NDC{TechTown} Higher order functions for ordinary C++ developers

Higher Order Functions – NDC{Techtown} 2018 © Björn Fahller @bjorn_fahller
1/74 compose([](auto const& s) { return s == "foo";}, std=:mem_fn(&foo=:name)) Higher Order Functions for Ordinary C++ Developers Björn Fahller

2/74 Very brief intro to higher order functions Gradually expanding example optional<T> (and extension) Higher Order Functions for Ordinary C++ Developers

3/74 Definition A higher-order function is a function that takes other functions as arguments or returns a function as result.

4/74 #include <algorithm> #include <vector> std=:vector<int> v; ==. if (std=:none_of(std=:begin(v), std=:end(v), [](int x) { return x == 0; })) { ==. }

5/74 #include <algorithm> #include <vector> std=:vector<int> v; ==. if (std=:none_of(std=:begin(v), std=:end(v), [](int x) { return x == 0; })) { ==. } template <typename Iterator, typename Predicate> bool none_of(Iterator i, Iterator e, Predicate predicate) { while (i == e) { if (predicate(*i)) return false; i=+; } return true; }

6/74 #include <algorithm> #include <vector> std=:vector<int> v; ==. if (std=:none_of(std=:begin(v), std=:end(v), [](int x) { return x == 0; })) { ==. } ==. int num; ==. while (std=:any_of(std=:begin(v), std=:end(v), [num](int x){ return x == num; })) { ==. }

7/74 [num](int x){ return x == num; }

8/74 auto equals(int num) { return [num](int x){ return x == num; }; }

9/74 auto equals(int num) { return [num](int x){ return x == num; }; } std=:function<bool(int)> equals(int num) { return [num](int x) { return x == num; }; }

10/74 template <typename T> auto equals(T key) { return [key](auto const& x){ return x == key; }; }

11/74 template <typename T> auto equals(T key) { return [key](auto const& x){ return x == key; }; } template <typename T> std=:function<bool(T)> equals(T key) { return [key](T x) { return x == key; }; }

12/74 template <typename T> auto equals(T key) { return [key](auto const& x){ return x == key; }; } template <typename T> std=:function<bool(T)> equals(T key) { return [key](T x) { return x == key; }; } Callable with anything equality comparable with T, for example C-string and std=:string

13/74 template <typename T> auto equals(T key) { return [key](auto const& x){ return x == key; }; } template <typename T> std=:function<bool(T)> equals(T key) { return [key](T x) { return x == key; }; } Callable with anything equality comparable with T, for example C-string and std=:string Only callable with T, because std=:function=> does not have a templated function call operator.

14/74 template <typename T> auto equals(T key) { return [key](auto const& x){ return x == key; }; }

15/74 #include <algorithm> #include <vector> std=:vector<int> v; ==. if (std=:none_of(std=:begin(v), std=:end(v), equals(0)) { ==. } ==. int num; ==. while (std=:any_of(std=:begin(v), std=:end(v), equals(num)) { ==. }

16/74 Live demo!

17/74 ✔Very brief intro to higher order functions Gradually expanding example optional<T> (and extension) Higher Order Functions for Ordinary C++ Developers

18/74 struct ip_address { ip_address(uint8_t i1, uint8_t i2, uint8_t i3, uint8_t i4) : num((uint32_t(i1) =< 24) | (uint32_t(i2) =< 16) | (uint32_t(i3) =< 8) | i4) {} ip_address(uint32_t n) : num(n) {} bool operator==(ip_address rh) const { return num == rh.num;} bool operator==(ip_address rh) const { return !(*this == rh);} uint32_t num; }; struct netmask : ip_address { using ip_address=:ip_address; }; inline ip_address operator&(ip_address lh, netmask rh) { return {lh.num & rh.num}; };

22/74 struct ip_address { ip_address(uint8_t i1, uint8_t i2, uint8_t i3, uint8_t i4) : num((uint32_t(i1) =< 24) | (uint32_t(i2) =< 16) | (uint32_t(i3) =< 8) | i4) {} ip_address(uint32_t n) : num(n) {} bool operator==(ip_address rh) const { return num == rh.num;} bool operator==(ip_address rh) const { return !(*this == rh);} uint32_t num; }; struct netmask : ip_address { using ip_address=:ip_address; }; inline ip_address operator&(ip_address lh, netmask rh) { return {lh.num & rh.num}; }; auto ip_matches(ip_address desired, netmask mask = netmask{255,255,255,255}) { return [desired, mask](ip_address actual) { return (desired & mask) == (actual & mask); }; }

