I like inclusive, exclusive.
Il giorno lunedì 27 gennaio 2014 12:18:37 UTC+11, skaller ha scritto:
>
>
> On 27/01/2014, at 11:49 AM, srean wrote:
>
> > Hi John,
> >
> > whats the intended behaviour for the following (note the use of i).
> >
> > proc f (i:int, n:int)
> > {
> > for var i in st upto n - 1 do
> > println$ i;
> > done
> > println$ i;
> > }
> > f(0,42);
>
> It should be an error, duplicate definition of i (parameter and loop
> variable).
> Parameters are ordinary local variables in the same scope as
> non-parameters.
>
> Ignoring that it should print the last value the control variable had
> before exit, in particular it should NOT go "one past the end".
> [So the final i should be n - 1 not n]
>
> The reason for that is simple enough: if you're scanning the full finite
> range of some type, you cannot go past the end, it would be an illegal
> operation.
>
> But see below....
>
> > I find it a little disconcerting that loop variables can be accessed
> > outside.
>
> Standard C. The "var" is just a convenience to avoid
>
> var i:int; for i in ...
>
> which has an uninitialised variable and you have to say its type.
>
> You can jump into a for loop, and an if then else chain. Remember
> these are *low level macros* and they're always constructed
> with gotos.
>
> You need to understand that Felix does not have "blocks" like C or C++.
> Blocks use up the machine stack for local variables, which is what
> protects the loop control variable in C++.
>
> Felix hates the machine stack. It prevents use of channels and all other
> service calls (because these are implemented by returning control to the
> scheduler, which implies the machine stack must hold the return address
> of the scheduler).
>
> If you want to you can do this:
>
> begin for var i ... end
>
> begin .. end is a block. more precisely its a procedure which is called
> exactly like
>
> { ... };
>
> in particular it means:
>
> (proc () { ... }) ();
>
> If you do this you can NOT return from the containing procedure, that is:
>
> begin for i in 0 upto 10 do
> return; // returns from the begin/end block NOT the
> containing function
> end;
> println "Hello";
>
>
> > Seems a like an inviting trap for me to fall into.
>
> You're too used to C++. This is standard C. Loop variables are
> accessible outside loops, in fact you have to declare them outside
> loops. This is useful. You can do this in C++ too.
>
> > Also a
> > summary of when are ranges "right" inclusive and when are they right
>
> At the moment ranges are inclusive EXCEPT in string slices which follow
> Python.
>
> And the handling of 0 is therefore suspect!
>
> I would like to change all this to be consistent (no matter what it
> breaks).
>
> The problem is I don't know how. In general a loop which can handle:
>
> (a) empty case
> (b) the full range of the type
> (c) signed values
> (d) unsigned values
> (e) both up and down counting
> (f) doesn't even make the counter go to an illegal value
>
> cannot exist with just two limits and a single specifier (upto, downto).
>
> C loops can handle it because the user choses the comparator,
> but this is fraught with difficulties. Its a common source of error.
> And it is very hard to optimise (because such a loop has no semantics).
>
> I have been tempted to do
>
> for i in 0 to n exclusive do ..
> for i in 0 to n inclusive do ...
>
> or
>
> for i in [0,n-1) do ..
> for i in [0,n-1] do ..
>
> or something (like math ranges) but really the lack of balanced parens
> there would
> make a serious mess of syntax highlighting.
>
> --
> john skaller
> ska...@users.sourceforge.net <javascript:>
> http://felix-lang.org
>
>
>
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