Re: How is this code supposed to work?
On Friday, 25 January 2019 at 16:19:38 UTC, Neia Neutuladh wrote:
On Fri, 25 Jan 2019 09:34:47 +, AndreasDavour wrote:
auto point3 = getResponse!Point!int("What's the point? ");
This could be:
getResponse!(Point!int)
or:
(getResponse!Point)!int
D requires this to be disambiguated at the parsing stage,
before the compiler works out what getResponse might be.
An example of the latter:
template getResponse(alias n)
{
alias getResponse = n;
}
getResponse!Point is just Point.
I'm not sure why that tutorial has it wrong, but the way to get
it fixed is to contact the author.
Thanks! I had some issues with the syntax, and getting that part
correct got me to the conceptual bits, which is where I got lost
for real. Syntax errors are such a drag, though.
Appreciated.
Re: How is this code supposed to work?
On Friday, 25 January 2019 at 14:25:33 UTC, AndreasDavour wrote:
https://run.dlang.io/is/a4oDFZ is an example of how this
looks. I feel like there's more to these templates than meet
the eye.
To clarify. I really don't understand the thinking behind these
templates, and wonder a bit about why a tutorial text like that
contains examples that doesn't compile. There must be a
conceptual unclarity somewhere I feel, not just where the
parenthesis goes.
Finally the penny dropped. So for the poor sods who will find
this in a web search in the future, I will add to my my monologue
and show off both my stupidity and maybe some hints on how to
understand what was going on. I know nothing of C++ and if
templates are a concept from there, so maybe that was why I did
not get it.
The chapter from "Programming D" I was referring to in my first
post did start to talk about function templates, and then moved
on to step by step show templating of structs, and functions to
work upon those. What I did not grasp was that these were all
part of the code. The author made me think, by his choice of
words or my preconceptions I do not know, that he showed
incremental additions to the function template and sequentially
the struct template, when he was in fact showing new templates
all together. So, if I included all the templates, specializing
on Point, and then string, and only T, and so on, it worked as
intended.
What I did not grasp with the code of this template:
Point!T getResponse(T : Point!T)(string question) {
writefln("%s (Point!%s): ", question, T.stringof);
auto x = getResponse!T(" x");
auto y = getResponse!T(" y");
return Point!T(x, y);
}
was that the "inner" call to getResponse() is not a private
method within this scope, and thus dispatching on the same type
(which would make it recursive), but if you had that other
templates specializing on other types it would call the correct
function. I had the templating system down as a one pass
search-and-replace, but it's clearly more dynamic than that. So I
was correct there was a conceptual un-clarity present. With me.
This brings up an interesting point. How do you write a text that
teaches these things? If you break up the code in small blocks
where you explain the concepts step by step, and show variants
and how you can expand the abilities of the code by using more
and more complex features, how do you do that so all the code is
self contained? Maybe it can not always be done, and should you
then have the final code in a block at the end, or do you note
clearly in the text that *this* block is an example, but *this
new* code block is complete and will work? It would be verbose if
you in every instance would quote boilerplate like the import
std.* and so on. I'm not sure I know how to best do it. Teaching
is hard.
Re: How is this code supposed to work?
On Friday, 25 January 2019 at 12:11:51 UTC, AndreasDavour wrote:
On Friday, 25 January 2019 at 12:09:34 UTC, AndreasDavour wrote:
On Friday, 25 January 2019 at 09:36:24 UTC, rikki cattermole
wrote:
On 25/01/2019 10:34 PM, AndreasDavour wrote:
[...]
auto point3 = getResponse!(Point!int)("What's the point? ");
[...]
I thought about that option, if it was unclear to the parser
that Point!int was the type to instantiate upon. Changing to
the other syntax made me wonder what would happen when
getResponse is calling itself, which in self was something I
am a bit unsure of how to understand. Because we get the same
syntax issue there and it does not seem to work.
https://run.dlang.io/is/a4oDFZ is an example of how this looks.
I feel like there's more to these templates than meet the eye.
To clarify. I really don't understand the thinking behind these
templates, and wonder a bit about why a tutorial text like that
contains examples that doesn't compile. There must be a
conceptual unclarity somewhere I feel, not just where the
parenthesis goes.
Re: How is this code supposed to work?
On Friday, 25 January 2019 at 12:09:34 UTC, AndreasDavour wrote:
On Friday, 25 January 2019 at 09:36:24 UTC, rikki cattermole
wrote:
On 25/01/2019 10:34 PM, AndreasDavour wrote:
[...]
auto point3 = getResponse!(Point!int)("What's the point? ");
[...]
I thought about that option, if it was unclear to the parser
that Point!int was the type to instantiate upon. Changing to
the other syntax made me wonder what would happen when
getResponse is calling itself, which in self was something I am
a bit unsure of how to understand. Because we get the same
syntax issue there and it does not seem to work.
https://run.dlang.io/is/a4oDFZ is an example of how this looks. I
feel like there's more to these templates than meet the eye.
Re: How is this code supposed to work?
On Friday, 25 January 2019 at 09:36:24 UTC, rikki cattermole
wrote:
On 25/01/2019 10:34 PM, AndreasDavour wrote:
How am I supposed to use that??
auto point3 = getResponse!Point!int("What's the point? ");
auto point3 = getResponse!(Point!int)("What's the point? ");
auto point4 = getResponse!Point!int("What's the point? ");
writeln("Distance: ", point3.distanceTo(point4));
generates the error:
struct_templates.d(48): Error: multiple ! arguments are not
allowed
struct_templates.d(49): Error: multiple ! arguments are not
allowed
Which makes me wonder about the syntax.
