Allow plugins to export custom functions

[thegedge]

It would be interesting to allow the set of functions allowed in interpolations to be an extension point for plugins. For example, one could write a plugin that exports a function to encode a string into base64, split a root CIDR block into a list of smaller CIDR blocks that covers the root block, or read a string from a URL.

[thegedge]

I should note that you can create a plugin that exports resources to compute things for you right now, but I think functions would be more appropriate than read-only + compute-once resources in many/most cases.

[phinze]

Yep totally - this is something I've thought about as well. The use case is absolutely clear, but the implementation is very important to get right so we don't shoot our future selves in the foot with a difficult-to-support ecosystem of configs. I will join @mitchellh in the thinking tag here. 😀

[thegedge]

Just glad to hear that there's interest in this. Looking forward to what you come up with :)

[steve-jansen]

+1 on CIDR functions

[apparentlymart]

Heh... this is the first time I came across this issue, but funny that two of your three example use-cases have been implemented in the mean time:

• implements base64{enc,dec} interpolation funcs #3325 implemented Base64 encoding
• Various interpolation functions for CIDR range manipulation. #3127 will implement CIDR manipulation once it's merged

No URL parsing yet, but I think that's suitably generic that it could be in core too.

I think continuing to add things to core Terraform will be sufficient at least until a function comes along that is truly specific to one particular set of resources, e.g. a function for parsing AWS IAM policy documents.

[polvoazul]

Why not support a "sh" function in variable interpolation, and then the user can run anything he wants?

[theist]

This is also something I'll like to have. My use case is to remove account section and region section of AWS ARNs to store them in policy documents. This is provider specific and can not be a generic function, and I want this because this allows to reuse policies with resources across regions.

For example if I use in the resource section "${aws_sqs_queue.my_queue.arn}" I get a computed ARN like 
arn:aws:sqs:us-west-1:000111223334455:my-queue and I want something to transform it to arn:aws:sqs:::my-queue. This fine string manipulation looks good for a plugin (imho)