try() function

[innovate-invent]

A mechanism is needed to access deep mapping and list values that may not exist, resulting in a default value.

I have found that the try() function in Terraform is quite robust in what it can accomplish.

It can be used to attempt to deeply access various data structures that may not exist and return the first one that does not cause an error. This can also be used for general error catching around other expressions that may fail for other reasons.

This would potentially compete with the current contains() functions use cases.

The syntax would be any try(&expression, &...), only evaluating the next expression if the previous fails. The return is the result of the first successful expression or the error of the last expression.

Examples:

• search({"a":1}, try(&b, `5`) -> 5
• search({"a":1}, try(&a, `5`) -> 1
• search({"a": {"b": {"c": ["x", "y", "z"]}}}, try(&a.b.c[1], "foo") -> "y"
• search({"a": {"b": {"c": ["x", "y", "z"]}}}, try(&a.b.c[3], "foo") -> "foo"
• search({"a": {"b": {"c": ["x", "y", "z"], "d": 7}}}, try(&a.b.c[3], &a.b.d, "foo") -> 7

[innovate-invent]

It looks like there is already a similar behavior in the way simple expressions are evaluated. They return null in the event the expression is invalid. This allows using chained OR operators instead of the try() argument list. Though this behavior is not consistent for all expressions and doesn't allow an expression to validly return a falsy value.

Because of this behavior of returning null for some invalid expressions, try() would not work in those cases.
We would need to either fully embrace the null return values for all expressions, or abandon this behavior in place of returning an error.

We could possibly convert erroring expressions not wrapped in try() to null to allow for backwards compatibility.
This behavior could be turned on and off by a config option, but that would be left to the implementation.