25/74 struct ip_address { ip_address(uint8_t i1, uint8_t i2, uint8_t i3, uint8_t i4) : num((uint32_t(i1) =< 24) | (uint32_t(i2) =< 16) | (uint32_t(i3) =< 8) | i4) {} ip_address(uint32_t n) : num(n) {} bool operator==(ip_address rh) const { return num == rh.num;} bool operator==(ip_address rh) const { return !(*this == rh);} uint32_t num; }; struct netmask : ip_address { using ip_address=:ip_address; }; inline ip_address operator&(ip_address lh, netmask rh) { return {lh.num & rh.num}; }; auto ip_matches(ip_address desired, netmask mask = netmask{255,255,255,255}) { return [desired, mask](ip_address actual) { return (desired & mask) == (actual & mask); }; } std=:vector<ip_address> v; ==. auto i = std=:remove_if(v.begin(), v.end(), ip_matches({192,168,1,1}, {255,255,0,0}));

26/74 class ipif { public: using state_type = enum { off, on }; ==. void set_state(state_type); state_type state() const { return state_; } ip_address address() const { return addr_; } netmask mask() const { return mask_; } ip_address gateway() const { return gw_; } private: ip_address addr_; netmask mask_; ip_address gw_; state_type state_; };

27/74 class ipif { public: using state_type = enum { off, on }; ==. void set_state(state_type); state_type state() const { return state_; } ip_address address() const { return addr_; } netmask mask() const { return mask_; } ip_address gateway() const { return gw_; } private: ip_address addr_; netmask mask_; ip_address gw_; state_type state_; };

28/74 class ipif { public: using state_type = enum { off, on }; ==. void set_state(state_type); state_type state() const { return state_; } ip_address address() const { return addr_; } netmask mask() const { return mask_; } ip_address gateway() const { return gw_; } private: ip_address addr_; netmask mask_; ip_address gw_; state_type state_; }; To match, for example the address of an ipif, we need to make the ip_matches() predicate work on a member.

29/74 class ipif { public: using state_type = enum { off, on }; ==. void set_state(state_type); state_type state() const { return state_; } ip_address address() const { return addr_; } netmask mask() const { return mask_; } ip_address gateway() const { return gw_; } private: ip_address addr_; netmask mask_; ip_address gw_; state_type state_; }; Given: f1(y) -> z and f2(x) -> y We want a composition f(x)->z as f1(f2(x))

30/74 class ipif { public: using state_type = enum { off, on }; ==. void set_state(state_type); state_type state() const { return state_; } ip_address address() const { return addr_; } netmask mask() const { return mask_; } ip_address gateway() const { return gw_; } private: ip_address addr_; netmask mask_; ip_address gw_; state_type state_; }; Given: f1(y) -> z ip_matches(ip_address) -> bool and f2(x) -> y select_address(ipif) -> ip_address We want a composition f(x)->z as f1(f2(x))

31/74 class ipif { public: using state_type = enum { off, on }; ==. void set_state(state_type); state_type state() const { return state_; } ip_address address() const { return addr_; } netmask mask() const { return mask_; } ip_address gateway() const { return gw_; } private: ip_address addr_; netmask mask_; ip_address gw_; state_type state_; }; template <typename F1, typename F2> auto compose(F1 f1, F2 f2) { return [=](auto const& x) { return f1(f2(x)); }; } Given: f1(y) -> z ip_matches(ip_address) -> bool and f2(x) -> y select_address(ipif) -> ip_address We want a composition f(x)->z as f1(f2(x))

32/74 class ipif { public: ==. ip_address address() const { return addr_;} ==. private: ip_address addr_; ==. }; std=:vector<ipif> interfaces; auto i = std=:find_if(interfaces.begin(), interfaces.end(), compose(ip_matches({192,168,1,1}), select_address));

