calling function templates

[Jos van Uden]

I noticed that I don't always have to use the bang notation for
function templates. I played around with that a little, but got
an error when I used a static array. I think it's because of a
casting problem or wrong type inference... I don't imagine it's
a bug. Just not possible.

module test;

import std.range;

void putNumbers(Range, T)(Range r, T start, T incr) {
T i = start;
while (!r.empty) {
r.put(i);  // line 8
i += incr;
}
}

void main() {

int[] arr = new int[20];
putNumbers!(int[])(arr, 0, 3); // dyn array, bang notation

int[] arr2 = new int[20];
putNumbers(arr2, 0, 3); // dyn array, regular notation

int[20] arr3;
putNumbers!(int[])(arr3, 0, 3); // stat array, bang notation

int[20] arr4;
putNumbers(arr4, 0, 3); // ERROR, stat array, regular notation

}

test.d(8): Error: cast(int[])r is not an lvalue
test.d(25): Error: template instance test.putNumbers!(int[20u],int) 
error instantiating

Re: calling function templates

[bearophile]

And a more general note Jos van Uden: don't use D2, it's unfinished, things 
don't work yet. Use a stable version of D1, like 1.042.

Bye,
bearophile

Re: calling function templates

[Jos van Uden]

bearophile wrote:

And a more general note Jos van Uden: don't use D2, it's unfinished, things 
don't work yet. Use a stable version of D1, like 1.042.

Bye,
bearophile


I'm not using the language. Just trying to learn it. Most code
examples I see, require D2.

Jos

Re: calling function templates

[bearophile]

Jos van Uden:
 I'm not using the language. Just trying to learn it.

To learn a programming language you have to use it some.


Most code examples I see, require D2.

Then don't look at them, and do your own experiments, etc.

Bye,
bearophile

Re: calling function templates

[Bill Baxter]

On Sun, Aug 9, 2009 at 9:30 AM, bearophilebearophileh...@lycos.com wrote:
 Jos van Uden:
 I'm not using the language. Just trying to learn it.

 To learn a programming language you have to use it some.


Most code examples I see, require D2.

 Then don't look at them, and do your own experiments, etc.


Aw come on.  I'd learn D2 if I were just getting into D now.  It's
where all the action is heading these days.
If you don't like dealing with the bleeding edge, then yeh, D1 is
better for now.   But there are reasons why a person might prefer to
learn either.

--bb

Re: calling function templates

[Daniel Keep]

Jos van Uden wrote:
 I noticed that I don't always have to use the bang notation for
 function templates. I played around with that a little, but got
 an error when I used a static array. I think it's because of a
 casting problem or wrong type inference... I don't imagine it's
 a bug. Just not possible.
 
 ...
 
 int[20] arr4;
 putNumbers(arr4, 0, 3); // ERROR, stat array, regular notation
 
 }
 
 test.d(8): Error: cast(int[])r is not an lvalue
 test.d(25): Error: template instance test.putNumbers!(int[20u],int)
 error instantiating

You're instantiating the template as putNumbers!(int[20u],int) because
arr4 is of type int[20u].

This means Range is int[20u].  Here's the code for put:

 void put(T, E)(ref T[] a, E e) {
 assert(a.length);
 a[0] = e; a = a[1 .. $];
 }

r.put(i) is rewritten by the compiler as put(r, i)... except that put
wants an int[], not an int[20u].  So it implicitly casts it to the
correct type, giving put(cast(int[])r, i).

But put ALSO expects its first argument to be passed by reference, and
you cannot pass the result of a cast by-reference.

(There are a LOT of things you can't pass by reference; it's a constant
thorn in my side, and many others' sides.)

The problem here is that put is fundamentally incompatible with
fixed-size arrays.  The solution is to change line 25 to read:

auto arr4_temp = arr4[]; putNumbers(arr4_temp, 0, 3);