property syntax strawman

[Michel Fortin]

On 2009-08-02 23:19:50 -0400, "Robert Jacques" <sandford at jhu.edu> said:

> On Sun, 02 Aug 2009 20:53:35 -0400, Michel Fortin  
> <michel.fortin at michelf.com> wrote:
> 
>> On 2009-08-02 20:18:51 -0400, "Robert Jacques" <sandford at jhu.edu> said:
>> 
>>> I also like the idea of omitting parenthesis for functions with no   argument.
>> 
>> I like it too. But the problem with the current approach D are:
>> 
>> 1. A syntax that permits function to be called both without and with  
>> empty parenthesis creates ambiguities when it returns a callable type 
>> (a  delegate or an object with an opCall member).
>> 
>> 2. We want properties to be nouns, and actions to be verbs. In english 
>> a  lot of words are both nouns and verbs, which makes it impractical to 
>>  distinguish a property from a function by its name alone.
>> 
>> Solving 1 involves having a way to say functions that can and must be  
>> called without parenthesis. Unless we want to force all functions with  
>> no parameter to be called without "()", we must have some kind of flag  
>> to tell us that a function expects or does not expect "()".
>> 
>> Solving 2 involves making a difference in the call syntax between  
>> property and action functions. Since the idea behind a property is to  
>> mimic a field, it follows that the best differentiator for properties 
>> is  the lack of parenthesis.
>> 
>> I'd be glad too if we could continue to skip parenthesis for calls to  
>> functions with no arguments, but I think fixing the two problems above  
>> is more important.
>> 
> 
> I agree 1) is an issue, but I view it as a corner case. (I use zero/one 
>  arg functions all the time and make use of the 'property' syntax left  
> right and center, but I've never run into the opCall problem) It would 
> be  nice if it were fixed, but we may be cutting off the nose to spite 
> the  face, as it were. (Or alternatively, taking the scientific instead 
> of  engineering approach)
> 
> Problem 2) I think is a major logical flaw: a property is a function. 
> So I  see no need to distinguish between the two. Properties, in fact, 
> can be a  very expensive functions that takes locks/do database 
> lock-up/etc. So  making syntax changes that make 'properties' look 
> cheap and 'functions'  look expensive is worse than what we have today. 
> Today, at least, everyone  knows that no parenthesis doesn't mean 
> necessarily cheap O(1) operation.

And I agree with you: properties are functions. But problem number 2 
isn't about whether properties are functions or not. It's about the 
meaning of a name. Say you have a "transform" function, you'll have 
some expectations about what it does based on the name. So I ask you: 
is a "transform" function an *action* (a verb) in the sense that it 
applies some transformation to an object, or is it a *property* (a noun 
or adjective) in the sense that it returns something like an affine 
transform associated with an object? The semantics of the two are 
completely different, yet they share the same name.

The real ambiguity here is the English language, were many nouns and 
adjectives are also verbs. We could switch to a natural language that 
does not have this problem for function name -- say French: "transform" 
becomes "transformer" (action, verb) or "transformation" (property, 
noun); "empty" becomes "vider" (action, verb) or "vide" (property, 
adjective) -- but that doesn't seem like a very practical option.

So, beside switching to a non-English language, we have two other 
options. First we could write a guideline saying properties (nouns and 
adjectives) should start with some prefix. I tried it[1]. I couldn't 
come with anything sane that works for non-boolean properties. In 
addition, most people don't read guidelines, and educating people to 
write "is", "has" or a modal verb in front of boolean properties would 
be a major effort. It works in Objective-C, but Objective-C function 
naming conventions are very different.

 [1]: http://prowiki.org/wiki4d/wiki.cgi?DProgrammingGuidelines

So the only option left, assuming we still want to solve the problem, 
is to introduce a formal syntax in D to distinguish properties (nouns 
and adjectives) from actions (verbs). A natural fit for that at the 
call site is to use the syntax without tailing empty parenthesis for 
properties, mimicking fields, and use the inherited function syntax 
from C-derived languages for actions. Choosing the right syntax for 
property definitions is a greater challenge since we want a simple 
syntax that helps people make the right choice depending on wether they 
want a property or an action, but we  (Andrei, me, some others) don't 
want to deviate too much from a regular function either.

As for whether properties should be cheap (not perform anything 
heavyweight) or not, that's another debate result in a guideline.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/