Ruby の Object#tap をD言語で書いてみた．デバッグ用途にどうぞ．

gistfile1.d

auto tap(alias fun, Range)(Range r)
{
    alias fun_ = unaryFun!fun;
    struct ResultType
    {
        this(Range r) { this.r = r; }
        @property auto front()
        {
            auto ret = r.front;
            fun_(ret);
            return ret;
        }

        void popFront() { r.popFront(); }
        @property auto empty() { return r.empty; }
    private:
        Range r;
    }
    return ResultType(r);
}

unittest
{
    {
        auto a = iota(3);
        auto b = iota(3).tap!(e => e);
        assert(equal(a, b));
    }

    {
        auto a = iota(3).map!"2*a";
        auto b = iota(3).tap!"3*a".map!"2*a";
        assert(equal(a, b));
    }
}