docs: function(callback) {
  
    if (isProviderInstalled) {
      // All is good.
      return Promise.resolve()
      .nodeify(callback);
  
    } else {
  
      // Query provider is installed value.
      return providerInstance().call('isInstalled')
      // Update cache and throw error if provider isn't installed.
      .then(function(isInstalled) {
        isProviderInstalled = isInstalled;
        if (!isInstalled) {
          console.log(art.needsInstall());
        }
      })
  
      // Return.
      .nodeify(callback);
  
    }
  
  }