New Issue: Should user-defined types be able to used in conditionals? And if so, how?

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17645, "bradcray", "Should user-defined types be able to used in conditionals? And if so, how?", "2021-05-06T00:11:58Z"

As a library author, I may want to define a type that would be convenient to use in a conditional. As a somewhat artificial example, if I define a type:

record height {
  var inches: int; 
}

I may want to permit a user to write:

if child.height then

rather than:

if child.height.inches > 0 then  // or `if child.height.getHeightInInches() > 0` or ...

In the modules developed by the core team, we have benefitted from this capability in the Regex module, for c_ptr, and for the syserr type, all of which are defined within library modules. Should we extend this capability to users as well?

Pros:

  • supports creating the ability to create types with succinct conditional forms

Cons:

  • like operator overloads, could be subject to abuse
  • somewhat similar to an implicit conversion, which some users have been averse to due to things happening invisibly

If we were to support this, how would we express it?

  • a well-defined method on the type?
  • a (new) operator on the type?

In either case, what would the name be?

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