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It goes much deeper than that. For example, again referencing Erlang since that's the only FP language I'm well-versed in, immutable data + pattern matching me

It goes much deeper than that. For example, again referencing Erlang since that's the only FP language I'm well-versed in, immutable data + pattern matching means that there are assertions on practically every line of code in production.

dragontamer 5y ago
In C++, any "const" data can only have "const" operations operate upon them. const int x = 20; x = 30 ; // Compile Error, not a const operation. int y = x + 20; // Allowed If you have a class: class Foo{ public: const int x; int y; Foo(int initial): x(initial), y(initial){ } void setY(int value){ y = value; } int getXPlusY() const{ return x + y; } }; Foo f(20); // Allowed f.x = 30; // Compile Error, not a const-operation int y = f.x + 30; // Allowed const Foo f2(20); // The f2 object is now const. Non-const routines are no longer allowed f2.setY(30); // Not a const-routine. int bar = f2.getXPlusY(); // Allowed, getXPlusY is a const-function ----------------- So yes. Every single line of code you write will be double-checked by the compiler to see if it is "const-correct". That's the advantage of multi-paradigm languages like C++. Its not "pure", but it implements ideas like immutability from other languages.
- Jtsummers 5y ago
  I think macintux is referring to something like this: get_exactly_n_or_crash(N) -> {N, Data} = get_data(N). Suppose get_data will get N or less pieces (if N pieces of data aren't available) of data and return, as a tuple, both the amount of data obtained and the data. The above assignment is actually part of Erlang's pattern matching, the second use of N (on the LHS of the =) is not actually an assignment, it's a check. If get_data returned less than N pieces of data, the program would crash (which you can catch and respond to) at that point.
- filoeleven 5y ago
  That gives you immutability on that instance, but the benefits are diminished since the language is not immutable-first. If it were, I’d expect f2.setY(30) to return a new immutable instance of Foo with x == 20 and y == 30. As it stands, there’s not a reliable way to get “an immutable Foo instance with everything the same except for y, which has this new value.” For a class with two properties, you can make a new constructor that initializes both, though it’s a bit unwieldy. It becomes unworkable when you have five or ten properties. A more data-oriented approach would be to use a key/value map, and have standard functions to “update” map members by returning new maps with the “updated” value(s). Those libraries probably exist for C++, but you have to enforce their use, whereas languages that are built around those concepts provide stronger guarantees from the outset.

dragontamer 5y ago

In C++, any "const" data can only have "const" operations operate upon them. const int x = 20; x = 30 ; // Compile Error, not a const operation. int y = x + 20; // Allowed If you have a class: class Foo{ public: const int x; int y; Foo(int initial): x(initial), y(initial){ } void setY(int value){ y = value; } int getXPlusY() const{ return x + y; } }; Foo f(20); // Allowed f.x = 30; // Compile Error, not a const-operation int y = f.x + 30; // Allowed const Foo f2(20); // The f2 object is now const. Non-const routines are no longer allowed f2.setY(30); // Not a const-routine. int bar = f2.getXPlusY(); // Allowed, getXPlusY is a const-function ----------------- So yes. Every single line of code you write will be double-checked by the compiler to see if it is "const-correct". That's the advantage of multi-paradigm languages like C++. Its not "pure", but it implements ideas like immutability from other languages.

Jtsummers 5y ago
I think macintux is referring to something like this: get_exactly_n_or_crash(N) -> {N, Data} = get_data(N). Suppose get_data will get N or less pieces (if N pieces of data aren't available) of data and return, as a tuple, both the amount of data obtained and the data. The above assignment is actually part of Erlang's pattern matching, the second use of N (on the LHS of the =) is not actually an assignment, it's a check. If get_data returned less than N pieces of data, the program would crash (which you can catch and respond to) at that point.
filoeleven 5y ago
That gives you immutability on that instance, but the benefits are diminished since the language is not immutable-first. If it were, I’d expect f2.setY(30) to return a new immutable instance of Foo with x == 20 and y == 30. As it stands, there’s not a reliable way to get “an immutable Foo instance with everything the same except for y, which has this new value.” For a class with two properties, you can make a new constructor that initializes both, though it’s a bit unwieldy. It becomes unworkable when you have five or ten properties. A more data-oriented approach would be to use a key/value map, and have standard functions to “update” map members by returning new maps with the “updated” value(s). Those libraries probably exist for C++, but you have to enforce their use, whereas languages that are built around those concepts provide stronger guarantees from the outset.