33/74 class ipif { public: ==. ip_address address() const { return addr_;} ==. private: ip_address addr_; ==. }; ip_address select_address(ipif const& interface) { return interface.address(); } std=:vector<ipif> interfaces; auto i = std=:find_if(interfaces.begin(), interfaces.end(), compose(ip_matches({192,168,1,1}), select_address));

34/74 class ipif { public: ==. ip_address address() const { return addr_;} ==. private: ip_address addr_; ==. }; auto address_matches(ip_address addr, netmask mask = {255,255,255,255}) { return compose(ip_matches(addr, mask), select_address); } std=:vector<ipif> interfaces; auto i = std=:find_if(interfaces.begin(), interfaces.end(), compose(ip_matches({192,168,1,1}), select_address);

35/74 class ipif { public: ==. ip_address address() const { return addr_;} ==. private: ip_address addr_; ==. }; auto address_matches(ip_address addr, netmask mask = {255,255,255,255}) { return compose(ip_matches(addr, mask), select_address); } std=:vector<ipif> interfaces; auto i = std=:find_if(interfaces.begin(), interfaces.end(), address_matches({192,168,1,1}));

36/74 class ipif { public: using state_type = enum { off, on }; ==. void set_state(state_type); state_type state() const { return state_; } ip_address address() const { return addr_;} netmask mask() const { return mask_; } ip_address gateway() const { return gw_; } ==. }; inline ip_address select_gateway(ipif const& interface) { return interface.gateway(); } inline ipif=:state_type select_state(ipif const& interface) { return interface.state(); }

37/74 auto i = find_if(v.begin(), v.end(), when_all(address_matches({192,168,1,1},{255,255,0,0}), state_is(ipif=:off)));

38/74 template <typename ==. Predicates> auto when_all(Predicates ==. ps) { return [=](auto const& x) { return (ps(x) =& ==.); }; } auto i = find_if(v.begin(), v.end(), when_all(address_matches({192,168,1,1},{255,255,0,0}), state_is(ipif=:off)));

39/74 template <typename ==. Predicates> auto when_all(Predicates ==. ps) { return [=](auto const& x) { return (ps(x) =& ==.); }; } auto address_matches(ip_address addr, netmask mask=netmask{255,255,255,255}) { return compose(ip_matches(addr, mask), select_addr); } auto i = find_if(v.begin(), v.end(), when_all(address_matches({192,168,1,1},{255,255,0,0}), state_is(ipif=:off)));

40/74 template <typename ==. Predicates> auto when_all(Predicates ==. ps) { return [=](auto const& x) { return (ps(x) =& ==.); }; } auto address_matches(ip_address addr, netmask mask=netmask{255,255,255,255}) { return compose(ip_matches(addr, mask), select_addr); } auto state_is(ipif=:state_type state) { return compose(equals(state), select_state); } auto i = find_if(v.begin(), v.end(), when_all(address_matches({192,168,1,1},{255,255,0,0}), state_is(ipif=:off)));

41/74 for_each(v.begin(), v.end(), if_then(when_all(address_matches({192,168,1,1}, {255,255,0,0}), state_is(ipif=:off)), set_state(ipif=:on)));

42/74 template <typename Predicate, typename Action> auto if_then(Predicate predicate, Action action) { return [=](auto=& x) { if (predicate(x)) { action(std=:forward<decltype(x)>(x)); } }; } for_each(v.begin(), v.end(), if_then(when_all(address_matches({192,168,1,1}, {255,255,0,0}), state_is(ipif=:off)), set_state(ipif=:on)));

43/74 template <typename Predicate, typename Action> auto if_then(Predicate predicate, Action action) { return [=](auto=& x) { if (predicate(x)) { action(std=:forward<decltype(x)>(x)); } }; } auto set_state(ipif=:state_type state) { return [=](ipif& interface) { interface.set_state(state); }; } for_each(v.begin(), v.end(), if_then(when_all(address_matches({192,168,1,1}, {255,255,0,0}), state_is(ipif=:off)), set_state(ipif=:on)));

44/74 Live demo!

45/74 ✔Very brief intro to higher order functions ✔Gradually expanding example optional<T> (and extension) Higher Order Functions for Ordinary C++ Developers

46/74 std=:optional<T>

47/74 std=:optional<T> • The class template std=:optional manages an optional contained value, i.e. a value that may or may not be present.

