Argentum: Variable declarations in IFs and truthy/falsy problem

In many languages it's considered OK to have non-bool values in if statement conditions. Python has 16+ different values and scenarios when objects of different types are considered false, Java Script - 9, C++ - 3. Rust allows to define user conversion with truthy::Truthy. This is obscure and error prone. That's why Argentum allows only one type as condition discriminator - optional; and only one value that considered false - optional_nothing.

One of the main reasons for non-bool values as conditions is to adorn a conditional operator with certain computations in its condition, and make result accessible within the conditional branches via temporary local variable. For instance, in C++:

if (auto v = expression()) use(v);

For this to work, v must be convertible to bool. However, it quickly became apparent that converting the same type to a bool type could be done in various ways and trythy/falsy rules aren't sufficient for all cases. Therefore, in C++17, a convenient variant appeared:

if (auto v = expression; predicate(v)) use(v);

How does it work?

  • It evaluates expression,
  • it placed result is in the local variable v,
  • Then it evaluates its predicate, which converts it into a bool.
  • Than basing on this bool, it choses the appropriate branch, and makes the value of v accessible within that branch. For example:
if (auto i = myMap.find(name); i != myMap.end())

Here is how this is achieved in Argentum without any additional syntax:

predicate(expression) ? use(_)


  • expression yields a value of type T,
  • predicate analyzes it and converts it into ?T with a value inside.
  • The ? operator, checks the ?T and calls use with name "_" holding the value of type T.


    ? log(_)
    : log("username is so @#$ity that I can't even say it");


  • Argentum implements this functionality without introducing new language constructs.
  • Argentum disallows implicit conversion of objects to bool, requiring each condition to specify which aspect of the object is being checked in each if-statement.

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