为什么.array必须在.group之后才能使用.sort？

Question

拥有以下代码:

import std.algorithm : filter, canFind, map, splitter, group, sort;
import std.stdio : File, writefln;
import std.range : array;

void main(string[] args)
{
    string filename = "/var/log/dpkg.log";

    string term = args[1];
    auto results = File(filename, "r")
                    .byLine
                    .filter!(a => canFind(a, term))
                    .map!(a => splitter(a, ":").front)
                    .group
                    .array  // why is this crucial ?
                    .sort!((a,b) => a[1] > b[1]);

    foreach (line; results)
        writefln("%s => %s times", line[0], line[1]);
}

我发现我迫切需要.array后.group.谁能告诉我为什么会这样？

一旦我摆脱它,我得到以下编译器错误:

main.d(16): Error: template std.algorithm.sorting.sort cannot deduce function from argument types !((a, b) => a[1] > b[1])(Group!("a == b", MapResult!(__lambda3, FilterResult!(__lambda2, ByLine!(char, char))))), candidates are:
/usr/include/dmd/phobos/std/algorithm/sorting.d(1830):        std.algorithm.sorting.sort(alias less = "a < b", SwapStrategy ss = SwapStrategy.unstable, Range)(Range r) if ((ss == SwapStrategy.unstable && (hasSwappableElements!Range || hasAssignableElements!Range) || ss != SwapStrategy.unstable && hasAssignableElements!Range) && isRandomAccessRange!Range && hasSlicing!Range && hasLength!Range)

Answer 1

结果group是一个延迟评估的序列,但sort要求其完整输入完全在内存中,例如一个数组.该array函数采用由group它产生的延迟序列并将其存储到sort能够作用的数组中.