On 22/02/2016 11:16, Steven D'Aprano wrote:
On Mon, 22 Feb 2016 12:16 am, BartC wrote:
[...]
No need for anyone to re-invent the
wheel! ;-)
I keep seeing this in the thread. Python has all this capability, yet it
still requires a lot of fiddly code to be added to get anywhere near as
simple as this:
read f, a, b, c
I can't say I really know what that means. My guess is that reads three
things (a, b and c) from a file f, but what those things are (strings,
binary blobs, floats, ints, something else?) is a mystery.
Yes, I mentioned that point. In the OP's language, the variables can be
declared as a particular type; in Python they could perhaps be
'declared' by first assigning with 0, 0.0 or "" for example. But that
would need reference parameters to make use of tidily.
But even when declared, sometimes additional formatting info is needed
(numbers might be in hex or binary for example).
One of the major problems with output parameters is that it isn't obvious
what is an output parameter and what isn't. When it comes to read, perhaps
you can guess, but when it comes to arbitrary functions, who can tell what
is output and what is input?
fromaginate m, x, y, z, a, b
Whereas a functional design makes it obvious: output is on the left of the
assignment symbol, input is on the right.
x, y, z = fromaginate m, a, b
Old-style /statements/ such as 'read' aren't functional. Then it is easy
to specify that the parameters need to be l-values.
readline(f, a, b, c)
I can't see a straightforward way of making this possible while still
keeping a, b and c simple integer, float or string types (because
Python's reference parameters don't work quite the right way).
Python doesn't have reference parameters.
Please feel free to discuss further if you disagree, or want additional
explanation.
Well, I've just finished reimplementing a language so that it uses
CPython-like references for objects (not, however, for types such as
small integers which stay as value types).
But that language retains the use of pointers which can be used /as well
as/ references to provide pass-by-reference, even for integers.
Then it is possible to write a function which can be called like:
readline(f,a,b,c) and which would update the caller's a,b,c variables.
Python can't do this, even with 100% reference objects. This is because
most types are immutable and their values can be shared across unrelated
variables. Try and change one of those, and all would be changed!
In any case, doing an assignment to a parameter just replaces its local
value. Caller's data can only be changed via an in-place modification of
the parameter, which I believe only works for lists.
(Maybe, something like this can be done with classes, but I did say you
can't do it with simple types.)
--
Bartc
--
https://mail.python.org/mailman/listinfo/python-list
