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website: "coalesce" function unifies its argument types 

In order to be able to predict a result type even if arguments are not yet
known, coalesce requires all of its arguments to be of the same type. Our
usual automatic conversion rules mean that in some cases the result is
a silent type conversion rather than an explicit error, so we'll at least
document that so that folks who encounter it can understand what is
causing the likely-surprising behavior.

If we were building this function over again today I expect we'd make it
always return an error under type mismatch, but to do so now would be a
breaking change and the potential cost of that seems too high for
something that doesn't seem to arise incredibly often in practice.
This commit is contained in:
Martin Atkins 2020-11-17 16:05:55 -08:00
parent eac85b506b
commit df47da1f8e
1 changed files with 21 additions and 0 deletions
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website/docs/configuration/functions/coalesce.html.md
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@ -16,6 +16,11 @@ earlier, see
`coalesce` takes any number of arguments and returns the first one
that isn't null or an empty string.
All of the arguments must be of the same type. Terraform will try to
convert mismatched arguments to the most general of the types that all
arguments can convert to, or return an error if the types are incompatible.
The result type is the same as the type of all of the arguments.
## Examples
```
@ -35,6 +40,22 @@ symbol to expand the list as arguments:
b
```
Terraform attempts to select a result type that all of the arguments can
convert to, so mixing argument types may produce surprising results due to
Terraform's automatic type conversion rules:
```
> coalesce(1, "hello")
"1"
> coalesce(true, "hello")
"true"
> coalesce({}, "hello")
Error: Error in function call
Call to function "coalesce" failed: all arguments must have the same type.
```
## Related Functions
* [`coalescelist`](./coalescelist.html) performs a similar operation with