New Tools for a More Functional C++

New Tools for a More
Functional C++
Sumant Tambe
Sr. Software Engineer, LinkedIn
Microsoft MVP
SF Bay Area ACCU
Sept 28, 2017

Blogger (since 2005)
Coditation—Elegant Code for Big Data
Author (wikibook) Open-source contributor
Since 2013 (Visual Studio and Dev Tech)

Functional Programming
in C++
by Ivan Cukic
(ETA early 2018)
Reviewer

Sum Types and (pseudo) Pattern Matching

Modeling Alternatives
Inheritance vs std::variant
• States of a game of Tennis
• NormalScore
• DeuceScore
• AdvantageScore
• GameCompleteScore

Modeling game states using std::variant
struct NormalScore {
Player p1, p2;
int p1_score, p2_score;
};
struct DeuceScore {
Player p1, p2;
};
struct AdvantageScore {
Player lead, lagging;
};
struct GameCompleteScore {
Player winner, loser;
int loser_score;
};
using GameState = std::variant<NormalScore, DeuceScore,
AdvantageScore, GameCompleteScore>;

Print GameState (std::variant)
struct GameStatePrinter {
std::ostream &o;
explicit GameStatePrinter(std::ostream& out) : o(out) {}
void operator ()(const NormalScore& ns) const {
o << "NormalScore[" << ns.p1 << ns.p2 << ns.p1_score << ns.p2_score << "]";
}
void operator ()(const DeuceScore& ds) const {
o << "DeuceScore[" << ds.p1 << "," << ds.p2 << "]";
}
void operator ()(const AdvantageScore& as) const {
o << "AdvantageScore[" << as.lead << "," << as.lagging << "]";
}
void operator ()(const GameCompleteScore& gc) const {
o << "GameComplete[" << gc.winner << gc.loser << gc.loser_score << "]";
}
};
std::ostream & operator << (std::ostream& o, const GameState& game) {
std::visit(GameStatePrinter(o), game);
return o;
}

std::visit spews blood when you miss a case

Print GameState. Fancier!
std::visit(overloaded {
[&](const NormalScore& ns) {
o << "NormalScore" << ns.p1 << ns.p2 << ns.p1_score << ns.p2_score;
},
[&](const DeuceScore& gc) {
},
[&](const AdvantageScore& as) {
},
[&](const GameCompleteScore& gc) {
}
}, game);
return o;
}

Passing Two Variants to std::visit
std::visit(overloaded {
[&](const NormalScore& ns, const auto& other) {
o << "NormalScore" << ns.p1 << ns.p2 << ns.p1_score << ns.p2_score;
},
[&](const DeuceScore& gc, const auto& other) {
},
[&](const AdvantageScore& as, const auto& other) {
},
[&](const GameCompleteScore& gc, const auto& other) {
}
}, game, someother_variant);
return o;
}
There can be arbitrary number of arbitrary variant types.
The visitor must cover all cases

A Template to Inherit Lambdas
template <class... Ts>
struct overloaded : Ts... {
using Ts::operator()...;
explicit overloaded(Ts... ts) : Ts(ts)... {}
};
A User-Defined Deduction Guide
explicit overloaded(Ts... ts) : Ts(ts)... {}
template <class... Ts> overloaded(Ts...) -> overloaded<Ts...>;

Next GameState Algorithm (all cases in one place)
GameState next (const GameState& now,
const Player& who_scored)
{
return std::visit(overloaded {
[&](const DeuceScore& ds) -> GameState {
if (ds.p1 == who_scored)
return AdvantageScore{ds.p1, ds.p2};
else
return AdvantageScore{ds.p2, ds.p1};
},
[&](const AdvantageScore& as) -> GameState {
if (as.lead == who_scored)
return GameCompleteScore{as.lead, as.lagging, 40};
else
return DeuceScore{as.lead, as.lagging};
},
[&](const GameCompleteScore &) -> GameState {
throw "Illegal State";
},
[&](const NormalScore& ns) -> GameState {
if (ns.p1 == who_scored) {
switch (ns.p1_score) {
case 0: return NormalScore{ns.p1, ns.p2, 15, ns.p2_score};
case 15: return NormalScore{ns.p1, ns.p2, 30, ns.p2_score};
case 30: if (ns.p2_score < 40)
return NormalScore{ns.p1, ns.p2, 40, ns.p2_score};
else
return DeuceScore{ns.p1, ns.p2};
case 40: return GameCompleteScore{ns.p1, ns.p2, ns.p2_score};
default: throw "Makes no sense!";
}
}
else {
switch (ns.p2_score) {
case 0: return NormalScore{ns.p1, ns.p2, ns.p1_score, 15};
case 15: return NormalScore{ns.p1, ns.p2, ns.p1_score, 30};
case 30: if (ns.p1_score < 40)
return NormalScore{ns.p1, ns.p2, ns.p1_score, 40};
else
return DeuceScore{ns.p1, ns.p2};
case 40: return GameCompleteScore{ns.p2, ns.p1, ns.p1_score};
default: throw "Makes no sense!";
}
}
}
}, now);
}

Modeling Alternatives with Inheritance
class GameState {
std::unique_ptr<GameStateImpl> _state;
public:
void next(const Player& who_scored) {}
};
class GameStateImpl {
Player p1, p2;
public:
virtual GameStateImpl * next(
const Player& who_scored) = 0;
virtual ~GameStateImpl(){}
};
class NormalScore : public GameStateImpl {
public:
GameStateImpl * next(const Player&);
};
class DeuceScore : public GameStateImpl {
public:
};
class AdvantageScore : public GameStateImpl {
int lead;
public:
};
class GameCompleteScore :public GameStateImpl{
int winner, loser_score;
public:
};

