36 activations of term rewriting macro?

[kaushalmodi]

I took an example from the Nim Manual's TRM section: https://nim-lang.org/docs/manual.html#term-rewriting-macros

The example is almost as-is except that I added a debugEcho so that I know if/when the TRM is getting activated:

template mulIsCommutative{`*`(a, b)}(a, b: int): int =
  debugEcho("-> Term rewriting activated!")
  b * a

let x = 3
echo "Calculating x * 2:"
echo x * 2

Surprisingly that prints:

Calculating x * 2:
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
6

Ref: https://gitter.im/nim-lang/Nim?at=5bbe2eb6bbdc0b25051a4d02

what happens is that diverging rewrite rules get cancelled at some point - Araq

/cc @Araq

[kaushalmodi]

To add to the complication, if I add another multiplication after that first one, the TRM stops activating altogether:

template mulIsCommutative{`*`(a, b)}(a, b: int): int =
  debugEcho("-> Term rewriting activated!")
  b * a

let x = 3
echo "Calculating x * 2:"
echo x * 2

echo ""
echo "Calculating x * 2 again:"
echo x * 2

echo ""
echo "Calculating x * 3:"
echo x * 3

Output:

Calculating x * 2:
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
-> Term rewriting activated!
6

Calculating x * 2 again:
6

Calculating x * 3:
9

[andreaferretti]

Duplicate of #8376 and #2901

[andreaferretti]

The confusion stems from the fact that term rewriting macros are applied recursively (up to a limit). I think this should be documented in the manual and all 3 issues closed

[kaushalmodi]

(up to a limit).

That limit is 36?

I think this should be documented in the manual and all 3 issues closed

I can understand that for the first multiplication overflow happened, and TRM stopped activating after 36 loops. But then why does it not get activated at all for the next 2 multiplications? Once the limit is reached, the TRM stops working altogether for the rest of the program?

[andreaferretti]

I presume so, according to your experiments

[kaushalmodi]

It would be more ensuring to get a better answer. "presume so" sounds like you also don't know for sure if that's the case. I'd like to know the answers to my 3 questions based on spec or design choice rather than based on my toy example.