Conditional Execution

daggle provides two mechanisms for conditional step execution: when and preconditions.

`when:` – Skip steps

Skip a step based on an R expression or shell command. If the condition fails, the step is silently skipped (not marked as failed).

- id: deploy-prod
  script: deploy.R
  when:
    command: 'test "$DAGGLE_ENV" = production'
  depends: [test]

With an R expression:

- id: notify
  r_expr: 'slackr::slackr_msg("Done")'
  when:
    r_expr: 'Sys.getenv("SLACK_TOKEN") != ""'
  depends: [report]

A when condition succeeds if exit code is 0 (shell) or the R expression completes without error.

Health checks that must pass before a step runs. If any precondition fails, the step fails immediately (not skipped).

- id: query-db
  script: etl/query.R
  preconditions:
    - command: 'pg_isready -h $DB_HOST'
    - r_expr: 'stopifnot(requireNamespace("DBI"))'

Use preconditions for expensive steps where you want to check prerequisites first.

	`when`	`preconditions`
On failure	Step is skipped	Step fails
Use case	Optional steps	Required checks before expensive work
Downstream	Skipped steps don’t block dependents	Failed steps block dependents
Count	One condition	Array of conditions