48/74 std=:optional<T> • The class template std=:optional manages an optional contained value, i.e. a value that may or may not be present. • Since C++17 – available for older versions in various open source libraries

49/74 std=:optional<T> • The class template std=:optional manages an optional contained value, i.e. a value that may or may not be present. • Since C++17 – available for older versions in various open source libraries • Very useful in lookup functions

50/74 std=:optional<T> • The class template std=:optional manages an optional contained value, i.e. a value that may or may not be present. • Since C++17 – available for older versions in various open source libraries • Very useful in lookup functions • T must not be a reference – Minor inconvenience, use std=:reference_wrapper<T>

51/74 template <typename Predicate> std=:optional<std=:reference_wrapper<ipif=> lookup(std=:vector<ipif>& ifs, Predicate pred) { auto iter = std=:find_if(ifs.begin(), ifs.end(), pred); if (iter == ifs.end()) return {}; return {*iter}; }

52/74 template <typename Predicate> std=:optional<std=:reference_wrapper<ip_if=> lookup(std=:vector<ipif>& ifs, Predicate pred) { auto iter = std=:find_if(ifs.begin(), ifs.end(), pred); if (iter == ifs.end()) return {}; return {*iter}; } void do_stuff(ipif&); void some_func() { =/==. auto found = lookup(ip_interfaces, address_matches({192,168,1,1})); if (found) { do_stuff(found.value()); } =/ }

53/74 Simon Brand @tartanllama https:=/github.com/TartanLlama/optional C++11/14/17 std=:optional with functional-style extensions and reference support https:=/optional.tartanllama.xyz

54/74 Simon Brand @tartanllama https:=/github.com/TartanLlama/optional C++11/14/17 std=:optional with functional-style extensions and reference support https:=/optional.tartanllama.xyz Standards proposal P0798R0: Monadic operations for std::optional.

55/74 Simon Brand @tartanllama https:=/github.com/TartanLlama/optional C++11/14/17 std=:optional with functional-style extensions and reference support https:=/optional.tartanllama.xyz #include <tl/optional.hpp> int i = 3; tl=:optional<int&> v; =/ no value v = i; =/ v refers to i

56/74 Simon Brand @tartanllama https:=/github.com/TartanLlama/optional C++11/14/17 std=:optional with functional-style extensions and reference support https:=/optional.tartanllama.xyz #include <tl/optional.hpp> int i = 3; tl=:optional<int&> v; =/ no value v = i; =/ v refers to i v.value() = 4; =/ i == 4

57/74 Simon Brand @tartanllama https:=/github.com/TartanLlama/optional C++11/14/17 std=:optional with functional-style extensions and reference support https:=/optional.tartanllama.xyz #include <tl/optional.hpp> int i = 3; tl=:optional<int&> v; =/ no value v = i; =/ v refers to i v.value() = 4; =/ i == 4 int j = 2; v = j; =/ i == 4. v bound to j

58/74 Simon Brand @tartanllama https:=/github.com/TartanLlama/optional C++11/14/17 std=:optional with functional-style extensions and reference support https:=/optional.tartanllama.xyz #include <tl/optional.hpp> int i = 3; tl=:optional<int&> v; =/ no value v = i; =/ v refers to i v.value() = 4; =/ i == 4 int j = 2; v = j; =/ i == 4. v bound to j ==. v.and_then([&v](int& n) { n = 2; return v;}); v.or_else([](){std=:cout =< "nothing\n"; }); v.or_else([](){return tl=:optional<int&>{i};});

59/74 Simon Brand @tartanllama https:=/github.com/TartanLlama/optional C++11/14/17 std=:optional with functional-style extensions and reference support https:=/optional.tartanllama.xyz #include <tl/optional.hpp> int i = 3; tl=:optional<int&> v; =/ no value v = i; =/ v refers to i v.value() = 4; =/ i == 4 int j = 2; v = j; =/ i == 4. v bound to j ==. v.and_then([&v](int& n) { n = 2; return v;}) .or_else([](){std=:cout =< "nothing\n"; }) .or_else([](){return tl=:optional<int&>{i};});

