What's the best way to compare two lists of string in terraform?
["arm64"] == ["arm64"] evaluates to false
My use case is constructing lambda extension ARNs based on aws_lambda_function's architectures, which has valid values ["arm64"] and ["x86_64"]. I fell back to comparing architectures[0], but that's not quite what I wanted to say: if the valid values expands in the future by allowing multiple values in the list, it may not fail fast with a clear error.
I suspect that what you are actually doing is comparing a tuple type with a list type.
Here's some terraform console evaluations to demonstrate:
> type(["arm64"])
tuple([
string,
])
> type(tolist(["arm64"]))
list(string)
> ["arm64"] == ["arm64"]
true
> tolist(["arm64"]) == ["arm64"]
false
> tolist(["arm64"]) == tolist(["arm64"])
true
Two values in Terraform can only be equal (as defined by ==) if they are of exactly the same type. Terraform's automatic type conversion rules mean that in most cases we don't need to worry too much about the difference between a tuple type and a list type, but the == operator doesn't give Terraform any hints about what types you might have intended and so it must compare the values exactly as given, without any automatic type conversions.
If you force both expressions to be of type list(string) using the tolist function, as I did in the last expression from my console session above, then they will compare equal if both lists contain the same strings in the same order.
terraform console existed (though I wanted it to!). When I get a chance I'll see if I can verify this is what I was hitting, but I bet it was.Since the lexographical order of a list matters when you compare two lists, I would advice to always use sort(). Furthermore, they need to be of the same type, comparing a tuple with a list will always render false.
["b", "a"] == ["a", "b"] => false, different lexographical order
sort(["b", "a"]) == sort(["a", "b"]) => true, same lexographical order
sort(["b", "a"]) == tolist(["a", "b"]) => true, sort returns a list as well
tolist(["a", "b"]) == tolist(["a", "b"]) => true, both are list
["a", "b"] == tolist(["a", "b"]) => false, comparing a tuple with a list
@Martin Atkins provided a very insightful answer, answering indeed your question of how to compare two lists.
On another side, I have a feeling that what you may want to do in your code is something like that
contains(aws_lambda_function.this.architectures, "arm64") ? "something" : "something_else"
"This module's logic is not ready to support > 1 lengths of the architectures field." or something like that
["arm64"] == ["arm64"]evaluates to false" I just attempted to reproduce that with Terraform 1.7.5 and it evaluated totrue. Please demonstrate otherwise with a MCVE, and include your version of Terraform.["arm64", "x86_64"]and["x86_64", "arm64"}to fail or not? From your question I think that you don't care about ordering, but I'm not sure