Prior to Terraform 0.7, lists in Terraform were just a shallow abstraction
on top of strings with a magic delimiter between items. Wrapping a single
string in brackets in the configuration was Terraform's prompt that it
needed to split the string on that delimiter during interpolation.
In 0.7, when first-class lists were added, this convention was preserved
by flattening lists-of-lists by one level when they were encountered in
configuration. However, there was an oversight in that change where it
did not correctly handle the case where the inner list was unknown.
In #14135 we removed some code that was flattening partially-unknown lists
into fully-unknown (untyped) values. This inadvertently exposed the missed
case from the previous paragraph, causing issues for list-wrapped splat
expressions with unknown members. While this worked fine for resources,
due to some fixup done inside helper/schema, this did not work for other
interpolation contexts such as module blocks.
Various attempts to fix this up and restore the flattening behavior
selectively were unsuccessful, due to a proliferation of assumptions all
over the core code that would be too risky to change just to fix this bug.
This change, then, takes the different approach of removing the
requirement that splats be presented inside list brackets. This
requirement didn't make much sense anymore anyway, since no other
list-returning expression had this constraint and so the rest of Terraform
was already successfully dealing with both cases.
This leaves us with two different scenarios:
- For resource arguments, existing normalization code in helper/schema
does its own flattening that preserves compatibility with the common
practice of using bracketed splats. This change proves this with a test
within the "test" provider that exercises the whole Terraform core and
helper/schema stack that assigns bracketed splats to list and set
attributes.
- For arguments in other blocks, such as in module callsites, the
interpolator's own flattening behavior applies to known lists,
preserving compatibility with configurations from before
partially-computed splats were possible, but those wishing to use
partially-computed splats are required to drop the surrounding brackets.
This is less concerning because this scenario was introduced only in
0.9.5, so the scope for breakage is limited to those who adopted this
new feature quickly after upgrading.
As of this commit, the recommendation is to stop using brackets around
splats but the old form continues to be supported for backward
compatibility. In a future _major_ version of Terraform we will probably
phase out this legacy form to improve consistency, but for now both
forms are acceptable at the expense of some (pre-existing) weird behavior
when _actual_ lists-of-lists are used.
This addresses #14521 by officially adopting the suggested workaround of
dropping the brackets around the splat. However, it doesn't yet allow
passing of a partially-unknown list between modules: that still violates
assumptions in Terraform's core, so for the moment partially-unknown lists
work only within a _single_ interpolation expression, and cannot be
passed around between expressions. Until more holistic work is done to
improve Terraform's type handling, passing a partially-unknown splat
through to a module will result in a fully-unknown list emerging on
the other side, just as was the case before #14135; this change just
addresses the fact that this was failing with an error in 0.9.5.
* Document the data source interpolation usage
Also briefly mentions how to use counts and splat syntax as with resources to further document the usage of counts for data sources (see https://github.com/hashicorp/terraform/pull/14143).
* Output -> Attribute
As per feedback.
This new function allows using a search within one list to filter another list. For example, it can be used to find the ids of EC2 instances in a particular AZ.
The interface is made slightly awkward by the constraints of HIL's featureset.
#13847
Adds `basename` and `dirname` interpolation. I want to add a `stack` tag to our infrastructure, the value of which is set to `${basename(path.cwd)}`. We currently use `${replace(path.cwd, "/^.+\\//", "")}` instead, but this is extremeley unreadable. The existance of a `basename` function would be very useful for this use case.
I don't have an immediate use case for a `dirname` function, but it seemed reasonable to add it as well.
* Pass over the Interpolation page
Fixes some grammar, typos and structure. Updated some headings and fixed
a couple of spelling mistakes.
* Added proper note syntax
* Turned some notes into actual notes
* Couple of minor typos just noticed
This commit adds a new interpolation function, zipmap, which produces a
map given a list of string keys and a list of values of the same length
as the list of keys.
The name comes from the same operation in Clojure (and likely other
functional langauges).
This page did not show how to actually use a list as a list. The
variables page states that "The usage of maps, list, strings, etc. is
documented fully in the interpolation syntax page", but that wasn't the
case.
I've split them out to list them explicitly and provide examples of
each.
Closes#9037
Jasmin Gacic