Redundant function calls in filtered joined range

FreeSlave freeslave93 at
Sat Jul 18 19:32:06 UTC 2020

Consider the following code. It counts the number of 
subdirectories in directories provided via commandline args.

import std.stdio;
import std.algorithm;
import std.file;
import std.range;
import std.exception;

@trusted bool isDirNothrow(string dir) nothrow
     bool ok;
     collectException(dir.isDir(), ok);
     return ok;

auto subDirs(Range)(Range searchDirs)
if(is(ElementType!Range : string))
     return searchDirs
         .filter!(isDirNothrow).map!(function(dir) {
             return dir.dirEntries(SpanMode.shallow)

int main(string[] args)
     auto r = args[1..$].subDirs;
     writefln("Found %s subdirs", count(r));
     return 0;

Using strace I inspect how many stat calls it uses. Like this:

strace -C -e stat path/to/binary

strace reports some duplicated stat calls, specifically on the 
first provided directory and first subdirectory of each 

How can I avoid redundant stat calls? Adding array() after each 
filter() removes duplicates but it leads to allocations 
obviously. Changing the cache() to array() before joiner also 
solve the issue.

I suspect there's something wrong in combination of filters and 

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