[CUE](https://cuelang.org/) の練習で CloudFormation のテンプレート定義を適当に書き下してみた。

cfn.cue

package cfn

Template:: close({
  AWSTemplateFormatVersion?: "2010-09-09"
  Transform?: string | [...string]
  Description?: string
  Metadata?: [string]: Value
  Parameters?: [string]: Parameter
  Mappings?: [string]: [string]: [string]: _
  Conditions?: [string]: Condition
  Resources: [string]: Resource
  Outputs?: [string]: Output
})

Parameter:: {}  //TODO

Condition:: {} //TODO

Mapping:: {} //TODO

Resource:: {
  Type: string
  DependsOn?: string | [...string]
  DeletionPolicy?: "Delete" | "Retain" | "Snapshot"
  UpdateReplacePolicy?: "Delete" | "Retain" | "Snapshot"
  CreationPolicy?: close({
    AutoScalingCreationPolicy?: close({ MinSuccessfulInstancesPercent: uint & <=100 | *100 })
    ResourceSignal?: close({
      Count?: uint | *1
      Timeout?: =~#"^PT(\d+H)?(\d+M)?(\d+S)?$"# | *"PT5M"
    })
  })
  UpdatePolicy?:
    (Type == "AWS::AutoScaling::AutoScalingGroup" & {
      AutoScalingReplacingUpdate?: close({ WillReplace?: bool })
      AutoScalingRollingUpdate?: close({
        AutoScalingRollingUpdate?: uint & >=1 | *1
        MinInstancesInService?: uint | *0
        MinSuccessfulInstancesPercent?: uint & <=100 | *100
        PauseTime?: =~#"^PT(\d+H)?(\d+M)?(\d+S)?$"# | *"PT0S" | *"PT5M" & WaitOnResourceSignals == true
        SuspendProcesses?: [...("Launch"|"Terminate"|"HealthCheck"|"ReplaceUnhealthy"|"AZRebalance"|"AlarmNotification"|"ScheduledActions"|"AddToLoadBalancer")] & [_, ...]
        WaitOnResourceSignals?: bool | *false
        if MinSuccessfulInstancesPercent != null | PauseTime != null | WaitOnResourceSignals != null { MinSuccessfulInstancesPercent:_, PauseTime:_, WaitOnResourceSignals:_ }
      })
      AutoScalingScheduledAction?: close({IgnoreUnmodifiedGroupSizeProperties: bool|*true})
    }) |
    (Type == "AWS::Lambda::Alias" & {CodeDeployLambdaAliasUpdate: {AfterAllowTrafficHook?: String, ApplicationName: String, BeforeAllowTrafficHook?: String, DeploymentGroupName: String}}) |
    (Type == "AWS::ElastiCache::ReplicationGroup" & {UseOnlineResharding: Bool})
  MetaData?: {
    [string]: Value
    "AWS::CloudFormation::Authentication"?: [string]:
      close({ type: "basic", username: String, password: String, uris: [...String] }) |
      close({ type: "S3", accessKeyId: String, secretKey: String, buckets: [...String], roleName: String })
    "AWS::CloudFormation::Init"?: _ //TODO
    "AWS::CloudFormation::Interface"?: close({
      ParameterGroups?: [...close({ Label: Label, Parameters: [...Value] })]
      ParameterLabels?: [string]: Label
      Label:: close({default?: string})
    })
  }
}

Output:: close({
  Description?: string
  Value: Value
  Condition?: Value
  Export?: close({Name: Value})
})

Value:: null | bool | number | string | [...Value] | { [string]: Value } | Ref
Bool:: bool | "true" | "false" | Ref | BoolFn
String:: string | Ref | StringFn
Number:: number | Ref | NumberFn
Ref:: close({Ref: !="^AWS::" & String}) | PseudoParameter
PseudoParameter:: close({Ref: "AWS::AccountId" | "AWS::Region" | "AWS::NotificationARNs" | "AWS::NoValue" | "AWS::Partition" | "AWS::Region" | "AWS::StackId" | "AWS::StackName" | "AWS::URLSuffix"})
xBuiltinFn:: close({[=~"^Fn::"]: _})
BuiltinFn:: // https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference.html
  {"Fn::Base64": String} |
  {"Fn::Cidr": [String, Numner, Number]} |
  {"Fn::FindInMap": [String, String, String]} |
  {"Fn::GetAtt": [(string|Ref), string]} |
  {"Fn::GetAZs": String | "" | {Ref: "AWS::Region"}} |
  {"Fn::ImportValue": String} |
  {"Fn::Join": [String, [...String]]} |
  {"Fn::Select": [Number, Value]} |
  {"Fn::Split": [String, String]} |
  {"Fn::Sub": String | [String, {[string]: Value}]} |
  {"Fn::Transform": close({Name: String, Parameters?: [string]: Value}) } |
  BoolFn | Ref
BoolFn:: close({[=~"^Fn::"]: _})
BoolFn:: // https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-conditions.html
  {"Fn::And": [...(Bool|{Condition:string})]} |
  {"Fn::Equals": [Value, Value]} |
  {"Fn::If": [string, Value, Value]} |
  {"Fn::Not": [Bool]} |
  {"Fn::Or": [...(Bool|{Condition:string})]}
StringFn=BuiltinFn //TODO
NumberFn=BuiltinFn //TODO