wip: Start adding lambdas lecture

Author: niosus

lectures/lambdas.md

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+Lambdas
+--
+
+<p align="center">
+  <a href="https://youtu.be/blah"><img src="https://img.youtube.com/vi/blah/maxresdefault.jpg" alt="Video" align="right" width=50% style="margin: 0.5rem"></a>
+</p>
+
+We've talked about so many things, like classes and functions but there is one more thing that modern C++ has that we did not really touch upon - lambdas.
+
+Here's what they are useful for. Imagine we have a vector of people, represented as a struct `Person`, and we would like to sort them by age. We can use the standard [`std::sort`](https://en.cppreference.com/w/cpp/algorithm/sort) function for that.
+
+```cpp
+#include <algorithm>
+#include <iostream>
+#include <string>
+#include <vector>
+
+struct Person {
+  std::string name;
+  int age;
+};
+
+void Print(const std::vector<Person>& persons) {
+  for (const auto& person : persons) {
+    std::cout << person.name << " " << person.age << "\n";
+  }
+}
+
+int main() {
+  std::vector<Person> people{
+      {"Gendalf", 55'000}, {"Frodo", 33}, {"Legolas", 2'931}, {"Gimli", 139}};
+  Print(people);
+  std::sort(
+      people.begin(), people.end(),
+      [](const auto& left, const auto& right) { return left.age < right.age; });
+  std::cout << "------ sorted --------" << std::endl;
+  Print(people);
+}
+```
+The third argument to the `std::sort` function here is the lambda expression that essentially stands in for a comparison operator between the objects of the person class.
+
+So let's talk about lambdas! What they are, how to write them to stay safe and efficient and, yes, how they make this valid C++ code:
+```cpp
+int main() {
+  [](){}();
+}
+```
+
+<!-- Intro -->
+
+## Before lambdas we had functors
+The concept of something "callable" that we can pass into a function or even store for a while is not new to C++. It existed long before lambdas were introduced into the language.
+
+Let's pause for a moment and talk a bit about what it means that something is "callable". Essentially it means that we can call it through a `()` operator with the expected number of arguments. So if we write a function `less(const Person&, const Person&)` and pass its pointer to `std::sort` it will do the trick:
+```cpp
+#include <algorithm>
+#include <iostream>
+#include <string>
+#include <vector>
+
+struct Person {
+  std::string name;
+  int age;
+};
+
+void Print(const std::vector<Person>& persons) {
+  for (const auto& person : persons) {
+    std::cout << person.name << " " << person.age << "\n";
+  }
+}
+
+bool less(const Person& p1, const Person& p2) { return p1.age < p2.age; }
+
+int main() {
+  std::vector<Person> people{
+      {"Gendalf", 55'000}, {"Frodo", 33}, {"Legolas", 2'931}, {"Gimli", 139}};
+  Print(people);
+  std::sort(people.begin(), people.end(), &less);
+  std::cout << "------ sorted --------" << std::endl;
+  Print(people);
+}
+```
+
+But what if this is not enough? What if we need to have a certain state? For example, we wouldn't want to sort the people by their absolute age, but by the difference of their age with respect to some number, say `4242`.
+
+Behold **function objects**, or **functors**. These are objects for which the function call operator is defined, or, in other words, that define an operator `()`.