Should I avoid nested if statements in Perl?

Question

I'm doing some Perl and seeing my nested "if" statements is driving me mad. I managed to reduce some of them with guard blocks in another section, but I'm stuck here.

Do you think I may leave the code as is, or is there a "proper" way to refactor the following ? (I also admit being relatively new to Perl)

This is actually a subroutine asking for user input on each parameters of a list (external file). $[3] is the matching pattern, $[2] is the default value for the considered parameter (NULL if there is none), $_[1] specifies if it is mandatory or not. the 'next' statement refers to the next parameter read (while loop).

With everyone's help (thanks !), here's the newest version.

100         if ( $input ne '' && ( $input !~ $match || $input =~ /'.+'/ ) ) {
101             print "! Format not respected. Match : /$match/ (without \' \')\n";
102             next;
103         }
104         if ( $input eq '' ) {
105             if ( $default eq 'NULL' ) {
106                 if ( $manda eq 'y' ) {
107                     print "! Mandatory parameter not filled in\n";
108                     next;
109                 }
110                 print "+ Ignoring parameter.\n";
111                 $input = '';
112             }
113             else {
114                 print "+ Using default value\n";
115                 $input = $default;
116             }
117         }

---

 98        if($input eq ''){
 99             if($_[2] eq 'NULL'){
100                 if($_[1] eq 'y'){
101                     print "! Mandatory parameter not filled in\n";
102                     next;
103                 }
104                 else{
105                     print "+ Ignoring parameter.\n";
106                     $input = '';
107                 }
108             }
109             else{
110                 print "+ Using default value\n";
111                 $input = $_[2];
112             }
113         }
114         elsif($input !~ $_[3] || $input =~ /'.+'/){
115                 print "! Format not respected. Match : /$_[3]/ (without \' \')\n"; 
116                 next;
117             }
118         }

Answer 1

Here's a slightly more readable version of chaos' answer:

# Set sane variable names...
my ($is_required, $default, $pattern) = @_

# Convert the external string convention for simpler evaluation...
$default = undef if $default eq 'NULL'

# Refuse problematic input before going any further...
if ($input ne '' && $input !~ $pattern || $input =~ /'.+'/) {
    print "! Format not respected. Match : /$pattern/ (without \' \')\n"; 
    next;
}


# If there was no input string...
if($input eq '') {

    # Set the default, if one was provided...
    if( $default ) {
        print "+ Using default value\n";
        $input = $default;
    } 
    # otherwise, complain if a default was required...
    else {
        if( $is_required eq 'y' ){
            print "! Mandatory parameter not filled in\n";
            next;
        }
        print "+ Ignoring parameter (no input or default provided).\n";
    }
}

The key points are:

• You don't need else if you are exiting the current loop with next
• Exceptional cases should be handled first
• You can greatly improve readability by using named variables

Answer 2

An alternative approach that sometimes helps with readability is to put some or all of the branches in well-named code references. Here is a start on the idea:

$format_not_respected = sub {
    return 0 if ...;
    print "! Format not respected....";
    return 1;
}
$missing_mandatory_param = sub {
    return 0 if ...;
    print "! Mandatory parameter not filled in\n";
    return 1;
}

next if $format_not_respected->();
next if $missing_mandatory_param->();
etc...