Choices and defaults

In HCL, a default and the list of allowed values are two separate statements.

variable "class" {
  type    = string
  default = "standard"
  validation {
    condition     = contains(["standard", "ia", "glacier"], var.class)
    error_message = "class must be standard, ia or glacier."
  }
}

Change the list and the default does not follow. A lookup(var.bucket, "class", "standard") elsewhere repeats the default a third time.

In CUE, a | between values offers a choice. A * in front of one choice marks the default: the value the field takes when nothing else is written.

Warm-up

Offer a choice with a default

Work in an empty directory. Write estate.cue. The bucket has a class with three choices.

package estate

bucket: {
	name:  "logs"
	class: *"standard" | "ia" | "glacier"
}

bucket: {
	name:  "logs"
	class: "standard"
}

Nothing sets class, so it takes the default, "standard". The other two choices stay allowed. The eight lines of HCL above say what this one line says.

Write the field again with one of the other choices. A field written twice adds a constraint.

package estate

bucket: {
	name:  "logs"
	class: *"standard" | "ia" | "glacier"
}

bucket: class: "ia"

bucket: {
	name:  "logs"
	class: "ia"
}

The new value matches one choice, so the default is not used.

Break it

Write a value that is not one of the choices.

package estate

bucket: {
	name:  "logs"
	class: *"standard" | "ia" | "glacier"
}

bucket: class: "cold"

Error: evaluating .:
bucket.class: 3 errors in empty disjunction:
bucket.class: conflicting values "glacier" and "cold":
    ./estate.cue:5:30
    ./estate.cue:8:16
bucket.class: conflicting values "ia" and "cold":
    ./estate.cue:5:23
    ./estate.cue:8:16
bucket.class: conflicting values "standard" and "cold":
    ./estate.cue:5:10
    ./estate.cue:8:16

Read the error from the top.

• The path is bucket.class.
• "3 errors in empty disjunction" means pravo tested "cold" against each of the three choices. No choice is left.
• Each error pairs one choice with your value.
• Each error gives two locations. Line 5 wrote the choice. Line 8 wrote "cold".

Take a value from the command line

pravo has no workspaces. Select the environment with -t env=prod.

A tag is a field marked @tag(name). The flag -t name=value supplies its value. Add an env field with three choices and a default.

package estate

env: *"dev" | "staging" | "prod" @tag(env)

bucket: {
	name:  "logs"
	class: *"standard" | "ia" | "glacier"
	stage: env
}

stage: env copies the value of env into the bucket. A11 covers this.

env: "dev"
bucket: {
	name:  "logs"
	class: "standard"
	stage: "dev"
}
env: "prod"
bucket: {
	name:  "logs"
	class: "standard"
	stage: "prod"
}

Without a tag, env takes its default. With -t env=prod, the value is "prod". Now ask for an environment that does not exist.

Error: evaluating .:
env: 3 errors in empty disjunction:
env: conflicting values "dev" and "qa":
    ./estate.cue:3:7
env: conflicting values "prod" and "qa":
    ./estate.cue:3:27
env: conflicting values "staging" and "qa":
    ./estate.cue:3:15

The flag does not skip the rules. -t env=qa is the same as writing env: "qa" in a file. It meets the same three choices, and none matches. The refusal happens during evaluation, before any provider starts.

Two defaults cancel each other

A second file marks a different choice as the default for class. Each file marks one choice with *, and they disagree.

package estate

bucket: class: *"ia" | "standard" | "glacier"

env: "dev"
bucket: {
	name:  "logs"
	class: "standard" | "ia" | "glacier"
	stage: "dev"
}

Error: the value is not concrete:
bucket.class: incomplete value "standard" | "ia" | "glacier"

pravo eval succeeds and prints the three choices without a default. The flag -c fails unless every value is concrete. It reports incomplete value, because pravo cannot pick one choice. C2 lists every cause of an incomplete value.

Remove the second file. The default works again.

env: "dev"
bucket: {
	name:  "logs"
	class: "standard"
	stage: "dev"
}

Exercise

test "$(pravo eval -e region 2>/dev/null)" = '"eu-west-1"' || { echo "with no tag, region should be eu-west-1"; exit 1; }
test "$(pravo eval -t region=us-east-1 -e region 2>/dev/null)" = '"us-east-1"' || { echo "-t region=us-east-1 should select us-east-1"; exit 1; }
if pravo eval -t region=ap-south-1 > /dev/null 2>&1; then echo "ap-south-1 should be refused"; exit 1; fi
echo ok

with no tag, region should be eu-west-1

package estate

env: *"dev" | "staging" | "prod" @tag(env)

region: *"eu-west-1" | "eu-central-1" | "us-east-1" @tag(region)

bucket: {
	name:  "logs"
	class: *"standard" | "ia" | "glacier"
	stage: env
}

Error: evaluating .:
region: 3 errors in empty disjunction:
region: conflicting values "eu-central-1" and "ap-south-1":
    ./estate.cue:5:24
region: conflicting values "eu-west-1" and "ap-south-1":
    ./estate.cue:5:10
region: conflicting values "us-east-1" and "ap-south-1":
    ./estate.cue:5:41

package estate

a: *1 | 2 | 3
b: *1 | 2 | 3
b: 2
c: *1 | *2 | 3
d: 1 | 2 | 3

a: 1
b: 2
c: 1 | 2
d: 1 | 2 | 3

Recap

• A | offers choices. The value must be one of them.
• A * marks the default. Mark it once per field.
• A @tag(name) field takes its value from -t name=value. The choices still apply.

The next lesson decides what happens to a field that nobody sets.