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Many naming conventions recommend that methods returning a boolean (also called predicate methods) should be named after a question. My question is: don't they really mean the methods should be named after an assertion?

The difference may be subtle, but you end up with different names in some cases:

  • question: is_pixel_transparent(...)
  • assertion: pixel_is_transparent(...)

Sometimes, this makes no difference and the phrasing is the same:

  • question: end_of_file(...)
  • assertion: end_of_file(...)

Besides, it seems like most of the time, what people call "questions" are actually assertions.

  • key_exists(...) --> this is not a question, this is an assertion.
    Example usage: if (key_exists(...)) ...
  • array_contains_element(...) --> this is not a question, this is an assertion.
    Example usage: if (array_contains_element(...)) ...

So, to restate the question, is everyone meaning assertion when they say question?

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    don't all these things you call assertions become questions when adding a questionmark? Key_Exists? Commented Dec 19, 2013 at 12:11
  • 2
    'Jon Skeet is awesome' is an assertion. 'Is Jon Skeet awesome?' is a question. See the difference. See the difference? Commented Dec 19, 2013 at 16:15
  • @Pieter, no, not in proper English. The question mark makes a sentence a question? Commented Dec 19, 2013 at 17:23
  • @Steven, I think my first example addresses the first half of your comment, and the second example the second half of it. Am I missing anything? Commented Dec 19, 2013 at 17:25
  • @rick: an assertion must be true, otherwise the program is in an undefined/error state. A question may or may not be true. I think of it as control-failure vs control-flow. Commented Dec 19, 2013 at 20:41

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The point of naming conventions is not to make your code read like English, so you might be over-analyzing a bit. In many languages, it is customary to prefix a method or function returning a boolean result or a boolean variable with is, when it makes sense. There are other traditions (e.g. Lisp, Ruby), where a suffix ? is used instead. (An older Lisp tradition is the suffix -p for predicate).

  • In your pixel transparency example, is_transparent should be a method of a pixel object. If you are in a language that does not have objects, but want to simulate an OOP style, then the type would usually be the prefix: Pixel_is_transparent. Note that the prefix is is only used to highlight the boolean nature of this method; it is already implied by the method call (pixel.transparent works as well, but this can become too ambiguous with other property names).
  • To test for EOF, a method could be named at_eof. This interprets the end of file as a location in the stream, whereas stream.is_eof would work as well: here, the EOF is a specific state.
  • To tests whether an entry exists, collection.exists(key) would be better.
  • array_contains_element isn't a good method name, as it contains a type, and the unecessary element. Better: array.contains(elem).

All of the names I suggest are assertions, or more precisely: predicates. Using questions does not make any linguistic sense when these predicates are used in an if-then-else contex. The word “assertion” is probably not optimal here, as it is used to describe the testing of invariants in many languages. The word “predicate” would be better: a statement that is either true or false. A statement is phrased as if it were true, not as a question. The statement 1 ∈ {} – “1 is element of the empty set” or “the empty set contains 1” is a false statement. The question “does the empty set contain the number 1?” can be answered with yes or no, but it isn't true or false.

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    +1 for the note about how the term "assertion" not being the right term to use here because of possible confusion. Commented Dec 19, 2013 at 12:44

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