Thursday 5 July 2018

egg Boolean operators

Here's an interview-style question:
What are the Boolean operators in C++?
Of course, it's a trick question; there are two flavours of "Boolean operator":

  • Operators that take Boolean operands, and/or
  • Operators that return Boolean values

So let's be more concrete. Which standard C++ operators can you substitute for "◊" to make the following well-formed?

    // C++
    bool a = false;
    bool b = true;
    auto c = a ◊ b;

It turns out that any C++ operator that can be applied to integers can be substituted, including shift operators like "<<". Because of the history of the introduction of "bool" values into the language, they often get type-promoted to integers silently. For example, "c" has type "int" in the following:

    // C++
    bool a = false;
    bool b = true;
    auto c = a << b;

Whilst the following holds true:

    // C++
    assert((false - true) == -1);

The list of C++ operators that take Boolean operands and return Boolean values is much smaller:

  • a == b
  • a != b
  • a < b
  • a <= b
  • a >= b
  • a > b
  • a && b
  • a || b
  • !a

In Java, Boolean values seem to be treated more carefully. It adds the "&", "|" and "^" operators.

Let's not even think about JavaScript!

I've been extending my test coverage of egg scripts (using OpenCppCoverage) and have come across this inconsistency. As part of the effort to make egg easy to learn by reducing surprise, I'm shying away from automatic type promotion (although I suspect integer-to-float promotions will always be warranted). Therefore, I decided to add explicit Java-style Boolean operators to egg.

    // egg
    bool a = foo();
    bool b = bar();
    var c1 = a & b; // 'c1' is bool
    var c2 = a | b; // 'c2' is bool
    var c3 = a ^ b; // 'c3' is bool

And, while I'm at, I'll add the following operators for orthogonality:

    // egg
    bool a = foo();
    bool b = bar();
    a &&= b; // Same as 'if (a) { a = b; }'
    a ||= b; // Same as 'if (!a) { a = b; }'
    int? c = baz();
    c ??= 123; // Same as 'if (c == null) { c = 123; }'

The last operator is particularly useful when dealing with default parameters.

And then we have the rabbit-hole of the missing "^^" and "^^=" operators...


I also noticed today that spaceships are set to invade C++20. I will resist such an invasion of egg!

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