sprintf: fix number formatting quirks wrt float/int

[srenatus]

$ opa eval -fpretty 'sprintf("%f", [13])'
"%!f(int=13)"

This should output "13". Using %v is a workaround, but won't allow us to control the number of digits after the dot, like %.3f would allow.

The problem is this code: https://github.com/open-policy-agent/opa/blob/v0.39.0/topdown/strings.go#L427-L436

☝️ We're trying to find the best format of the arg to pass along to the golang stdlib Printf function. The heuristic breaks down when you try to format a number that could be an int or a float internally.

Here's another example:

$ opa eval -fpretty --import future.keywords '{ sprintf("%f", [x]) | some x in {1, 1.1}}'
[
  "%!f(int=1)",
  "1.100000"
]

The desired output would be [ "1", "1.100000"].

💭 One approach would be to examine the format string, and prepare the number argument accordingly: If the n-th format specifier is of the float-y type, make the n-th arg a float; if it's int-y, make it an int.

[damienjburks]

I'd like to take a stab at completing this! Please assign it to me.

[srenatus]

Thanks! LMK if you need assistance or if something is unclear or if you'd like to discuss some approach... 😃

[damienjburks]

Question: @srenatus Let's say I set the format to %.3f in the example you've shown above. Would this be the expected output?

$ opa eval -fpretty --import future.keywords '{ sprintf("%.3f", [x]) | some x in {1, 1.1}}'
[
  "001",
  "1.100"
]

OR would you prefer to see the exact same with the int value?

$ opa eval -fpretty --import future.keywords '{ sprintf("%.3f", [x]) | some x in {1, 1.1}}'
[
  "1",
  "1.100"
]

[srenatus]

Hmm I think I'd go with "1.000". Like here, https://go.dev/play/p/tNR1eM9TpSD

[hugorut]

Hi there, should we reopen this issue since the fix has been reverted? https://github.com/open-policy-agent/opa/pull/4794

Or is there another issue I should watch to track a fix for this behaviour?

---

Edit: maybe https://github.com/open-policy-agent/opa/issues/4771 is what I'm missing, apologies

[srenatus]

Yeah that other issue is what to watch now. We could reopen this one, anyways, you're right.

[hooksie1]

Is there any update on this? Just got bit by this recently.

[srenatus]

Nope, no work has happened on this since, AFAICT 😓