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.

When doAsMerge is used, a copy of the Loop transform’s input is passed to each transform and the outputs from each transform are merged using a set union.

When doAsChain is used, each transform is executed sequentially, receiving the previous transform’s output as its input. The first transform is to receive the Loop transform’s input as its input.

Syntax reference

type: Loop
on: <SpEL expression>
var: <string>
index: <string>
doAsChain: <transform>
doAsMerge: <transform>

• on must be a SpEL expression that evaluates a list. This is the list of elements to be iterated over.
• var is the name of the variable to be assigned to the current element on each iteration. (optional)
• index is the variable’s name to be assigned to the index of the current element on each iteration. (optional)
• doAsMerge is the transform to be executed for every element in the list, on a copy of the Loop transform’s input.
• doAsChain is the transform to be executed for every element in the list, passing the output of the transform as input to the next transform.

Both var and index are optional.

Only one of the doAsMerge or doAsChain variables is to be used in a Loop transform.

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.

type: Loop
on: "#modules"
var: m
doAsMerge:
  type: 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.

type: Loop
on: "#artifacts"
var: a
doAsChain:
  type: OpenRewriteRecipe
  recipe: org.openrewrite.maven.AddDependency
  options:
    groupId: "'org.springframework'"
    artifactId: "#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:
  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
engine:
  include: [pom.xml]
  chain:
    - type: Loop
      on: pluginsToAdd # Iterate on the pluginsToAdd collection
      var: p           # The variable "p" will contain each tuple in turn
      doAsChain:       # Will apply the second execution to the result of the first, and so on...
        type: OpenRewriteRecipe
        recipe: org.openrewrite.maven.AddPlugin
        options:
          groupId:    "#p['groupId']"
          artifactId: "#p['artifactId']"
          version:    "#p['version']"

For more information, see Using Custom Types.