Sharing State is easier with Inheritance
class GameState {
std::unique_ptr<GameStateImpl> _state;
public:
void next(const Player& who_scored) {}
Player& who_is_serving() const;
double fastest_serve_speed() const;
GameState get_last_state() const;
};
class GameStateImpl {
Player p1, p2;
int serving_player;
double speed;
GameState last_state;
public:
virtual GameStateImpl * next(
const Player& who_scored) = 0;
virtual Player& who_is_serving() const;
virtual double fastest_serve_speed() const;
virtual GameState get_last_state() const;
};

Sharing Common State is Repetitive with std::variant
Player who_is_serving = std::visit([](auto& s) {
return s.who_is_serving();
}, state);
Player who_is_serving = state.who_is_serving();
ceremony!
Player p1, p2;
int serving_player;
Player & who_is_serving();
};
struct DeuceScore {
Player p1, p2;
int serving_player;
};
int serving_player;
};
int loser_score;
int serving_player;
};

How about recursive std::variant?
Player p1, p2;
int serving_player;
};
struct DeuceScore {
Player p1, p2;
int serving_player;
};
int serving_player;
};
int loser_score;
int serving_player;
};
Not possible unless you use recursive_wrapper and dynamic allocation.
Not in C++17.
Dare I say, it’s not algebraic? It does not compose 
std::variant is a container. Not an abstraction.

std::variant disables fluent interfaces
{
using GameState = std::variant<NormalScore, DeuceScore,
AdvantageScore, GameCompleteScore>;
GameState state = NormalScore {..};
GameState last_state = std::visit([](auto& s) {
return s.get_last_state();
}, state);
double last_speed = std::visit([](auto& s) {
return state.fastest_serve_speed();
}, last_state);
double last_speed = state.get_last_state().fastest_serve_speed();
}
ceremony!

Combine Implementation Inheritance with
std::variant
{
using GameState = std::variant<NormalScore_v2, DeuceScore_v2,
AdvantageScore_v2, GameCompleteScore_v2>;
GameState state = NormalScore_v2 {..};
Player who_is_serving = std::visit([](SharedGameState& s) {
return s.who_is_serving();
}, state);
Player who_is_serving = state.who_is_serving();
}
SharedGameState
who_is_serving()
NormalScore_v2 DeuceScore_v2 AdvantageScore_v2 GameCompleteScore_v2
ceremony!

Modeling Alternatives
Inheritance std::variant
Dynamic Allocation No dynamic allocation
Intrusive Non-intrusive
Reference semantics (how will you copy a
vector?)
Value semantics
Algorithm scattered into classes Algorithm in one place
Language supported
Clear errors if pure-virtual is not implemented
Library supported
std::visit spews blood on missing cases
Creates a first-class abstraction It’s just a container
Keeps fluent interfaces Disables fluent interfaces. Repeated std::visit
Supports recursive types (Composite) Must use recursive_wrapper and dynamic
allocation. Not in the C++17 standard.

const is shallow
struct X {
void bar();
};
struct Y {
X* xptr;
explicit Y(X* x) : xptr(x) {}
void foo() const {
xptr->bar();
}
};
{
const Y y(new X);
y.foo(); // mutates X??
}

Deep Immutability: propagate_const<T>
struct X {
void bar();
void bar() const; // Compiler error without this function
};
struct Y {
X* xptr;
propagate_const<X *> xptr;
explicit Y(X* x) : xptr(x) {}
void foo() const {
xptr->bar();
}
};
{
const Y y(new X);
y.foo(); // calls X.bar() const
}

Deep Immutability: propagate_const<T>
#include <experimental/propagate_const>
using std::experimental::propagate_const;
{
propagate_const<X *> xptr;
propagate_const<std::unique_ptr<X>> uptr;
propagate_const<std::shared_ptr<X>> shptr;
const propagate_const<std::shared_ptr<X>> c_shptr;
shptr.get() === X*
c_shptr.get() === const X*
*shptr === X&
*c_shptr === const X&
get_underlying(shptr) === shared_ptr<X>
get_underlying(c_shptr) === const shared_ptr<X>
shptr = another_shptr; // Error. Not copy-assignable
shptr = std::move(another_shptr) // but movable
Library fundamental TS v2

The Named Parameter Idiom (mutable)
class configs {
std::string server;
std::string protocol;
public:
configs & set_server(const std::string& s);
configs & set_protocol(const std::string& s);
};
start_server(configs().set_server(“localhost”)
.set_protocol(“https”));

The Named Parameter Idiom (immutable)
class configs {
public:
configs set_server(const std::string& s) const {
configs temp(*this); temp.server = s; return temp;
}
configs set_protocol(const std::string& proto) const {
configs temp(*this); temp.protocol = proto; return temp;
}
};
Avoid
copy-constructors?

The Named Parameter Idiom (immutable*)
class configs {
public:
}
}
configs set_server(const std::string& s) && {
server = s; return *this;
}
configs set_protocol(const std::string& proto) && {
protocol = proto; return *this;
}
};
&
&

The Named Parameter Idiom (immutable*)
class configs {
public:
}
}
configs&& set_server(const std::string& s) && {
server = s; return *this; std::move(*this);
}
configs&& set_protocol(const std::string& proto) && {
protocol = proto; return *this; std::move(*this);
}
};
&
&

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