Loop transform

This topic tells you about the Application Accelerator Loop transform in Tanzu Application Platform (commonly known as TAP).

The Loop transform iterates over elements in a list and applies the provided transform for every element in that list.

Depending on the style you use, denoted by the optional + marker, the transform behaves either behaves as doAsMerge (+) or doAsChain (no +):

• When you use doAsMerge, a copy of the input for the Loop transform is passed to each transform and the outputs from each transform are merged using a set union.

• When you use doAsChain, each transform is executed sequentially, receiving the output from the previous transform as its input. The first transform is to receive the input from the Loop transform as its input.

Syntax reference

for VAR-IDENTIFIER [, INDEX-IDENTIFIER] in SPEL-EXPRESSION [+] {
  ... // other transforms
}

• SPEL-EXPRESSION is a SpEL expression that evaluates a list. This is the list of elements to be iterated over.
• VAR-IDENTIFIER is the name of the variable to be assigned to the current element on each iteration.
• (Optional) INDEX-IDENTIFIER is the name of the variable to be assigned to the index of the current element on each iteration.

Behavior

Consider the following when choosing doAsMerge or doAsChain:

• doAsMerge executes the transform on the same input files for every iteration and merges the resulting outputs. It is best suited when a transform is executed multiple times on the same input and does not have conflicts.

• doAsChain executes the transform on the initial input files once, and then passes the resulting output to the second iteration and so on. It is best suited when a transform must detect any changes that occurred in the previous iteration.

Examples

See the following examples using the Loop transform.

Example 1

Create a new directory for every module in modules (a list of strings) based on the contents of the template directory.

for m in #modules +{
  RewritePath(regex: "template/(.*)", rewriteTo: #m + '/' + #g1)
}

The following diagram shows how this example behaves:

Example 2

Add every artifactId in artifacts (a list of strings) as a Spring dependency.

for a in #artifacts {
  OpenRewriteRecipe(
    recipe: 'org.openrewrite.maven.AddDependency',
    options: {
      groupId: 'org.springframework',
      artifactIf: #a,
      version: '5.7.1
    }
  )
}

The following diagram shows how this example behaves:

Example 3

You can use Loop in combination with custom types, for example:

accelerator.yaml:

accelerator:
  types:
    - name: MavenPlugin
      struct:
        - name: groupId
        - name: artifactId
        - name: version
  options:
    - name: pluginsToAdd
      dataType: [MavenPlugin] # End users will be able to enter a collection of GAV tuples

accelerator.axl:

engine {
  Include({"pom.xml"})
  for p in #pluginsToAdd {
    OpenRewriteRecipe(
      recipe: 'org.openrewrite.maven.AddPlugin',
      options: {
        groupId:    #p['groupId'],
        artifactId: #p['artifactId'],
        version:    #p['version']
      }
    )
  }
}

For more information, see Using Custom Types.