelico wrote: > 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. You may have overlooked where Marko wrote: Actually, this is a more *direct* translation -- Antoon.

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