This is a library for creating AWS CloudFormation templates.

CloudFormation is a system that creates AWS resources from declarative templates. One common criticism of CloudFormation is its use of JSON as the template specification language. Once you have a large number of templates, possibly including cross-references among themselves, raw JSON templates become unwieldy, and it becomes harder to confidently modify them. Stratosphere alleviates this issue by providing an Embedded Domain Specific Language (EDSL) to construct templates.

The core datatype of stratosphere is the Template, which corresponds to a single CloudFormation template document. Users construct a template in a type-safe way using simple data types. The following example creates a template containing a single EC2 instance with a key pair passed in as a parameter:

instanceTemplate :: Template
instanceTemplate =
  template
  [ resource EC2Instance (
    EC2InstanceProperties $
    ec2Instance
    "ami-22111148"
    & eciKeyName ?~ (Ref KeyName)
    )
    & deletionPolicy ?~ Retain
  ]
  & description ?~ "Sample template"
  & parameters ?~
  [ parameter "KeyName" "AWS::EC2::KeyPair::KeyName"
    & description ?~ "Name of an existing EC2 KeyPair to enable SSH access to the instance"
    & constraintDescription ?~ "Must be the name of an existing EC2 KeyPair."
  ]

The types in stratosphere attempt to map exactly to CloudFormation template components. For example, a template requires a set of Resources, and optionally accepts a Description, Parameters, etc. For each component of a template, there is usually a set of required arguments, and a (usually large) number of optional arguments. Each record type has a corresponding constructor function that has the required parameters as arguments.

For example, since a Template requires a set of Resources, the template constructor has Resources as an argument. Then, you can fill in the Maybe parameters using lenses like & and ?~.

Once a Template is created, you can either use Aeson's encode function, or use our encodeTemplate function (based on aeson-pretty) to produce a JSON ByteString. From there, you can use your favorite tool to interact with CloudFormation using the template.