I'm wrong or Will we fix the ducks limp?
bc at freeuk.com
Thu Jun 9 06:42:17 EDT 2016
On 09/06/2016 11:03, Marko Rauhamaa wrote:
> Antoon Pardon <antoon.pardon at rece.vub.ac.be>:
>> Your challenge, shows that you don't fully understand what reference
>> variables are. The behaviour you see in Pascal, doesn't depend (alone)
>> on the parameter being a reference parameter. It also depends on the
>> fact that the assignment in pascal mutates the variable that is
>> assigned to. Variables are references if they are aliases, so that if
>> you mutate through one alias, the mutation is visible through other
>> aliases. So your challenge comes down to expecting me to mutate
>> something by means that in python don't allow mutation.
> I think bringing Pascal in this discussion is only confusing matters.
> Let me repeat the abstract Python data model I gave a couple of days
> - there are labeled *pegs* ("variables")
> - there are *puppies* ("objects")
> - each peg has one *leash* hanging from it
> - each leash is tied to a puppy
> - each puppy can have zero one or more leashes tied to it
> - some puppies can hold leashes in their *mouths*
> - some puppies can take hold of new leashes and let go of leashes
> I'm not joking. Everybody is arguing about preconceived notions tied to
> terminology. The peg-leash-puppy model is accurate and extensive.
> We can now give semantics to Python's execution model. For example,
> - every rvalue expression evaluates to a leash
> - the lvalue expression identifies a peg or a mouth
> - the assignment statement hangs a leash on a peg or in a mouth
And to implement swap() you need a leash that can be tied to a peg.
(Here, I'm assuming 'hanging from' and 'tied to' suggest that a leash is
uni-directional. But it's possible to imagine that a different part of a
peg is used to hang an out-going leash, compared to tying an incoming
one. Just like the puppies have necks to tie incoming leashes to (via
collars to avoid cruelty) and mouths for out-going ones.)
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