Puppet require vs. include vs. class

According to puppet reference include and require:

• Both include and require are functions;
• Both include and require will "Evaluate one or more classes";
• Both include and require cannot handle parametered classes
• require is a superset of include, because it "adds the required class as a dependency";
• require could cause "nasty dependency cycle";
• require is "largely unnecessary"; See puppet language guide.
  Puppet also has a require function, which can be used inside class definitions and which does implicitly declare a class, in the same way that the include function does. This function doesn’t play well with parameterized classes. The require function is largely unnecessary, as class-level dependencies can be managed in other ways.
• We can include a class multiple times, but cannot declare a class multiple times.

  class inner {
    notice("I'm inner")
  
    file {"/tmp/abc":
      ensure => directory
    }
  }
  
  class outer_a {
    # include inner
    class { "inner": }
  
    notice("I'm outer_a")
  }
  
  class outer_b {
    # include inner
    class { "inner": }
  
    notice("I'm outer_b")
  }
  
  include outer_a
  include outer_b
  

  Duplicate declaration: Class[Inner] is already declared in file /home/bewang/temp/puppet/require.pp at line 11; cannot redeclare at /home/bewang/temp/puppet/require.pp:18 on node pmaster.puppet-test.com
  

• You can safely include a class, first two examples pass, but you cannot declare class inner after outer_a or outer_b like the third one:

  class inner {
  }
  
  class outer_a {
    include inner
  }
  
  class outer_b {
    include inner
  }
  
  class { "inner": }
  include outer_a
  include outer_b
  

  class { "inner": }
  class { "outer_a": }
  class { "outer_b": }
  

  include outer_a
  include outer_b
  class { "inner": } # Duplicate redeclaration error