Puppet: substitute parameter in fact name

Here is custom fact:

require 'etc'

Etc.passwd do |user|
    Facter.add("homedir_#{user.name}".intern) do
        setcode { user.dir }
    end
end

It determine users home dir locations.

Here is defined type:

define ssh_keys::key ($user) {

    $home = $::homedir_#{user}

    if $home != undef {

    file { "$home/.ssh/":
        ensure => directory,
    }

    file { "$home/.ssh/authorized_keys":
        ensure => file,
        source => "puppet:///modules/ssh_keys/$user",
        }
    }

}

It does not work with $::homedir_#{user}, but works with exactly defined fact: $::homedir_root. What is wrong here?