Re: New to std.algorithm and generics, no idea how to make simple things to work

[John Colvin]

On Friday, 7 March 2014 at 19:53:04 UTC, newguy wrote:
I really can't wrap my head around these. I fought whole day
trying to figure out how to do the simplest thing one can
imagine: remove an element from a doubly linked list. Here's 
what
I've tried, see if there is a recurring mistake of thought or
something:
import std.stdio;
import std.algorithm;
import std.range;
import std.container;

struct Point {
     int x;
     int y;
}

void main() {
     DList!Point points;
     points.insert(Point(0,0));
     points.insert(Point(10,10));
     points.insert(Point(5,5));
     points.insert(Point(20,20));

     points.remove(takeOne(find!(p => p.x > 7)(points[])));
     // test.d(18): Error: function
std.container.DList!(Point).DList.remove (Range r) is not
callable using argument types (Result)

     points.linearRemove(takeOne(find!(p => p.x > 
7)(points[])));
     // test.d(21): Error: template
std.container.DList!(Point).DList.linearRemove cannot deduce
function from argument types !()(Result), candidates are:
     // /usr/include/dmd/phobos/std/container.d(2234):
std.container.DList!(Point).DList.linearRemove(R)(R r) if (is(R
== Range))
     // /usr/include/dmd/phobos/std/container.d(2240):
std.container.DList!(Point).DList.linearRemove(R)(R r) if (is(R
== Range))
     // /usr/include/dmd/phobos/std/container.d(2253):
std.container.DList!(Point).DList.linearRemove(R)(R r) if (is(R
== Take!Range))

     points.remove(find!(p => p.x > 7)(points[]));
     // 0

     points.remove(takeOne(filter!(p => p.x > 7)(points[])));
     // test.d(30): Error: function
std.container.DList!(Point).DList.remove (Range r) is not
callable using argument types (Result)

     points.remove(filter!(p => p.x > 7)(points[]));
     // test.d(33): Error: function
std.container.DList!(Point).DList.remove (Range r) is not
callable using argument types (FilterResult!(__lambda3, Range))

     points.linearRemove(filter!(p => p.x > 7)(points[]));
     // test.d(36): Error: template
std.container.DList!(Point).DList.linearRemove cannot deduce
function from argument types !()(FilterResult!(__lambda1,
Range)), candidates are:
     // /usr/include/dmd/phobos/std/container.d(2234):
std.container.DList!(Point).DList.linearRemove(R)(R r) if (is(R
== Range))
     // /usr/include/dmd/phobos/std/container.d(2240):
std.container.DList!(Point).DList.linearRemove(R)(R r) if (is(R
== Range))
     // /usr/include/dmd/phobos/std/container.d(2253):
std.container.DList!(Point).DList.linearRemove(R)(R r) if (is(R
== Take!Range))

     foreach (Point p; points) {
         if (p.x > 7) {
             //points.remove(/* Somehow get the range */);
             break;
         }
     }

     foreach (Point p; points) {
         writeln(p.x);
     }
}


Purpose is to remove one element that matches predicate, or any
amount really. Now DList.remove is defined as "Range 
remove(Range
r)" and filter is "auto filter(Range)(Range rs) if
(isInputRange!(Unqual!Range))" with explanation "The call
filter!(predicate)(range) returns a new range only containing
elements x in range for which predicate(x) is true." So if I
understand correctly, filter should return a range that I can
remove from the list. Why isn't this working?

std.container isn't great and hasn't received enough attention. 
There are plans to improve it, but I believe we're waiting on 
std.allocator and possibly a higher level layer on top of it 
before any significant revamp is made. It is perfectly usable in 
its current state however, just not always totally pleasant.
Don't let this put you off std.algorithm and std.range, which are 
both very high quality.