Conditional Parameter

Conditional parameters is a lono concept. It encapsulates a common pattern with the usage of parameter, condition and ref.

Let’s say you would like to have an “optional” parameter on an EC2 instance resource. You can try this.

parameter("KeyName", Default: "")
resource("Instance", "AWS::EC2::Instance",
  KeyName: ref("KeyName")
)

However, this does not work because the KeyName is an empty string. The KeyName actually needs to exists.

What’s needed is for the KeyName: ref("KeyName") assignment to be conditional. When a KeyName parameter is blank string , the assignment should be KeyName: ref("AWS::NoValue").

Condition to the Rescue

One way to achieve an “optional” parameter is with a condition. Example:

parameter("KeyName", Default: "")
condition("HasKeyName", not!(equals("KeyName", "")))
resource("Instance", "AWS::EC2::Instance",
  KeyName: if!("HasKeyName", ref("KeyName"), ref("AWS::NoValue"))
)

This is what a lono conditional parameter is all about.

Lono Conditional Parameters

Lono conditional parameters encapsulates the parameter, condition, and ref pattern. Here’s an example.

parameter("KeyName", Conditional: true)
resource("Instance", "AWS::EC2::Instance",
  KeyName: ref("KeyName", Conditional: true)
)

The Conditional: true on the parameter method results in automatically creating a condition:

condition("HasKeyName", not!(equals("KeyName", "")))

The Conditional: true on the ref method results in automatically adding the if method with HasKeyName as the condition and ref("AWS::NoValue") as the else value.

For a lot of parameters, using the conditional parameter pattern can really help DRY up your code.

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