One of the things I'm looking forward to in Perl6 is greatly improved sub/method signatures.

I'm hoping that this will eliminate the need for anything like Params::Validate, which IMO is a nasty hack to make up for a serious weakness in Perl5.


I'm going to go over the various features in P::V and see if there are equivalents in Perl6, and bring up any questions I have. I think this will be interesting for folks still new to P6 (like myself) and existing P::V users (I think there's a fair number, but maybe I flatter myself ;)


Mandatory vs. Optional Parameters

This is a pretty straightforward one in P6, I think. Parameters can be marked as required with "is required" like this:

 sub date ($year, ?$month, ?$day) # positional

 sub date (+$year is required, +$month, +$day) #named

This is ok but frankly I'm not too keen on the fact that for positional subs, the default is required, but for named params it's the other way around.

Wouldn't it clearer to just use a leading "?+" for optional named params?


Default Values

 sub date ($year, ?$month = 1, ?$day = 1)

 sub date (+$year is required, +$month = 1, +$day = 1)

Again, nice and straightforward.


Type Validation, "isa", & "can"

Params::Validate allows for several ways to check the _value_ of a parameter. One way is to specify a primitive type like "SCALAR" or "ARRAYREF". In P6 we have that with this:

 sub date (Scalar +$year is required, ...)

I'm not sure is "Scalar" is a valid type though, but that's ok because we have better types built in. In this case we should really use "Int" for a year.

P::V also allows one to specify a class membership ("isa)" or one or more methods ("can") a given object/class must have. In Perl6 we can just specify a class:

 sub transmit (Receiver $receiver)

If I understand this correctly, Receiver is a role here, and one that many different classes may use/implement. This basically combines the isa & can concepts. Instead of specifying required _methods_, we specify the role, which seems conceptually cleaner anyway.


Regexes and Callbacks

P::V lets you validate a value based on a regex or one or more callbacks. I think these can be replaced with subtypes, which is one of the most exciting new P6 features (for me).

So instead of this (Perl5 P::V):

  { size => { callbacks =>
              { 'is a valid month' => sub { $_[0] >= 1 && $_[0] <= 12 }

we can now do this:

  my subtype Month where { 1 <= $^s <= 12 }

This is so freaking awesome!

  Then we just reference the subtype in our sub declaration:

  sub date { $year, Month ?$month = 1, Day ?$day = 1 }

Subtypes can also be defined as regexes:

  my subtype PerlIdentifier where / <+<alpha>+[_]> <+<alpha>+<digit>+[_]>* /;

(not 100% sure on that rule but you get the idea)


Dependencies, Exclusions, and "Require one-of"

With P::V I can do this:

  { credit_card_number =>
    { optional => 1,
      depends => [ 'credit_card_expiration', 'credit_card_holder_name' ] },

    credit_card_expiration => { optional => 1 },

    credit_card_holder_name => { optional => 1 },
  }

I have no idea how I might do this in Perl6, but I would love to see it supported as part of parameter declarations

Similarly, something I've wanted to add to P::V is exclusions:

  { credit_card_number =>
    { optional => 1,
      excludes => [ 'ach_bank_account_number' ] },
  }

Another thing that would be really nice would be to require one of a set of parameters, or one set out of multiple sets, so we could say "we need credit card info _or_ bank account info".

For example, with the examples above, I probably want to require _either_ a credit card number _or_ a bank account number. Providing both is an error, but so is not providing either.

Again, no idea how to do this in Perl6, but it seems like something that could be supported via sub declarations

Since all of these can be checked at compile time (sometimes), it seems like they'd be useful parts of the language, as opposed to something user-created. Of course, I understand that there will be more ways to mess with the compilation in Perl6.


Transformations

Another potential future feature for P::V is the ability to specify some sort of transformation callback for a parameter. This is handy if you want to be flexible in what inputs you take, but not explicitly write code for all cases:

  { color => { regex => qr/^(?:green|blue|red)$/i,
               transform => sub { lc $_[0] } }
  }

I suspect that this could be implemented by a user-provide trait like "is transformed":

  sub print_error ($color where m:i/^ [green | blue | red] $/ is transformed { 
lc })

Presumably this can be done with the existing language. It doesn't add anything at compile time, so it really doesn't need to be part of the language.



Anyway, I'd love to hear feedback on this. What did I get right? What did I get wrong? Did I miss a more elegant way to do something? What other types of param validation do other folks use/want to see?



-dave

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