Dermot

I believe it inherits from SimpleStr which allows zero length.

FWIW here's my code, which exercises the coercion when the string is empty, but does not seem to apply it. ( Update: added debug and showed output )

use strict;
use warnings;
use feature 'say', 'state';
use Types::Standard 'Undef';
use Types::Common::String 'SimpleStr','StrLength';
use Type::Params 'compile';

state $check = compile (
    Undef | (SimpleStr & (StrLength[1,4]))->plus_coercions(
        SimpleStr, sub { warn 'coercing'; length ? $_ : undef },
    )
);

#no warnings 'uninitialized';

for my $str ( undef, '', 'abcd', 'abcde' ) {
    my $label = ! defined($str) ? 'undef' : ! length ($str) ? 'empty' 
+: $str;
    my $checked = $check->($str);

    say sprintf('%s : >%s<', $label, $str);
}
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This outputs:

Use of uninitialized value in sprintf at 11130005.pl line 20.
undef : ><
coercing at 11130005.pl line 10.
empty : ><
abcd : >abcd<
coercing at 11130005.pl line 10.
Value "abcde" did not pass type constraint "Undef|__ANON__" (in $_[0])
+ at 11130005.pl line 18
    "Undef|__ANON__" requires that the value pass "Undef" or "__ANON__
+"
    Value "abcde" did not pass type constraint "Undef"
        Value "abcde" did not pass type constraint "Undef"
        "Undef" is defined as: (!defined($_))
    Value "abcde" did not pass type constraint
        "__ANON__" is a subtype of "SimpleStr&StrLength[1,4]"
        "SimpleStr&StrLength[1,4]" requires that the value pass "Simpl
+eStr" and "StrLength[1,4]"
        Value "abcde" did not pass type constraint "StrLength[1,4]"
        "StrLength[1,4]" expects length($_) to be between 1 and 4
        length($_) is 5
[download]

Hope this helps!



The way forward always starts with a minimal test.