Automatic typing

[Ary Borenszweig]

On 6/30/13 10:30 PM, JS wrote:
> On Monday, 1 July 2013 at 01:08:49 UTC, Kenji Hara wrote:
>> 2013/7/1 JS <js.mdnq at gmail.com>
>>
>>> I am simply talking about having the compiler enlarge the type if
>>> needed.
>>> (this is mainly for built in types since the type hierarchy is
>>> explicitly
>>> known)
>>>
>>
>> Just a simple matter, it would *drastically* increase compilation time.
>>
>> void foo()
>> {
>>     auto elem;
>>     auto arr = [elem];
>>
>>     elem = 1;
>>     ....
>>     elem = 2.0;
>>     // typeof(elem) change should modify the result of typeof(arr)
>> }
>>
>> Such type dependencies between multiple variables are common in the
>> realistic program.
>>
>> When `elem = 2.0;` is found, compiler should run semantic analysis of the
>> whole function body of foo _once again_, because the setting type of elem
>> ignites the change of typeof(arr), and it would affect the code meaning.
>>
>> If another variable type would be modified, it also ignites the whole
>> function body semantic again.
>>
>> After all, semantic analysis repetition would drastically increase.
>>
>> I can easily imagine that the compilation cost would not be worth the
>> small
>> benefits.
>>
>> Kenji Hara
>
> No, this would be a brute force approach. Only one "preprocessing pass"
> of (#lines)  would be required. Since parsing statement by statement
> already takes place, it should be an insignificant cost.

Believe me, it's not. Look at this:

---
int foo(int elem) {
   return 1;
}

char foo(float elem) {
   return 'a';
}

auto elem;
elem = 1;
auto other = foo(elem);
elem = other + 2.5;
---

Explain to me how the compiler would work in this case, step by step.