YAML "!eval" tag implementation

[kknicker]

I would like to implement a YAML "!eval" tag which will evaluate a yq expression and assign the result to the key value.

vars:
  x: 1
  y: 2

z: !eval .vars | .x + .y

The relative assignment operator changes the context so expressions relative to the entire document do not work. I was able to work around this by first saving the document to a variable, and then use that in the relative assignment RHS.

yq '. as $root | (.. | select(tag == "!eval")) |= (. tag = "" | . as $expr | $root | eval($expr))' t.yaml

vars:
  x: 1
  y: 2
z: 3

This seems like this could be inefficient if the document is large. Is there a better way to do this?

[kknicker]

I just discovered the "ref" operator. So, the following may work without coping the entire document to a variable. I don't have access to my development environment at the moment, but I will try this as soon as I do.

yq '. ref $root | (.. | select(tag == "!eval")) |= (. tag = "" | . as $expr | $root | eval($expr))' t.yaml

[kknicker]

Ok, ". ref $root" appears to work as well as ". as $root", and hopefully is a more efficient way to evaluate expressions relative to the whole document.

This "!eval" tag implementation might make a good example use of the eval operator.

vars:
  x: 1
  y: 2

z: !eval .vars | .x + .y

yq '. ref $root | (.. | select(tag == "!eval")) |= (. tag = "" | . as $expr | $root | eval($expr))' t.yaml

vars:
  x: 1
  y: 2
z: 3

[kknicker]

The previous "!eval" tag implemenation requires some ugly quoting if the expression is a string to be interpolated.

vars:
  x: 1
  y: 2

z: !eval '"The answer is \(.vars | .x + .y)"'

The following complementary "!streval" tag implementation avoids this.

vars:
  x: 1
  y: 2

z: !streval The answer is \(.vars | .x + .y)

yq '. ref $root | (.. | select(tag == "!streval")) |= (. tag = "" | . as $expr | $root | eval("\"" + $expr + "\""))' t.yaml

vars:
  x: 1
  y: 2
z: The answer is 3

[kknicker]

Possible yq enhancement. Add a "evalsubst" operator similar to "envsubst", but to perform yq expression substitutions instead of environment variables.