README.md.dhall

let version = ./version.dhall

let config = ./config.dhall

let fold =
      https://github.com/dhall-lang/dhall-lang/raw/v20.0.0/Prelude/List/fold.dhall

let exampleText = ./examples/readme.gen.dhall as Text

let exampleResult = ./examples/readme.gen.json as Text

in  ''
# Dhall AWS CloudFormation

`dhall-aws-cloudformation` contains [Dhall](https://github.com/dhall-lang/dhall-lang) bindings to AWS CloudFormation, so you can generate CloudFormation template from Dhall expressions. This will let you easily typecheck, template and modularize your CloudFormation definitions.

## :mag: [References](https://gh.1punch.dev/dhall-aws-cloudformation/package.dhall.html)
## :bulb: [Examples](https://gh.1punch.dev/dhall-aws-cloudformation/examples/index.html)

## :book: Usage

### Use resource schema
AWS Cloudformation has massive amount of specifications, to load all
dhall remotely will be very slow and impractical.

One simply way to make import faster is by only importing just each resource you need
#### Remote import resource
```dhall
${exampleText}
```

to convert to CloudFormation JSON file just

```
dhall-to-json < ./template.dhall > ./template.json
```

which generates

```json
${exampleResult}
```

Other way around is build the binary of subset of the resources using nix

#### Build and load package.dhall binary to local cache

Have something like `./examples/example0.nix`, and the dhall file you want to compile `./examples/example0.dhall`

```nix
${./examples/example0.dhall as Text}
```
Add all the resources you need to `cf-preset`, run nix-build, if the subset is not too large it will be very quick, and you will see something like:
```
 ⚠ ─ If error occured, you may need to update the sha256 in your dhall file e.g.
 │ let aws = missing sha256:a04e4db67b092e40987639cca5cd845f452b3984ee7ec77172f815a31e830325
```

Actually the first time it will fail since you can't guess the correct sha of the subset binary, now if you update example0.dhall with the correct sha,
it should then be able to compile to `./result/example0.yaml`
```dhall
let aws =
missing
sha256:a04e4db67b092e40987639cca5cd845f452b3984ee7ec77172f815a31e830325

let Function = aws.Cloudformation.`AWS::Lambda::Function`
```

### Intrinsic Function

The following intrinsic functions are implemented, you can find examples of using intrinsic function in [Fn.dhall document](https://oyanglul.us/dhall-aws-cloudformation/Fn.dhall.html)
- [x] Fn::Base64
- [x] Fn::Cidr
- [x] Condition functions
- [x] Fn::FindInMap
- [x] Fn::GetAtt
- [x] Fn::GetAZs
- [x] Fn::ImportValue
- [x] Fn::Join
- [x] Fn::Select
- [x] Fn::Split
- [x] Fn::Sub
- [x] Fn::Transform
- [x] Ref

### Type Safe `Fn::GetAttr`
Instead of manually looking for AWS documents to make sure the resource has what attributes, we can just use `<Resource>.GetAttr.<attribute name>`:

```dhall
render (Role.GetAttr.Arn "HelloWorldFunctionRole")
```

So the compiler can just help you find the correct attributes available.

### Sam Policy Templates
Cloudformation's Policy document is loosy type as just JSON, it is hard to get the policy right and too many boilerplates to create a Dhall JSON data
thanks to [AWS SAM](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-policy-templates.html) there are some common policy documents we can laverage

these templates are translated into Dhall functions, so you don't need to use SAM to be able to use these policy documents.

```dhall
let Policy = https://github.com/jcouyang/dhall-aws-cloudformation/raw/${version}/cloudformation/AWS::IAM::Role/Policy.dhall
let Sam/Policy = https://github.com/jcouyang/dhall-aws-cloudformation/raw/${version}/sam/policy-template/package.dhall
...
  Policies = Some [Policy::{
    , PolicyDocument = Sam/Policy.DynamoDBReadPolicy (Fn.String "DBName")
    , PolicyName = s "dynamo read only"
  }]
...
```
will generates
```json
{
  "Policies": [
    {
      "PolicyDocument": {
        "Statement": [
          {
            "Action": [
              "dynamodb:GetItem",
              "dynamodb:Scan",
              "dynamodb:Query",
              "dynamodb:BatchGetItem",
              "dynamodb:DescribeTable"
            ],
            "Effect": "Allow",
            "Resource": [
              {
                "Fn::Sub": [
                  "arn:''${AWS::Partition}:dynamodb:''${AWS::Region}:''${AWS::AccountId}:table/''${tableName}",
                  {
                    "tableName": "DBName"
                  }
                ]
              },
              {
                "Fn::Sub": [
                  "arn:''${AWS::Partition}:dynamodb:''${AWS::Region}:''${AWS::AccountId}:table/''${tableName}/index/*",
                  {
                    "tableName": "DBName"
                  }
                ]
              }
            ]
          }
        ]
      },
      "PolicyName": "dynamo read only"
    }
  ]
}
```
## :coffee: Contribute
### Build and Test

```
> nix-shell
$ cabal build
$ cabal test
```
### Generate Type Definitions
e definitions are generated from config file `./config.dhall` which contains specifications used by [AWS CDK](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/cfnspec/build-tools/update.sh) as well:
${fold
    { mapKey : Text, mapValue : Text }
    config.specifications
Text
    ( λ(x : { mapKey : Text, mapValue : Text }) →
      λ(y : Text) →
        ''
        - [${x.mapKey}](${x.mapValue})
        ${y}''
    )
    ""}
regenerate types definition files, simply run
```
$ cabal run
```
if you just want to regenerate dhall files without setting up haskell dev environment, just
```sh
docker run --rm -v $(pwd):/data -w /data ghcr.io/jcouyang/dhall-aws-cloudformation
```
## :warning: Known Issue
The following CloudFormation definitions will raise assertion error due to invalid type definition such as empty type or cyclic import
${fold
    Text
    config.excludes
    Text
    ( λ(x : Text) →
      λ(y : Text) →
        ''
        - `${x}`
        ${y}''
    )
    ""}
''