On 28.02.2017 17:54, M.-A. Lemburg wrote:
> On 28.02.2017 17:35, David Mertz wrote:
>> Clearly there is SOME semantics that is consistent and coherent since many
>> languages have either a lazy or eager declaration syntax, with different
>> defaults between languages but both being built in. There are a lot of
>> ways that Python isn't Haskell, obviously. But both Scheme and OCaml are
>> eager by default with a lazy declaration (and Haskell or Miranda have an
>> eager declaration correspondingly).
>>
>> It might be worth looking at their semantics in the PEP.
>
> Scheme, for example, uses an explicit approach to turning
> a promise into a value:
>
> http://www.shido.info/lisp/scheme_lazy_e.html
>
> This makes a lot of sense, but you can already have the
> same in Python using generators.
Here's an example similar to OCaml's lazy evaluation, which
uses a simple lazy proxy object.
import sys
import time
### OCaml like lazy evaluation
class Lazy:
def __init__(self, code, frame):
self.code = code
self.globals = frame.f_globals
self.locals = frame.f_locals
def force(self):
return eval(self.code, self.globals, self.locals)
def lazy(code):
return Lazy(code, sys._getframe(1))
###
def log(level, b, c):
if level > 100:
return
if isinstance(c, Lazy):
c = c.force()
print ('%04i: %s' % (level, b % c))
def expensive(x):
time.sleep(2.0)
return x
value = 1
log(1000, 'Hello %i', lazy("expensive(value)"))
log(10, 'Error %i', lazy("expensive(value)"))
###
Everything is nice and explicitly defined in the example.
You can see where the deferred evaluation is requested
and where it's eventually run. There are no surprises.
--
Marc-Andre Lemburg
eGenix.com
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