How to pass hash-like object which does associative role to a constructor expecting a hash?

问题

I am experimenting with customized hashes. The following is trying to implement a simpler lookup for config-like hashes:

use v6;

class X::Config::KeyNotFound is Exception {
    method message() {
        "Key not found!";
    }
}

# A hash that allows for nested lookup using a '.' to separate keys.
# (This means that keys themselves cannot contain a dot)
# For example:
#
#   %h = Config.new(%(a => %(b => 1)));
#   my $foo = %h<a.b>;  # <-- $foo = 1
#
class Config does Associative[Cool,Str] {
    has %.hash;

    multi method AT-KEY ( ::?CLASS:D: $key) {
        my @keys = $key.split('.');
        my $value = %!hash;
        for @keys -> $key {
            if $value{$key}:exists {
                $value = $value{$key};
            }
            else {
                X::Config::KeyNotFound.new.throw;
            }
        }
        $value;
    }

    multi method EXISTS-KEY (::?CLASS:D: $key) {
        my @keys = $key.split('.');
        my $value = %!hash;
        for @keys -> $key {
            if $value{$key}:exists {
                $value = $value{$key};
            }
            else {
                return False;
            }
        }
        return True;
    }

    multi method DELETE-KEY (::?CLASS:D: $key) {
        X::Assignment::RO.new.throw;
    }

    multi method ASSIGN-KEY (::?CLASS:D: $key, $new) {
        X::Assignment::RO.new.throw;
    }

    multi method BIND-KEY (::?CLASS:D: $key, $new){
        X::Assignment::RO.new.throw;
    }
}

my %hash = a => %(aa => 2, ab => 3), b => 4;
my %cfg := Config.new( hash => %hash );

# A dummy class to illustrate the problem:    
class MyTest {
    has %.config;
}

# Now this code does not work:
MyTest.new(
    config  => %cfg,
);

The output is:

Odd number of elements found where hash initializer expected:
Only saw: Config.new(hash => {:a(${:aa(2), :ab(3)}), :b(4)})
  in block <unit> at ./p.p6 line 70

(Line 70 is the line MyTest.new( )

The code works fine if I pass a normal hash to the constructor instead, for example using %hash instead of %cfg:

MyTest.new(
    config  => %hash,
);

回答1:

The class also needs to do the Iterable role:

class Config does Associative[Cool,Str] does Iterable {
    ...
}

Which requires that an iterator method be implemented. In this case, it's probably easiest to delegate to the iterator of the nested hash:

method iterator() { %!hash.iterator }

With this, the error is resolved. (The default iterator gives an iterator that is a 1-item sequence containing the object itself, thus the error observed.)

The iterator is required because the semantics of constructing an object with a hash attribute are assignment, not binding. When we assign into a hash, then we obtain an Iterator from the thing we're assigning from, and iterate it to get the values to assign. I mention this in case your expectation was binding - that is, that MyTest will reference the instance of Config. For that, one would need to write a custom BUILD that does binding in MyTest, or to declare it as has $.config instead, which means it will just reference the Config instance rather than copying the values out of it into a new hash.

来源：https://stackoverflow.com/questions/55557297/how-to-pass-hash-like-object-which-does-associative-role-to-a-constructor-expect

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