Expression that can't fail, but fail

[jgournet]

Sorry, this is probably a silly thing I'm doing, but I don't get it:
Using "vector vrl":

Happy path:

$ .test = 123
123

$ .test = to_string(.test)
"123"

Basic failure:

$ .test = {"hello": "world"}
{ "hello": "world" }

$ .test = to_string(.test)

error[E103]: unhandled fallible assignment
  ┌─ :1:9
  │
1 │ .test = to_string(.test)
  │ ------- ^^^^^^^^^^^^^^^^ this expression is fallible because at least one argument's type cannot be verified to be valid
  │ │
  │ or change this to an infallible assignment:
  │ .test, err = to_string(.test)
  │
  = see documentation about error handling at https://errors.vrl.dev/#handling
  = see functions characteristics documentation at https://vrl.dev/expressions/#function-call-characteristics
  = learn more about error code 103 at https://errors.vrl.dev/103
  = see language documentation at https://vrl.dev
  = try your code in the VRL REPL, learn more at https://vrl.dev/examples


$

=> ok, fair enough: let me add ", err":

$ .test = {"hello": "world"}
{ "hello": "world" }

$ .test, err = to_string(.test)
"function call error for \"to_string\" at (13:29): unable to coerce { hello: string } into string"

ok, it's happy - but let me pass 123 again now:

$ .test = 123
123

$ .test, err = to_string(.test)

error[E104]: unnecessary error assignment
  ┌─ :1:8
  │
1 │ .test, err = to_string(.test)
  │ -----  ^^^   ---------------- because this expression can't fail
  │ │      │
  │ │      this error assignment is unnecessary
  │ use: .test = to_string(.test)
  │
  = see documentation about error handling at https://errors.vrl.dev/#handling
  = learn more about error code 104 at https://errors.vrl.dev/104
  = see language documentation at https://vrl.dev
  = try your code in the VRL REPL, learn more at https://vrl.dev/examples

$

sooooo : "to_string(.test)" is both faillible and can't fail ?
=> what am I doing wrong ? and how to make it work for both cases ?

Notes:

• I guess I can add a "if is_string(.test) or is_float(.test) [...etc]" before, but that sounds silly
• I tried with "to_string(.test) ?? """ with the same results

Vector version:

/ # vector --version
vector 0.35.0 (x86_64-unknown-linux-musl e57c0c0 2024-01-08 14:42:10.103908779)

[jszwedko]

Hi @jgournet ,

This is admittedly a bit confusing, but the fallibility of to_string depends on the type of the input argument, and if it is known to the compiler. In your first case, the compiler knows it is an object and so to_string could fail (in this case it'll always fail). In your second example it knows the input is an integer and so will never fail. Does that make sense?

[jgournet]

Thanks for saying it is confusing - I was not sure if it was me :)
Yes, it makes some sense.
As a side note, I was using vector vrl to test my syntax and outputs before using those rules in our config file - this is why it confused me, as when running rules against our logs, I do not control the input (actually, those rules are to control the output, whatever is sent as input). Anyway, the result did confuse me, as I had thus no idea what to write in the rules :)

Thanks for your help !