I thought about that option, if it was unclear to the parser that
Point!int was the type to instantiate upon. Changing to the other
syntax made me wonder what would happen when getResponse is
calling itself, which in self was something I am a bit unsure of
how to understand. Because we get the same syntax issue there and
it does not seem to work.
How is this code supposed to work?
I'm reading the "Programming D" here:
http://ddili.org/ders/d.en/templates.html
and am a bit confused by the section on templates.
with a struct template like this:
struct Point(T) {
T x;
T y;
T distanceTo(Point that) const {
immutable real xDistance = x - that.x;
immutable real yDistance = y - that.y;
immutable distance = sqrt((xDistance * xDistance) +
(yDistance * yDistance));
return cast(T) distance;
}
}
and a function template like this:
Point!T getResponse(T : Point!T)(string question) {
writefln("%s (Point!%s): ", question, T.stringof);
auto x = getResponse!T(" x");
auto y = getResponse!T(" y");
return Point!T(x, y);
}
How am I supposed to use that??
auto point3 = getResponse!Point!int("What's the point? ");
auto point4 = getResponse!Point!int("What's the point? ");
writeln("Distance: ", point3.distanceTo(point4));
generates the error:
struct_templates.d(48): Error: multiple ! arguments are not
allowed
struct_templates.d(49): Error: multiple ! arguments are not
allowed
Which makes me wonder about the syntax.
Re: Reversing a string
On Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote: On Friday, 11 January 2019 at 09:41:30 UTC, bauss wrote: On Friday, 11 January 2019 at 08:25:41 UTC, Seb wrote: On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote: Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...] Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC What a terrible name. Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1...@digitalmars.com?page=1 -=mike=- Look at that. Incidentally, I kind of like "foreach(i; 99 .. 0 : -1)".
Re: Reversing a string
On Friday, 11 January 2019 at 09:41:30 UTC, bauss wrote: On Friday, 11 January 2019 at 08:25:41 UTC, Seb wrote: On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote: Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...] Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC What a terrible name. Well, it was not the first one I would have searched for, no. Good to know it exists as well, though. Thanks!
Re: Reversing a string
On Friday, 11 January 2019 at 08:45:12 UTC, JN wrote: On Friday, 11 January 2019 at 08:15:01 UTC, rikki cattermole wrote: Note the immutable, it means you cannot modify individual values. Which is a problem for reverse because it modifies in place. The error message is kind of unfortunate. This is a simple usecase and the error message is undecipherable already. It'd be cool if the compiler could try to strip immutability, and if the type matches then, throw an error something like "Cannot pass immutable char[] to reverse, did you mean char[]?". That would help a lot, as I got "rikki cattermole"'s answer at once, when my eyes were brought to the "immutable" part. I come from the lisp world, so I'm kind of familiar with the idea of copying and/or modifying in place to limit consing. I guess I should I have realised this would be a perfect example of that kind of situation.
Reversing a string
Hi.
I've just started to learn some D, so maybe this question is
extremely stupid, but please bear with me.
I have been trying to reverse a string, and through some googling
found the std.algorithm.mutation.reverse method. But, when I try
to use it I get some very verbose errors that just makes my eyes
glaze over. What on earth do they mean? How do I reverse a
string, really?
This is my code:
import std.stdio;
import std.algorithm;
void main() {
auto test = "This is my text";
string next_test = reverse(test);
writeln(test);
writeln(next_test);
}
This is my error:
[ante@tiny ~/src/D/tutorial]$ rdmd string4.d
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorithm/mutation.d(2513):
Error: template `std.algorithm.mutation.reverse` cannot deduce function from
argument types `!()(immutable(ubyte)[])`, candidates are:
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorithm/mutation.d(2485):
`std.algorithm.mutation.reverse(Range)(Range r) if (isBidirectionalRange!Range &&
(hasSwappableElements!Range || hasAssignableElements!Range && hasLength!Range &&
isRandomAccessRange!Range || isNarrowString!Range && isAssignable!(ElementType!Range)))`
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorithm/mutation.d(2521):
Error: template `std.algorithm.mutation.reverse` cannot deduce function from
argument types `!()(immutable(ubyte)[])`, candidates are:
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorithm/mutation.d(2485):
`std.algorithm.mutation.reverse(Range)(Range r) if (isBidirectionalRange!Range &&
(hasSwappableElements!Range || hasAssignableElements!Range && hasLength!Range &&
isRandomAccessRange!Range || isNarrowString!Range && isAssignable!(ElementType!Range)))`
string4.d(6): Error: template instance
`std.algorithm.mutation.reverse!string` error instantiating
Failed: ["/usr/home/ante/src/dmd2/freebsd/bin64/dmd", "-v",
"-o-", "string4.d", "-I."]
--
Can some unpack that for me, please?
I might add that the most intuitive way to reverse a string, I
think, would be to consider it a range and just do "newstring =
oldstring[$ .. 0]" but that gives an error about being out of
bounds, so I guess the compiler expect values to increase from
left to right, and does not want to increment beyond $. Makes
sense, but that syntax would be very neat.
