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machine.js

  // Available variables:
  // - Machine
  // - interpret
  // - assign
  // - send
  // - sendParent
  // - spawn
  // - raise
  // - actions
  // - XState (all XState exports)
  
  // We should move the defaults in (if possible, delays and guards), but only if this helps readability
  // We should know how to get / override arguments (maxRetries, time??, see useQuery). Some are needed and some have a default
  // We should allow cancellation?
  // Maybe START / PAUSE / CANCEL events?
  
  const fetchMachine = Machine({
    id: 'backoff',
    initial: 'init',
    context: {
      attempts: 0
    },
    states: {
      init: { 
        on: {CONFIGURE: [
          { target: 'ready', cond: 'validateConf' },
          { target: 'failure' }
          ]},
      entry: ['setConfiguration']
          
      },
      ready: { on: {
        START: 'started',
      }},
      started: {
        on: { '': [
          { target: 'requesting', cond: 'canRetry'},
          { target: 'failure' }
        ]}
      },
      requesting: {
        invoke: {
        id: 'fetch',
        src: (context, event) => new Promise((_, reject) => reject("rejected")),
        onDone: {
          target: 'success',
          actions: assign({ user: (context, event) => event.data })
        },
        onError: {
          target: 'waiting',
          actions: [assign({ error: (context, event) => event.data }),
          assign({
              attempts: (context, event) => context.attempts + 1
            })
          ]
        }
      }
      },
      success: {
        type: 'final'
      },
      failure: {
        type: 'final'
      },
      waiting: {
        after: {
          DELAY: {
            target: 'started',
          }
        }
      }
    }
  }, {
    delays: {
      DELAY: (context, event) => {
        return Math.min(1000 * 2 ** context.attempts, 30000);
      },
    },
    guards: {
      canRetry: (context, event) => {
        return context.attempts < 3;
      },
      validateConf: (context, event) =>  !!event.promise //maybe??
      
    },
    actions: {
      setConfiguration: assign({ promise: (_, event) => event.promise })
    }
    
  });