How to create one variable from another variable in terraform variable.tf file?

Question

For example, In variable.tf file we have a code like below

variable "variable1" {
    type    = string
    default = "ABC"
}

variable "variable2" {
    type    = string
    default = "DEF"
}

variable "variable3" {
    type    = string
    default = "$var.variable1-$var.variable2"
}

Expected output :

variable3 = ABC-DEF

Answer 1

you can use local instead

locals {
  variable3 = var.variable1+"-"+var.variable2
}

and then to call it instead of using var. use local. like this!

resource "example" "example" {

   example = local.variable3

}

ref : https://www.terraform.io/docs/configuration/locals.html

Answer 2

To my knowledge what you want is not doable with default.

However you can create variable3 and just not assign it a default value and then in your call set variable3 = var.variable1-var.variable2

Not sure this solves your problem but to my knowledge the way you want to do it, won't work.

Also I would recommend upgrading to v0.12.

Answer 3

You can't do this. Docs clearly states:

The default argument requires a literal value and cannot reference other objects in the configuration.

But you could probably use locals for variable3.

Answer 4

You're looking for fixed size buffers.

You can create a structure that contains a number of bytes inline:

struct Structure
{
    unsafe fixed byte largeField[256];
}

They are not actual arrays, C# stores all those bytes inline in the struct. The accesses to its elements are simply pointer offsets. It's unsafe, since it is not a real array, there is no bounds checking, you can overrun the stack – see this question.

It gets compiled roughly to the equivalent of:

struct Structure
{
    [StructLayout(LayoutKind.Sequential, Size = 256)]
    [CompilerGenerated]
    [UnsafeValueType]
    public struct <largeField>e__FixedBuffer
    {
        public byte FixedElementField;
    }

    [FixedBuffer(typeof(char), 128)]
    public <largeField>e__FixedBuffer largeField;
}

As you can see the result contains no reference types, so you can use that with MemoryMappedViewAccessor.