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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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closed as primarily opinion-based by gnat, Glenn Nelson, Dan Pichelman, Dynamic, MichaelT Dec 20 '13 at 2:54

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question.

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don't all these things you call assertions become questions when adding a questionmark? Key_Exists? –  Pieter B Dec 19 '13 at 12:11
    
'Jon Skeet is awesome' is an assertion. 'Is Jon Skeet awesome?' is a question. See the difference. See the difference? –  Steven A. Lowe Dec 19 '13 at 16:15
    
@Pieter, no, not in proper English. The question mark makes a sentence a question? –  rick Dec 19 '13 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? –  rick Dec 19 '13 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. –  Steven A. Lowe Dec 19 '13 at 20:41

1 Answer 1

up vote 12 down vote accepted

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. –  Pieter B Dec 19 '13 at 12:44

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