Consider the following CloudFormation template:

AWSTemplateFormatVersion: "2010-09-09"

Resources:
  MyMediaBucket:
    Type: AWS::S3::Bucket
    Properties:
      ObjectLockEnabled: true

It creates an S3 bucket and sets the ObjectLockEnabled property, which has a type of Boolean in the CloudFormation spec. Now imagine that you want to parameterize the ObjectLockEnabled property, so that the value can be determined when the CloudFormation stack is launched using a template parameter.

There is no Boolean type for template parameters. The closest thing is a String parameter with two boolean-like values in its AllowedValues list. Thankfully, CloudFormation resource properties tend to be very good at type coercion. In nearly all cases, for example, if a property expects a numeric String (like "42") but you pass in a Number, or vice versa, the system will handle things correctly.

This is true of boolean properties as well. A string value of "true" or "false" will be treated as a boolean value when passed to a Boolean resource property. Thus, the template above would also work with a String literal value for ObjectLockEnabled:

AWSTemplateFormatVersion: "2010-09-09"

Resources:
  MyMediaBucket:
    Type: AWS::S3::Bucket
    Properties:
      ObjectLockEnabled: 'true'

This makes it very easy to use a String template parameter to control Boolean resource properties:

# This is a good way of creating boolean template parameters
AWSTemplateFormatVersion: "2010-09-09"

Parameters:
  BucketObjectLockEnabled:
    Type: String
    AllowedValues:
      - true
      - false

Resources:
  MyMediaBucket:
    Type: AWS::S3::Bucket
    Properties:
      ObjectLockEnabled: !Ref BucketObjectLockEnabled

It’s worth noting that the AllowedValues in the previous example are actual YAML bool values. They are being coerced to strings in the context of the parameter, and then the string value is being coerced back to a Boolean at the resource property. This method is not sneakily creating a real boolean parameter. Using string literals in the parameter list behaves the same way, and probably makes what is actually happening more obvious.

# This way works, too, with one less coercion
Parameters:
  BucketObjectLockEnabled:
    Type: String
    AllowedValues:
      - 'true'
      - 'false'

Interestingly, AllowedValues appears to resolve all possible YAML bool values to "true" and "false". YAML allows for a number of different reserved words to act as bools: y, yes, on, off, no, etc. You can use any of those values within AllowedValues, and if you create a stack in the Console, the dropdown selector will always show "true" and "false".

# This works as well, but probably isn't recommended, given the number of
# conversions the values will go through: yes to true; true to "true"; "true"
# to true
Parameters:
  BucketObjectLockEnabled:
    Type: String
    AllowedValues:
      - yes
      - no

The type coercion on the resource properties cannot handle all the various forms of YAML booleans. Passing a string like "yes" or "off" to a Boolean property will fail.

# This would not work
Parameters:
  BucketObjectLockEnabled:
    Type: String
    AllowedValues:
      - 'yes'
      - 'no'
# Neither would this
Resources:
  MyMediaBucket:
    Type: AWS::S3::Bucket
    Properties:
      ObjectLockEnabled: 'off'

If the idea of the YAML parser and CloudFormation messing with your value types makes you uncomfortable, there is always the option of dealing with values only in their native types. It’s much more verbose, and does not appear to be necessary, but it is also a viable option.

AWSTemplateFormatVersion: "2010-09-09"

Parameters:
  BucketObjectLockEnabled:
    Type: String
    AllowedValues:
      # Using string literal values for the String parameter. These can be any
      # arbitrary values you want.
      - Yes_please # Probably use something like 'True' or Enabled instead
      - No_thank_you

Conditions:
  # Create a condition that matches on the parameter string value being treated
  # as true/on.
  HasObjectLockEnabled: !Equals [!Ref BucketObjectLockEnabled, Yes_please]

Resources:
  MyMediaBucket:
    Type: AWS::S3::Bucket
    Properties:
      # Conditionally pass the appropriate YAML `bool` value to the resource
      # property.
      ObjectLockEnabled: !If [HasObjectLockEnabled, true, false]