Fun with Lambdas: C++14 Style (part 1)

Sumant Tambe, Ph.D.
Microsoft Visual C++ MVP
Senior Software Research Engineer
Real-Time Innovations, Inc.
@sutambe
SFBay Association of C/C++ Users
March 12, 2014

Author
Blogger Open-Source Contributor
LEESA
Rx4DDS.NET
Reflection for DDS-XTypes

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» SkillsMatter

» Lambda Expressions
˃ expr.prim.lambda
» Anonymous functions
void abssort(float* x, unsigned N)
{
std::sort(x, x + N,
[](float a, float b) {
return std::abs(a) < std::abs(b);
});
}

class Comp {
float a;
public:
Comp(float x) {
a = x;
}
bool compare(float b) const {
}
};
float array[5] = { 0, 1, 2, 3, 4 };
float a = 3;
Comp f(a);
for(float item : array)
std::cout << std::boolalpha << f.compare(item);

class Comp {
float a;
public:
Comp(float x) {
a = x;
}
bool operator () (float b) const {
}
};
float array[5] = { 0, 1, 2, 3, 4 };
float a = 3;
Comp f(a);
std::cout << std::boolalpha << f(item);

class ##### {
float a;
public:
Foo(float x) {
a = x;
}
}
};
float array[5] = { 0, 1, 2, 3, 4 };
float a = 3;
auto f = #####(a);

class ##### {
float a;
public:
Foo(float x) {
a = x;
}
}
};
float array[5] = { 0, 1, 2, 3, 4 };
float a = 3;
auto f = #####(a);
auto f = [a](float b) { return std::abs(a) < std::abs(b) };

» Anonymous functions
» Written exactly in the place where it's needed
» Can access the variables available in the
enclosing scope (closure)
» May maintain state (mutable or const)
» Can be passed to a function
» Can be returned from a function
» Deduce return type automatically
» Accept generic parameter types (only in C++14)
[a](auto b) { return std::abs(a) < std::abs(b) };

Thanks to Douglas Crockford. Link to Doug’s JavaScript talk

Fun with Lambdas: C++14 Style (part 1)

Write an Identity function that takes
an argument and returns the same
argument.
Identity(3) //3

auto Identity = [](auto x) {
return x;
};

Write 3 functions add, sub, and mul
that take 2 parameters each and return
their sum, difference, and product
respectively.
add(3, 4) // 7
sub(4, 3) // 1
mul(4, 5) // 20

auto add = [](auto x, auto y) {
return x + y;
};
auto sub = [](auto x, auto y) {
return x - y;
};
int mul (int x, int y) {
return x * y;
};

Write a function, identityf, that
takes an argument and returns a
callable that returns that argument
auto idf = identityf(5);
idf() // 5

auto identityf = [](auto x) {
class Inner {
int x;
public: Inner(int i): x(i) {}
int operator() () { return x; }
};
return Inner(x);
};
idf() // 5

return [](){ /* must remember x */ };
};

return [=]() { return x; };
};
idf() // 5

» A lambda is just an anonymous function.
» A closure is a function which closes over the
environment in which it was defined.
» Not all closures are lambdas and not all
lambdas are closures.
» Closures are just function objects in C++
» C++ closures do not extend the lifetime of their
context. (If you need this use shared_ptr)

Write a function that produces a
function that returns values in a range
fromto(0, 10)

auto fromto = [](auto start, auto finish) {
return
};
auto range = fromto(0, 10)
range() // 0
range() // 1
[=]() mutable {
if(start < finish)
return start++;
else
throw std::runtime_error(“Complete");
};

Write a function that adds from two
invocations
addf(5)(4) // 9

auto addf = [](auto x) {
return [=](auto y) {
return x+y;
};
};
addf(5)(4) // 9

Write a function swap that swaps the
arguments of a binary function
swap(sub)(3, 2) // -1

auto sub = [](auto x, auto y) {
return x–y;
};
auto swap =[](auto binary) {
return [=](auto x, auto y) {
return binary(y, x);
};
};
swap(sub)(3, 2) // -1

Write a function twice that takes a
binary function and returns a unary
function that passes its argument to
the binary function twice.
twice(add)(11) // 22

auto twice =[](auto binary) {
return [=](auto x) {
return binary(x, x);
};
};
twice(add)(11) // 22

Write a function that takes a binary
function and makes it callable with two
invocations
applyf(mul)(3)(4) // 12

auto applyf = [](auto binary) {
return [binary](auto x) {
return [binary,x](auto y) {
return binary(x, y);
};
};
};
auto a = applyf(mul);
auto b = a(3);
auto c = b(4) // 12

Write a function that takes a function
and an argument and returns a
function that takes the second
argument and applies the function
curry(mul, 3)(4) // 12

auto curry = [](auto binary, auto x) {
return [=](auto y) {
};
};
curry(mul, 3)(4) // 12

» Currying is the technique of transforming a
function that takes multiple arguments in such
a way that it can be called as a chain of
functions, each with a single argument.
» In lambda calculus functions take a single
argument only.
» Must know Currying to understand Haskell
» Currying != Partial function application

auto addFour = [](auto a, auto b,
auto c, auto d) {
return a+b+c+d;
};
auto partial = [](auto func, auto a,
auto b) {
return [=](auto c, auto d) {
return func(a, b, c, d);
};
};
partial(addFour,1,2)(3,4); //10

Without creating a new function show
3 ways to create the inc function
inc(4) // 5

auto inc = curry(add, 1);
auto inc = addf(1);
auto inc = applyf(add)(1);
inc(4) // 5

Write a function composeu that takes
two unary functions and returns a
unary function that calls them both
composeu(inc, curry(mul, 5))(3) // 20

auto composeu =[](auto f1, auto f2) {
return [=](auto x) {
return f2(f1(x));
};
};
composeu(inc1, curry(mul, 5))(3) // 20

Write a function that returns a function
that allows a binary function to be
called exactly once
once(add)(3, 4) // 7
once(add)(3, 4) // error

auto once = [](auto binary) {
bool done = false;
return [=](auto x, auto y) mutable {
if(!done) {
done = true;
}
else
throw std::runtime_error("once!");
};
};
once(add)(3, 4) // 7
once(add)(3, 4) // exception

Write a function that takes a binary
function and returns a function that
takes two arguments and a callback
and invokes the callback on the result
of the binary function.

auto binaryc = [](auto binary) {
return [=](auto x, auto y, auto callbk) {
return callbk(binary(x,y));
};
};
binaryc(mul)(5, 6, inc) // 31
binaryc(mul)(5,6,[](int a) { return a+1; });

Write 3 functions
1. unit – same as Identityf
2. stringify – that stringifies its
argument and applies unit to it
3. bind – that takes a result of unit
and returns a function that takes a
callback and returns the result of
callback applied to the result of
unit.

auto unit = [](auto x) {
return [=]() { return x; };
};
auto stringify = [](auto x) {
std::stringstream ss;
ss << x;
return unit(ss.str());
};
auto bind = [](auto u) {
return [=](auto callback) {
return callback(u());
};
};

std::cout << "Left Identity "
<< stringify(15)()
<< "=="
<< bind(unit(15))(stringify)()
<< std::endl;
std::cout << "Right Identity "
<< stringify(5)()
<< "=="
<< bind(stringify(5))(unit)()
<< std::endl;
Verify

Fun with Lambdas: C++14 Style (part 1)

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