Terraform variables per-workspace

README.md

$ terraform workspace new staging   
Created and switched to workspace "staging"!

You're now on a new, empty workspace. Workspaces isolate their state,
so if you run "terraform plan" Terraform will not see any existing state
for this configuration.

$ terraform apply

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

count_api = 4
count_www = 1
shared_all = shared
size_api = n1-standard-1
size_www = n1-standard-1

$ terraform workspace new production
Created and switched to workspace "production"!

You're now on a new, empty workspace. Workspaces isolate their state,
so if you run "terraform plan" Terraform will not see any existing state
for this configuration.

$ terraform apply                   

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

count_api = 18
count_www = 2
shared_all = shared
size_api = n1-standard-4
size_www = n1-standard-4

$ terraform workspace new testing   
Created and switched to workspace "testing"!

You're now on a new, empty workspace. Workspaces isolate their state,
so if you run "terraform plan" Terraform will not see any existing state
for this configuration.

$ terraform apply

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

count_api = 1
count_www = 1
shared_all = shared
size_api = n1-standard-1
size_www = n1-standard-1

output.tf

output "count_api" {
  value = local.workspace["instance_count_api"]
}

output "count_www" {
  value = local.workspace["instance_count_www"]
}

output "size_api" {
  value = local.workspace["instance_size_api"]
}

output "size_www" {
  value = local.workspace["instance_size_www"]
}

output "shared_all" {
  value = var.shared_all
}

variables.tf

locals {
  env = {
    defaults = {
      instance_count_api = 1
      instance_count_www = 1
      instance_size_api  = "n1-standard-1"
      instance_size_www  = "n1-standard-1"
    }

    production = {
      instance_count_api = 18
      instance_count_www = 2
      instance_size_api  = "n1-standard-4"
      instance_size_www  = "n1-standard-4"
    }

    staging = {
      instance_count_api = 4
      instance_count_www = 1
    }
  }

  workspace = merge(local.env["defaults"], contains(list(local.env), terraform.workspace) ? local.env[terraform.workspace] : local.env["defaults"])
}

variable "shared_all" {
  description = "This variable is shared by all workspaces"
  default     = "shared"
}

Line 23 modified for recent versions of Terraform:

workspace = merge(
    local.env["defaults"], 
    contains(keys(local.env), terraform.workspace) ? 
      local.env[terraform.workspace] : 
      local.env["defaults"]
  )

@ivanpetrushev - this could be simplified so that the final map is empty if there is no match for the current workspace.

workspace = merge(
    local.env["defaults"], 
    contains(keys(local.env), terraform.workspace) ? 
      local.env[terraform.workspace] : 
      tomap({})
  )

Basically, there is no need to merge local.env["defaults"] with itself, an empty map will do just fine.