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[docs] Add limitations section to for_each (#27299) 

* Add limitations section to for_each

Move limitations from a note to their own section,
to allow for expansion on disallowing sensitive values
in for_each
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Pam Selle 2020-12-17 11:03:14 -05:00 committed by GitHub
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website/docs/configuration/meta-arguments/for_each.html.md
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@ -36,12 +36,6 @@ instance for each item in that map or set. Each instance has a distinct
infrastructure object associated with it, and each is separately created,
updated, or destroyed when the configuration is applied.
-> **Note:** The keys of the map (or all the values in the case of a set of strings) must
be _known values_, or you will get an error message that `for_each` has dependencies
that cannot be determined before apply, and a `-target` may be needed. `for_each` keys
cannot be the result (or rely on the result of) of impure functions, including `uuid`, `bcrypt`,
or `timestamp`, as their evaluation is deferred during the main evaluation step.
Map:
```hcl
@ -103,6 +97,31 @@ This object has two attributes:
- `each.value` — The map value corresponding to this instance. (If a set was
provided, this is the same as `each.key`.)
## Limitations on values used in `for_each`
The keys of the map (or all the values in the case of a set of strings) must
be _known values_, or you will get an error message that `for_each` has dependencies
that cannot be determined before apply, and a `-target` may be needed.
`for_each` keys cannot be the result (or rely on the result of) of impure functions,
including `uuid`, `bcrypt`, or `timestamp`, as their evaluation is deferred during the
main evaluation step.
Sensitive values, such as [sensitive input variables](https://www.terraform.io/docs/configuration/variables.html#suppressing-values-in-cli-output),
[sensitive outputs](https://www.terraform.io/docs/configuration/outputs.html#sensitive-suppressing-values-in-cli-output),
or [sensitive resource attributes](https://www.terraform.io/docs/configuration/expressions/references.html#sensitive-resource-attributes)
(if the `provider_sensitive_attrs` experiment is enabled), cannot be used as arguments
to `for_each`. The value used in `for_each` is used to identify the resource instance
and will always be disclosed in UI output, which is why sensitive values are not allowed.
Attempts to use sensitive values as `for_each` arguments will result in an error.
If you transform a value containing sensitive data into an argument to be used in `for_each`, be aware that
[most functions in Terraform will return a sensitive result if given an argument with any sensitive content](https://www.terraform.io/docs/configuration/expressions/function-calls.html#using-sensitive-data-as-function-arguments).
In many cases, you can achieve similar results to a function used for this purpose by
using a `for` expression. For example, if you would like to call `keys(local.map)`, where
`local.map` is an object with sensitive values (but non-sensitive keys), you can create a
value to pass to `for_each` with `toset([for k,v in local.map : k])`.
## Using Expressions in `for_each`
The `for_each` meta-argument accepts map or set [expressions](/docs/configuration/expressions/index.html).