[ruby-core:101974] [Ruby master Feature#17474] Interpreting constants at compile time
From:
eregontp@...
Date:
2021-01-07 13:36:30 UTC
List:
ruby-core #101974
Issue #17474 has been updated by Eregon (Benoit Daloze).
Dan0042 (Daniel DeLorme) wrote in #note-21:
> I think it would make sense to say that precisely "when" the expression is executed is undefined. So in the case of lazy parsing it would be parsed and evaled when the containing code is parsed, whenever that is.
"undefined" is for me a synonym of broken, incompatible, useless semantics.
The semantics need to be clearly defined, otherwise there will be incompatibilities.
Literally "parse time" doesn't make sense, because e.g., one can parse once to bytecode, and reuse the bytecode without parsing for future executions.
So I guess what is meant is "load/require time", similar to when constant would be defined, before evaluating any code in the file.
There is no good way to decide if e.g. `10 ** 6` is pure and safe to evaluate during load time, because `**` could be redefined and potentially check the caller.
It also seems very unnatural that the expression couldn't use anything in its evaluation context, not even constants, for example this wouldn't work if done at load time:
```ruby
module A
N = 10
def self.foo
$(N ** 6)
end
end
```
So that's why evaluating on first execution is somewhat sensible, but at parse/load time I believe it's not.
----------------------------------------
Feature #17474: Interpreting constants at compile time
https://bugs.ruby-lang.org/issues/17474#change-89826
* Author: jzakiya (Jabari Zakiya)
* Status: Open
* Priority: Normal
----------------------------------------
Ruby has borrowed concepts/idioms from allot of languages.
I am proposing borrowing a feature from Forth to provide for compile time interpretation of Constants.
This should make executed code faster|efficient, while maintaining source code brevity|clarity.
Below is actual code used in a large rubygem I have.
To develop this method, I had to do allot of test runs to determine the range values.
Once found, these values don't change, but I just kept the computed forms of the values, in case I want to upgrade them.
In Forth I can interpret those expressions that result in constants, which will be compiled as single values for run time.
See wikeipedia article on Forth below starting at **Mixing states of compiling and interpreting**.
https://en.wikipedia.org/wiki/Forth_(programming_language)
Forth was designed for, and is still used most frequently, in hardware controllers, and with microprocessors.
IMHO this feature would also make MRuby more code efficient and faster for this domain too, and IOT devices.
Below is an example of real code that would benefit from this.
While this example would result in numerical constant, string constants could also be interpreted.
```
def select_pg(endnum, startnum)
start_num = end_num
end_num = endnum; start_num = startnum
range = end_num - start_num
pg = 5
if start_num <= Integer.sqrt(end_num) # for one array of primes upto N
pg = 7 if end_num > 50 * 10**4
pg = 11 if end_num > 305 * 10**5
else # for split array cases
pg = 7 if ((10**6 ... 10**7).include?(range) && start_num < 10**8) ||
((10**7 ... 10**8).include?(range) && start_num < 46 * 10**8) ||
((10**8 ... 10**9).include?(range) && start_num < 16 * 10**10) ||
(range >= 10**9 && start_num < 26 * 10**12)
pg = 11 if ((10**8 ... 10**9).include?(range) && start_num < 55 * 10**7) ||
(range >= 10**9 && start_num < 45 * 10**9)
end
primes = [2, 3, 5, 7, 11, 13].select { |p| p <= pg }
{primes, primes.reduce(:*)} # [excluded primes, modpg] for PG
end
```
Allowing for compile time interpretation, the code could be rewritten as below.
```
def select_pg(endnum, startnum)
start_num = end_num
end_num = endnum; start_num = startnum
range = end_num - start_num
pg = 5
if start_num <= Integer.sqrt(end_num) # for one array of primes upto N
pg = 7 if end_num > [50 * 10**4]
pg = 11 if end_num > [305 * 10**5]
else # for split array cases
pg = 7 if (([10**6] ... [10**7]).include?(range) && start_num < [10**8]) ||
(([10**7] ... [10**8]).include?(range) && start_num < [46 * 10**8]) ||
(([10**8] ... [10**9]).include?(range) && start_num < [16 * 10**10])||
(range >= [10**9] && start_num < [26 * 10**12])
pg = 11 if (([10**8] ... [10**9]).include?(range) && start_num < [55 * 10**7]) ||
(range >= [10**9] && start_num < [45 * 10**9])
end
primes = [2, 3, 5, 7, 11, 13].select { |p| p <= pg }
{primes, primes.reduce(:*)} # [excluded primes, modpg] for PG
end
```
This maintains the original form, so if I need/want to change the range limits again
I can just change the calculation inline, without having to remember where those values came from.
As 3.0 has introduced many new features and idioms, this could be introduced with no breaking change too.
Old code would work as before, while new code could take advantage of this feature.
Thanks is advance of giving this proposal serious consideration.
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