Allow me to do math on variables

[bobtfish]

I want to be able to make a module which generates ASGs of the same min and desired size, but with a max size of the desired size+1, and I want to be able to pass the size in as a variable (i.e. a string), however math only works on ints.

Therefore I wrote a toint function, to allow me to be able to write code like this:

max_size = "${toint(var.cluster_size)+1}"

Full example:
https://github.com/bobtfish/terraform-aws-coreos-kubernates-cluster/blob/master/nodes.tf#L28

I'm not sure if this is the right way forward (it'd be nice if we were to implicitly convert strings to ints), but it appears to work for my use-case right now, and was trivial to implement, so I thought I'd share it to prompt some discussion :)

[sethvargo]

I think it might be clearer if this was to_int for UX. "toint" sounds like a single word to be and could be confusing. What do you think?

[phinze]

+1 - toint is also only 1 character off from taint which is a different important concept within TF

[bobtfish]

If you like the explicit conversion (and don't think Terraform should be doing this implicitly) then I also like to_int more - I will rename and add tests some time in the next couple of days. :)

[bobtfish]

Updated to rename the function as requested, and add docs for the website.

[knuckolls]

code looks good to me and i like the idea. probably needs tests before merging to ensure future regressions don't occur.

[mitchellh]

Fixed through #1521.

We always meant for these type conversions to actually be implicit. In fact, in 0.4.x you can do this: ${1+var.foo} and it'll work. There was a bug in the type inference which wasn't working if the first operand was not a primitive. We've fixed this in the above PR.

I understand the value of explicit types (after all, we do code in Go), but I think for the TF configuration language, it is actually preferred to have dynamic typing rather than static.

[ghost]

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