add exists template function

[jsipprell]

When using levant with a common template but different vars files, it would be useful to be able to test for the existence of a key in a map. This PR adds an exists template function to test for key existence without failing due to the template missingkey option being set.

Example usage:

job "foo" {
[[ if exists . "constraint" ]]
  constraint {
    attribute = "${[[ .constraint.attribute ]]}"
    value = "[[ .constraint.value ]]"
  }
[[- end ]]

  ...
}

[jsipprell]

Addendum: I recognize this is possible by using the index function, but this may not be obvious or trivial to someone unfamiliar with go templating. (Plus, an index test wouldn't actually test for key existence, but rather whether the value is non-zero)

[jrasell]

Hi @jsipprell thanks a lot for this addition, I really like it. I am curious if the name exists is too broad here, and potentially mapExists or similar might be better as the function is scoped to maps only? Also if you have the time, could you add a quick docs update to include this func also

[jsipprell]

Sure, I can update the docs.

How about varExists? To me, that seems to map on to the idioms levant uses and indicates intuitively that one is testing for the existence of a variable (which would be the common usage).

[hashicorp-cla]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

0 out of 2 committers have signed the CLA.

• Jesse Sipprell
• jsipprell

---

Jesse Sipprell seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA.
If you have already a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[angrycub]

@jrasell Since the sprig functions are merged into levant, would this be a case that is now covered by sprig's hasKey function?