make (a < b < c) illegal?

[Andrei Alexandrescu (See Website For Email)]

Derek Parnell wrote:
> On Wed, 07 Feb 2007 17:09:35 -0800, Andrei Alexandrescu (See Website For
> Email) wrote:
> 
>> Derek Parnell wrote:
>>> On Wed, 07 Feb 2007 16:55:15 -0800, Walter Bright wrote:
>>>
>>>> Right now, in D (as well as C and C++), when you see the expression:
>>>>
>>>> 	if (a < b < c)
>>>>
>>>> what is your first thought? Mine is that it was written by a newbie who 
>>>> didn't realize that (a < b) returns true or false, and that it does NOT 
>>>> mean ((a < b) && (b < c)). The odds approach certainty that this is a 
>>>> logic error in the code, and even if it was intentional, it raises such 
>>>> a red flag that it shouldn't be used anyway.
>>>>
>>>> Andrei has proposed (and I agreed) that this should be done away with in 
>>>> the language, i.e. comparison operators should no longer be associative. 
>>>>   It's a simple change to the grammar. If one really did want to write 
>>>> such code, it could be done with parentheses:
>>>>
>>>> 	if ((a < b) < c)
>>>>
>>>> to get the original behavior. At least, that looks intentional.
>>>>
>>>> I don't think this will break existing code that isn't already broken.
>>> First thought: Yes, your proposed change makes sense.
>>>
>>> Second thought: Why not make it do what the coder is wanting it to do?
>>>    Namely, make the idiom:
>>>
>>>      expression1 relopA expression2 relopB expression3
>>>
>>>    translate as
>>>
>>>      ( auto temp = expression2,
>>>       (expression1 relopA temp) && (temp relopB expression3) )
>> What's the intended meaning of:
>>
>> a < b == c < d
> 
> Well that *obvious*!
> 
>  (((a < b) && (b == c)) < d)  <G>
> 
> Okay, okay, I see your point. But it would be useful (one day) to easily
> code the idiom  (a op b) && (b op c),  no? 
> 
> How about someone knocking up a mixin template for expressions of this
> format? I haven't got a clue how it could be done as the
> mixin/template/meta-programming syntax and semantics of D is still so
> obtuse and confusing to me that I can only do the very simplest things and
> then only after many false starts.

It's very easy to implement as a straight template function:

ordered(a, b, c)

returns true iff a <= b <= c, and

strictly_ordered(a, b, c)

returns true iff a < b < c. The functions can be nicely accommodate 
multiple arguments. Other functions can define STL-style in-range.


Andrei