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.IX Title "MooseX::SetOnce 3"
.TH MooseX::SetOnce 3 2026-08-26 "perl v5.44.0" "User Contributed Perl Documentation"
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.SH SYNOPSIS
.IX Header "SYNOPSIS"
Add the "SetOnce" trait to attributes:
.PP
.Vb 3
\&  package Class;
\&  use Moose;
\&  use MooseX::SetOnce;
\&
\&  has some_attr => (
\&    is     => \*(Aqrw\*(Aq,
\&    traits => [ qw(SetOnce) ],
\&  );
.Ve
.PP
\&...and then you can only set them once:
.PP
.Vb 1
\&  my $object = Class\->new;
\&
\&  $object\->some_attr(10);  # works fine
\&  $object\->some_attr(20);  # throws an exception: it\*(Aqs already set!
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
The \*(AqSetOnce\*(Aq attribute lets your class have attributes that are not lazy and
not set, but that cannot be altered once set.
.PP
The logic is very simple:  if you try to alter the value of an attribute with
the SetOnce trait, either by accessor or writer, and the attribute has a value,
it will throw an exception.
.PP
If the attribute has a clearer, you may clear the attribute and set it again.