Feature Request: Null Coalescing Operator

[filex]

Many languages offer syntactic sugar for defining default values in form of a null coalescing operator. The most common one seems to be ??. go-cty offers the coalesce() function for that purpose.

The function works, but with a little sugar everything is sweeter :)

HCL Template

map = {
  attribute1 = null ?? "default"
  attribute2 = "Hello ${null ?? "world" }"
}

(In reality, instead of null we would use an expression that could actually yield a non-null result.)

Expected behavior

attribute1 should be assigned default
attribute2 should be assigned Hello world

This is the same behaviour as of cty's coalesce() function:

map = {
  attribute1 = coalesce(null, "default")
  attribute2 = "Hello ${coalesce(null, "world")}"
}

Obviously, "falsish" values, such as 0 or the empty string, should not be considered null. 0 ?? 1 should evaluate to 0.

This is a feature proposal for hcl2. As ?? is a parse error at the moment, it should be possible to add the operator without breaking existing configurations.