variable if class

hi,

i read this -> 
https://cmdln.org/2018/05/24/what-is-evaluated-first-in-cfengine-3-vars-or-classes/  
and i tried to use this higher kind of wisdom, but ...noooo

just want to read in a file in a string var if a specific class is defined 
eg. "the file exists".

body file control
{
  inputs => { "$(sys.libdir)/stdlib.cf", };
             }



bundle agent var_if_test {

    vars:
     "user" string => "schast";

     "ssh_key_dir" string => "/root/cfe_testbed/user_ssh_keys";

     "ssh_key" string => readfile("$(ssh_key_dir)/$(user).cf"),
               if => ( "exists_ssh_key" );


    classes:


            "exists_ssh_key"          expression => 
fileexists("$(ssh_key_dir)/$(user).cf");

    reports:

        "CFEngine $(sys.cf_version)";

      "$(ssh_key)"
        if => "exists_ssh_key";


}



bundle agent __main__
{
  methods:
      "var_if_test";
}

error: 

root@aixtest01: /root/cfe_testbed # /var/cfengine/bin/cf-agent -KI  -f 
./var_if_test.cf
./var_if_test.cf:17:23: error: syntax error
               if => ( "exists_ssh_key" );
                      ^
./var_if_test.cf:17:23: error: Invalid r-value type '('
               if => ( "exists_ssh_key" );
                      ^
./var_if_test.cf:17:40: error: Check previous line, Expected ';', got 
'"exists_ssh_key"'
               if => ( "exists_ssh_key" );
                                       ^
./var_if_test.cf:17:42: error: Expected promiser string, got ')'
               if => ( "exists_ssh_key" );
                                         ^
   error: There are syntax errors in policy files
   error: Policy failed validation with command 
'"/var/cfengine/bin/cf-promises" -c "./var_if_test.cf"'
   error: Failsafe condition triggered. Interactive session detected, 
skipping failsafe.cf execution.
   error: Error reading CFEngine policy. Exiting...

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