v1_eager.d

// Eager version

void main(string[] args)
{
	import std.algorithm, std.file, std.path, std.range, std.uni;
	args[1..$].each!(fn =>
		fn
		.readText()
		.toLower
		.copy(File(fn.setExtension(".lower"), "wb").lockingTextWriter)
	);
}

v2_range_current.d

// Range version - current names

void main(string[] args)
{
	import std.algorithm, std.file, std.path, std.range, std.uni;
	args[1..$].each!(fn =>
		fn
		.readText()
		.toLowerCase()
		.copy(File(fn.setExt(".lower"), "wb").lockingTextWriter)
	);
}

v3_range_proposed1.d

// Range version - proposal 1

void main(string[] args)
{
	import std.algorithm, std.file, std.path, std.range, std.uni;
	args[1..$].each!(fn =>
		fn
		.readText()
		.lowerCased()
		.copy(File(fn.withExtension(".lower"), "wb").lockingTextWriter)
	);
}

v4_range_proposed2.d

// Range version - proposal 2

void main(string[] args)
{
	import std.algorithm, std.file, std.path, std.range, std.uni;
	args[1..$].each!(fn =>
		fn
		.readText()
		.toLowerLazy()
		.copy(File(fn.setExtensionLazy(".lower"), "wb").lockingTextWriter)
	);
}