On 20 February 2018 at 11:18, Antoon Pardon <antoon.par...@vub.be> wrote:
> Personnally I would prefer the type system of Pascal and Modula2 with
> their interval type
> above a Digit class in python. For the simple reason that once you had
> declared a variable
> like this:
> x: 1 .. 10;
>
> Each assignment to x would then implicitly do something like an assert
> to checks the constraint,
> so it would be impossible to ever assign 11 to x, without an error being
> thrown.
>
> There is no such possibility in Python. You can off course start with x
> = Digit(5), but the language
> won't stop you from doing x = 11 later.
All that is saying is that in Pascal, variables have types, whereas in
Python values have types but variables (names ;-)) don't. It's true,
but not particularly important in toy examples like this. In larger
scale programs, tracking the "type" of what gets assigned to a
variable can be really useful, and carefully managed types can help
with this. Pascal/Modula2 (and C/C++) have the compiler do this,
Python has a separate tool (MyPy). Early C compilers used to have
linters that did some checks outside of the compiler - maybe someday
Python will build MyPy into the compiler (although I suspect not).
Also, tracking "types" is only half the battle. There are probably
very few people who have never mistakenly assigned NULL to a pointer
in C that shouldn't be null. Or even in Java, which is supposed to not
allow things like that. And tracking const types in C is widely
acknowledged to be a mixed blessing at best. Where does "type" end and
"documented constraint or programming contract" start?
> I'm not proficient with C++, but IIUC, you could make a class in C++ and
> have the constructor and
> copy operator check for these kind of things. True it would be run-time
> checks but that would
> already be more than Python can give.
That (run-time checks) is exactly the same as Python gives. In C++,
variables have types so you could ensure that you could only assign a
value of your digit type to a given variable, but as I said above
that's a separate point.
It's somewhat unrelated (scoping is a different topic than assignment)
but you can do
{
int x = 2;
{
char *x = "hello";
}
}
in C, so names can have different types even in C (it's just
variables, or names within a specific scope, that have types
associated with them).
Summary: Different programming languages have different semantics.
Which really isn't that surprising...
Paul
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