terraform

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Martin Atkins 140c613ae8 lang/funcs: "one" function In the Terraform language we typically use lists of zero or one values in some sense interchangably with single values that might be null, because various Terraform language constructs are designed to work with collections rather than with nullable values. In Terraform v0.12 we made the splat operator [*] have a "special power" of concisely converting from a possibly-null single value into a zero-or-one list as a way to make that common operation more concise. In a sense this "one" function is the opposite operation to that special power: it goes from a zero-or-one collection (list, set, or tuple) to a possibly-null single value. This is a concise alternative to the following clunky conditional expression, with the additional benefit that the following expression is also not viable for set values, and it also properly handles the case where there's unexpectedly more than one value: length(var.foo) != 0 ? var.foo[0] : null Instead, we can write: one(var.foo) As with the splat operator, this is a tricky tradeoff because it could be argued that it's not something that'd be immediately intuitive to someone unfamiliar with Terraform. However, I think that's justified given how often zero-or-one collections arise in typical Terraform configurations. Unlike the splat operator, it should at least be easier to search for its name and find its documentation the first time you see it in a configuration. My expectation that this will become a common pattern is also my justification for giving it a short, concise name. Arguably it could be better named something like "oneornull", but that's a pretty clunky name and I'm not convinced it really adds any clarity for someone who isn't already familiar with it.		5 years ago
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