60/74 template <typename Predicate> tl=:optional<ipif&> lookup(std=:vector<ipif>& ifs, Predicate pred) { auto iter = std=:find_if(ifs.begin(), ifs.end(), pred); if (iter == ifs.end()) return {}; return {*iter}; } void do_stuff(ipif&); void some_func() { =/==. auto found = lookup(ip_interfaces, address_matches({192,168,1,1})); if (found) { do_stuff(found.value()); } =/ }

61/74 template <typename Predicate> tl=:optional<ipif&> lookup(std=:vector<ipif>& ifs, Predicate pred) { auto iter = std=:find_if(ifs.begin(), ifs.end(), pred); if (iter == ifs.end()) return {}; return {*iter}; } tl=:optional<ipif&> do_stuff(ipif&); void some_func() { =/==. lookup(ip_interfaces, address_matches({192,168,1,1})) .and_then(do_stuff); =/ }

62/74 template <typename Predicate> tl=:optional<ipif&> lookup(std=:vector<ipif>& ifs, Predicate pred) { auto iter = std=:find_if(ifs.begin(), ifs.end(), pred); if (iter == ifs.end()) return {}; return {*iter}; } void some_func() { =/==. lookup(ip_interfaces, address_matches({192,168,1,1})) .and_then(set_state(ipif=:off)); =/ }

63/74 template <typename Predicate> tl=:optional<ipif&> lookup(std=:vector<ipif>& ifs, Predicate pred) { auto iter = std=:find_if(ifs.begin(), ifs.end(), pred); if (iter == ifs.end()) return {}; return {*iter}; } void some_func() { =/==. lookup(ip_interfaces, address_matches({192,168,1,1})) .and_then(set_state(ipif=:off)) .or_else([](){std=:cerr =< “Failed\n”;}); =/ }

64/74 Live demo!

65/74 ✔Very brief intro to higher order functions ✔Gradually expanding example ✔optional<T> (and extension) Higher Order Functions for Ordinary C++ Developers

66/74 Generic library functions • equals(value) ... • compose(function...) • if_then(pred,action) • when_all(predicate...) • when_none(predicate...) • when_any(predicate...) • do_all(action...) Domain specific functions • ip_matches(ip, mask) • select_address(ipif) • select_gateway(ipif) • select_state(ipif) • set_state(ipif&) Composed domain specific functions • address_matches(ip) • state_is(state_type)

67/74 https://github.com/rollbear/lift Generic library functions • equals(value) ... • compose(function...) • if_then(pred,action) • when_all(predicate...) • when_none(predicate...) • when_any(predicate...) • do_all(action...) Domain specific functions • ip_matches(ip, mask) • select_address(ipif) • select_gateway(ipif) • select_state(ipif) • set_state(ipif&) Composed domain specific functions • address_matches(ip) • state_is(state_type)

68/74 https://github.com/rollbear/lift Generic library functions • equals(value) ... • compose(function...) • if_then(pred,action) • when_all(predicate...) • when_none(predicate...) • when_any(predicate...) • do_all(action...) Domain specific functions • ip_matches(ip, mask) • select_address(ipif) • select_gateway(ipif) • select_state(ipif) • set_state(ipif&) Composed domain specific functions • address_matches(ip) • state_is(state_type) As of Boost 1.67.0 (April 14th, 2018) boost::hof (higher order functions) https://www.boost.org/doc/libs/1_67_0/libs/hof/

69/74 Take away messages • Write functions that uses auto return type to create lambdas

70/74 Take away messages • Write functions that uses auto return type to create lambdas • Write functions that access or modify your state

71/74 Take away messages • Write functions that uses auto return type to create lambdas • Write functions that access or modify your state • Compose functions

72/74 Take away messages • Write functions that uses auto return type to create lambdas • Write functions that access or modify your state • Compose functions • And give names to the compositions

73/74 Take away messages • Write functions that uses auto return type to create lambdas • Write functions that access or modify your state • Compose functions • And give names to the compositions • Functional extensions to optional<T> and expected<T,E> removes the need for many conditionals

74/74 Björn Fahller https:=/github.com/rollbear/lift [email protected] @bjorn_fahller @rollbear cpplang, swedencpp Higher Order Functions for Ordinary C++ Developers

NDC{TechTown} Higher order functions for ordinary C++ developers

NDC{TechTown} Higher order functions for ordinary C++ developers

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