Concatenate strings in file_line resource - regex

Question

I'm trying to concatenate a string in a puppet manifest like so:
file_line {'Append to /etc/hosts':
ensure => present,
line => "${networking['ip'] + '\t'}${networking['fqdn'] + '\t'}${networking['hostname']}",
match => "${'^#?'+ networking['ip'] + '\s+' + networking['fqdn'] + '\s+' + networking['hostname']}",
path => '/etc/hosts'
}
I either get syntax errors or in the case above:
The value '' cannot be converted to Numeric
Which I'm guessing means that it doesn't like the plus operator.
So how do I interpolate the strings in the match and line attributes?

Answer 1

The problem here is that the operator + is restricted to only Numeric types (documentation). It cannot be used with String types. However, the spacing and regular expressions can still be used as normal without attempting a string concatenate. These merely need to be placed outside the variable interpolation. Therefore:
file_line { 'Append to /etc/hosts':
ensure => present,
line => "${networking['ip']}\t${networking['fqdn']}\t${networking['hostname']}",
match => "^#?${networking['ip']}\s+${networking['fqdn']}\s+${networking['hostname']}",
path => '/etc/hosts'
}
should resolve your issues with the type mismatch and the + operator.

Answer 2

i know you already got your answer, but you can make life easier using host to manage /etc/hosts:
host {
"localhost": ip => "127.0.0.1";
"$::puppet_server":
ip => "1.2.3.4",
host_aliases => [ "puppetserver" ],
;
"dns-01.tld":
ip => "1.2.3.5",
host_aliases => [ "dns-01" ],
;
"dns-02.tld":
ip => "1.2.3.6",
host_aliases => [ "dns-02" ],
;
}