Re: Design Pattern for Validation

On Tue, Oct 11, 2011 at 04:30:25PM +0200, Peter Gordon wrote:
> I am trying to find a decent design pattern for Moose validation of
> user input. All the Moose examples I have found either assume that 
> the data is correct or else dies.

...
> From an OOP perspective, it seems to me that the validation performed
> by the subtype should be part of the class and not something that is
> just floating around.

Agreed - since we already have the class's constraints right there in the
class definition, we can just use existing moose code to perform data
validation in an object instance.

>                      There is also no reasonable way to trap the
> error, and report back to the user that there was an error. And I am
> not a fan of try/catch.

Check out MooseX::Constructor::AllErrors - this is exactly what it was
designed for.  You still need to use a try block, but this is not a code
smell on its own -- and you will get *all* type violation errors back (as
well as other errors like missing attributes that are required=>1, etc), in
a tidy object that you can query to get the nature of the error(s) that
occured.

Example (uses TryCatch):

my $obj;
try
{
    $obj = $class->new(%args);
}
catch ($exception)
{
    my $meta = Class::MOP::class_of($exception);
    my $message = blessed $meta && $meta->isa('Moose::Meta::Class')
        ? join('; ', map { $_->message } $exception->errors)
        : $exception;

    log_error("failed to construct a $class: ", $message);
}


PS. As an aside, I gather from irc #moose that a few folks have been
looking into other nicely-formed exception classes for Moose - if the same
API could be used as in MX::C::AE, then the above code block will work in
other situations as well as at construction time.


-- 
     "Sometimes there is a silver bullet for boosting software engineering
     productivity. But you need to shoot the right person." - Craig Andera
            .             .            .            .             .
Karen Etheridge, [email protected]       GCS C+++$ USL+++$ P+++$ w--- M++