Exported variables are always evaluated as String.class .

test.dig

_export:
  a:
    b: []

+test:
  _export:
    c:
      d: ${a.b}
      e: ${a}
    

  +a:
    sh>: echo $a
  +b:
    sh>: echo $c

test.log

2017-10-11 00:30:38 +0000 [INFO] (XNIO-1 task-8) io.digdag.core.workflow.WorkflowExecutor: Starting a new session project id=1 workflow name=xxxx session_time=2017-10-11T00:30:38+00:00
2017-10-11 00:30:41 +0000 [INFO] (0059@[0:default]+xxxx+test+a) io.digdag.core.agent.OperatorManager: sh>: echo $a
{"b":[]}
2017-10-11 00:30:43 +0000 [INFO] (0059@[0:default]+xxxx+test+b) io.digdag.core.agent.OperatorManager: sh>: echo $c
{"d":"[]","e":"{\"b\":\"[]\"}"}