Why does Moo's builder method have access to only some of the other attributes?

Question

I was expecting a builder method to have access to all the other attributes provided by the caller. But it seems it only has access to those whose name is alphabetically less than the current attribute's. E.g. why can the builder for b here see the value of a but not c? (Both 'a' and 'c' are present in the final object)

Code:

#!/usr/bin/perl -w
use strict;
use Data::Dumper;
{
    package P;
    use Moo;

    printf "Moo version: %s\n", $Moo::VERSION;

    # a and c are defined in the same way
    has a => ( is => 'ro' );
    has c => ( is => 'ro' );
    has b => (
        is => 'ro',
        builder => '_build_b',
    );

    sub _build_b {
        my ($self) = @_;
        print Data::Dumper->new(
            [ $self ], [ 'self_during_build_b' ]
        )->Indent(1)->Sortkeys(1)->Dump;
        return "somebuiltvalue";
    }
}
my $p = P->new({ a => 1, c => 3 });
print Data::Dumper->new([$p],['p'])->Indent(1)->Sortkeys(1)->Dump;

Output:

Moo version: 2.003004
$self_during_build_b = bless( {
  'a' => 1
}, 'P' );
$p = bless( {
  'a' => 1,
  'b' => 'somebuiltvalue',
  'c' => 3
}, 'P' );

Sinan Ünür · Accepted Answer · 2018-02-21T14:53:21.067

6

In fact, you should not assume anything about any other fields in the builder for a specific member. If the value of some member of your class depends on the values of one or more other members, then the appropriate place to handle that would be the class BUILD method.

I don't know exactly what you are trying to achieve, but are you looking for lazy?

Or, maybe b needs to be a method in your class

edited Feb 21 '18 at 14:53

answered Feb 21 '18 at 14:46

Sinan Ünür

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Thanks Sinan for your quick answer. "you should not assume anything about any other fields in the builder for a specific member" - I don't see that documented anywhere. It is confusing that those alphabetically lower are available and the others not. Yeah, I worked around it using `lazy => 1`, although `BUILD` is probably a better place for it. – Peter V. Mørch Feb 21 '18 at 15:03
I really don't understand why I should not be able to rely on the attributes that actually were provided or that have default values. Wouldn't that make the builders more handy? – Peter V. Mørch Feb 21 '18 at 15:07
1

Is there a language where you can rely on the order of initialization of private non-static fields in the constructor? If a property needs access to the values of members of a fully initialized object, it needs to be implemented as a method. `lazy` makes that more efficient since the result of the method call is cached, but that's what's going on there. – Sinan Ünür Feb 21 '18 at 15:14
You can imagine a case where you have builder `a` and builder `b` defined as `b + 1` and `a + 1` respectively. Obviously there is going to be some order of which ones are initialized first. – AKHolland Feb 21 '18 at 15:30
@AKHolland And which order would that be? Should you be able to rely on a particular order? – Sinan Ünür Feb 21 '18 at 16:37
1

@Peter: I doubt if it's true that only attributes with a lexically smaller name are accessible. The construct used by `Moo` is a *hash*, which is inherently and deliberately *unsorted*, so I would expect the builder methods to be called in a random order. It's very possible that `Moo` sorts the attribute names, if only so that successive runs of the same initialisation code produce the same results. A dummy attribute `zz`, whose builder method would have access to every other attribute, but that's an unpleasant approach and an ordinary method is surely preferable. – Borodin Feb 21 '18 at 18:09
1

@PeterV.Mørch Re: "_why I should not be able to rely on the attributes_" -- you are in the middle of the initialization in the constructor, that's enough. Best assume that there is no data until _all_ that is done. Not only must you deal with that member only but you have to be careful with that in itself if it involves complex data structures or composition/inheritance of other objects (order of _their_ construction etc). It's in every language as Sinan says, and really more so in those with more involved OO machinery. – zdim Feb 21 '18 at 19:42
Ok, ok, ok. I get it. :-) – Peter V. Mørch Feb 22 '18 at 09:42

Why does Moo's builder method have access to only some of the other attributes?

1 Answers1