D's treatment of values versus side-effect free nullary functions

[Jim Balter]

"Don" <nospam at nospam.com> wrote in message 
news:i2ffm7$2qf3$1 at digitalmars.com...
> Jim Balter wrote:
>>
>> "Jonathan M Davis" <jmdavisprog at gmail.com> wrote in message 
>> news:mailman.435.1279700666.24349.digitalmars-d at puremagic.com...
>>> On Wednesday 21 July 2010 00:09:05 Don wrote:
>>>> While running the semantic on each function body, the compiler could
>>>> fairly easily check to see if the function is CTFEable. (The main
>>>> complication is that it has to guess about how many iterations are
>>>> performed in loops). Then, when a CTFEable function is called with
>>>> compile-time constants as arguments, it could run CTFE on it, even if 
>>>> it
>>>> is not mandatory.
>>>>
>>>> Incidentally, having the function being marked as 'pure' doesn't really
>>>> help at all for determining if it is CTFEable.
>>>
>>> Hmm. As I understood it, it did.
>>
>> You understood right.
>>
>>> But I guess that if it did, the compiler could
>>> technically determine whether a function was actually pure anyway by 
>>> looking at
>>> its body. All the pure attribute would do is save it the trouble. So, 
>>> the pure
>>> attribute wouldn't do much in that sense except save the compiler some
>>> computation.
>>
>> pure functions must halt, so the attribute let's you tell the compiler 
>> something that it could only figure out with difficulty, if at all.
>
> Being marked as 'pure' is no guarantee that the function will terminate.

A pure function has no side effects; one that doesn't terminate produces no 
return value either (throwing an exception is a form of termination). Thus a 
pure function that doesn't terminate is a bug. It would be perfectly 
reasonable for a compiler to assume that a pure function does terminate but 
to put a time limit on how long it spends evaluating it -- which it should 
do anyway, since even provably terminating functions can take longer than 
the time to the heat death of the universe to complete ... as I noted 
before, the halting problem isn't relevant in practice, because even if it 
were solvable, there are no time constraints on the solution.