reduce function doesn't return results from single iteration

[Daniel Kinon]

Hello,

    I'm attempting to use the reduce function(https://puppet.com/docs/puppet/6.2/function.html#reduce) to consolidate hiera facts into data structures I can use with puppet resources.  I've run into an issue that I'm not sure how to solve.  When the data passed to the reduce function results in 2 or more iterations, I get the data return I expect.  When the data passed only results in 1 iterations, I get back no results when I expect one result.  Here is the applicable code snippet (hiera lookups replaced with dummy data):

~~~

  $groups  = {

    "group1" => {

      "gid"     => 901,

      "members" => ["user1","user2","user3","user4"],

    },

    "group2" => {

      "gid"     => 902,

      "members" => ["user2"],

    }

  }

  $hostgroups = [ "group1", "group2" ]

  $usernames = ["user1","user2"]

  $usernames.each |$username| {

    $usergroups = $hostgroups.reduce([]) |$result, $group| {

      $members = $groups[$group]["members"]

      warning("[$username] $group: $members")

      if ( $username in $members ) {

        warning("[$username] Added $group")

        $result + $group

      }

    }

    warning("[$username] groups: $usergroups")

    warning()

  }

~~~

The output of this code shows user1 with an empty array of groups when it should have group1 while user2 has the expected 2 group output:

~~~

Warning: Scope(Class[Base::Test]): [user1] group1: [user1, user2, user3, user4]

Warning: Scope(Class[Base::Test]): [user1] Added group1

Warning: Scope(Class[Base::Test]): [user1] group2: [user2]

Warning: Scope(Class[Base::Test]): [user1] groups:

Warning: Scope(Class[Base::Test]):

Warning: Scope(Class[Base::Test]): [user2] group1: [user1, user2, user3, user4]

Warning: Scope(Class[Base::Test]): [user2] Added group1

Warning: Scope(Class[Base::Test]): [user2] group2: [user2]

Warning: Scope(Class[Base::Test]): [user2] Added group2

Warning: Scope(Class[Base::Test]): [user2] groups: [group1, group2]

Warning: Scope(Class[Base::Test]):

~~~

Let me know if I'm missing something otherwise this seems like it might be a bug.

Thanks,

-Dan

[Stephen Marlow]

Hey Dan,

 In each iteration of the reduce function the memo value is replaced by the result value of the block. The final return value of reduce is the result of the last iteration of the block. The result of the block is the value of the last statement executed. Your block has two possible results:

    1) $result + $group when the condition is met

    2) Undef otherwise (which I think might be the result of the if statement)

  Since user1 doesn't match the condition in the last iteration the result of the reduce is Undef.

  To correct this you could add an else clause that just returns $result, e.g.

  $usergroups = $hostgroups.reduce([]) |$result, $group| {

    $members = $groups[$group]["members"]

    warning("[$username] $group: $members")

    if ( $username in $members ) {

      warning("[$username] Added $group")

      $result + $group

    } else {

      $result

    }

  }

  Hope this helps.

    - Steve

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[Daniel Kinon]

Hey Steve,

    Ha, totally obvious now what the issue is.  This is exactly what I needed, thanks for the help and great explanation!
Cheers,

-Dan

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