"Can't modify non-lvalue subroutine call" when adding Moose attribute from method

I am battling with Moose these days, and I ran into the following problem. I create an object that has many required attributes on its creation. However, I wish to add attributes to it when a method is called. More specifically, I'd like to add the arguments for that method as a hash attribute. I'd like to do this so that subsequent calls to other methods know that the earlier method has already been called, with said parameters.

Example, but fictional code:

package Banana;

use Moose;

has ['peel', 'edible'] => (
  is  => 'ro',
  isa => 'Bool',
  required => 1,
);

has 'color' => (
  is  => 'ro',
  isa => 'Str',
  required => 1,
);

has 'grow_params' => (
  is  => 'ro',
  isa => 'HashRef',
);

sub grow {
  my ($self, $params) = @_;
  # params would be a hashref of method arguments
  $self->grow_params = $params;
  # Execute some code changing other, initial vars
}

This won't work, as the following error gets thrown:

Can't modify non-lvalue subroutine call of &Banana::grow_params

I've looked here on SO and on PerlMonks but I can't seem to find a general explanation of what the error means. Most answers simply re-write the original code and that's that. So what does the error mean, and can I accomplish what I'm trying to do? Or is this not the way to do it?

Solution

Moose object attributes are hidden behind functions of the same name, so when you have $self->grow_params that's a function. Trying to assign a value to it won't work, you need to call it like a function.

$self->grow_params($params);

But even then, as you've currently defined grow_params as read only, you can't change it's value once the object has been created, even within it's own methods.