Perl 6 – Getting beyond static vs. dynamic [pdf]

Right. I think it's reasonable to say that all P6 code is statically typed, using nominal typing.[1]

I've not actually checked the following code yet. I just wanted to post it now and will return to update after I've tested this stuff tonight, but here's what I'm currently thinking:

* The Any type is the default value type.

* Built-in literals are automatically converted in to more specific value types at the time they are encountered by the compiler:

  my $a;   # $a contains an `(Any)` aka an undefined Any
  $a = 42; # puts an Int value into $a

* User defined literals and run-time string input are also supposed to be optionally automatically converted from an Any into a more specific type on assignment, but that's not yet implemented in the Rakudo P6 compiler.

* Optional explicit types improve type-checking on assignment:

  my Int      $d = 22/7; # compile-time error

* Not yet implemented but due to land soon:

  my Int(Any) $e = 22/7; # coerces 22/7 (a Rat, thus an Any)
                         # to 3 (an Int) at compile-time

* And for calls:

  sub add (Int $x, Int $y) { $x + $y }

  my     $a = 42;
  my     $b = '42';
  my Str $c = '42';

  add($a, $a); # OK
  add($a, $b); # run-time error?
  add($a, $c); # compile-time error

* Optional explicit types enable multisub dispatch:

  sub add (Int $x, Str $y) { say "oops!" }

  add($a, $c); # resolves to above sub at compile-time
               # says "oops!" at run-time

* P6 allows users to introduce "subsets". Subsets combine a base type (always static/compile-time) with additional constraint logic. The additional constraint may force the subset to become partially dynamic rather than solely static.

* Subsets can be applied in an assignment or signature:

  my enum Day ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'];
  subset Weekday of Day where 'Mon' .. 'Fri';

  my Weekday $wd = Sat; # compile-time error

  sub foo (Weekday $wd) { ... }

* The additional constraint logic of a subset is a P6 "smart match" -- arbitrary code that accepts or rejects a specific value of the base type. In the Weekday subset above the constraint logic is `'Mon' .. 'Fri';`.

* If code analysis can deduce a finite set from the combination of base type and the additional constraint logic, then a compiler may choose to make the whole subset static and check and resolve calls and constrained assignments at compile-time. Otherwise the static part of a subset is checked at compile-time, which may lead to a particular call being among the candidates for a final dispatch, and then there may be a final round of call resolution and checking at run-time.

* Assignments and function calls that don't involve dynamic subsets, or where the compiler can eliminate them from consideration at compile-time, are type checked and dispatch resolved at compile-time.

* Methods are always dispatch resolved at run-time.

[1] http://en.wikipedia.org/wiki/Nominal_type_system

> the "unityped" argument.

Which is a gross, deliberate misunderstanding of how dynamic typing works.

Time, random, user input, self-modifying self-compiling code, undetermined function results, ...

I agree that there are no absolute rules. A fixed system is boring, and often very inefficient depending how you look at it. Sure my system is static, but inside the black box there is a lot of "dynamism" that I can't change without a recompile. The development environment is dynamic. Writing code, creating the rules, and creating the locked down static types are all dynamic actions. There is no need for a separation between developer/user. User can set the rules, create the workflow, compile the code on the fly. There is no need for a separation between compile and run time.

GHC is itself written in Haskell (utilizing a technique known as bootstrapping), but the runtime system for Haskell, essential to run programs, is written in C and C−−.

I don't know much about C--, but C, what Haskell's runtime is written in, is fairly dynamic.

Care to say a bit about your use cases that you consider inappropriate for static typing?

The reason is simple: Using a statically typed language (particularly one with reflection capabilities), it's trivial to do pretty much anything a dynamic language is capable of doing without extra tooling, perhaps with just a bit more verbosity in code. On the other hand, attempting to simulate static typing with an entirely dynamic language doesn't happen without extra tooling around the compiler.

Even languages with strict type systems like Haskell don't entirely throw away dynamic typing. It has `unsafeCoerce` which can attempt to cast a value at runtime, and fail if it's not the right type like any dynamic language would. However, when combined with Data.Typeable, storing type information in addition to the values you're interested in allows you to turn `unsafeCoerce` into something much safer: `fromDynamic`, which doesn't cause the runtime to fail, but instead returns a Maybe type - one that either contains a successfully coerced value or Nothing - and the language forces you to check this before you can use the value. You have the best of both worlds now - the ability to write whatever dynamic typed code you want, but with the ability to fall back to a strict type system to keep things in check.

Because they've been investigating MetaObjects (and MetaObject Protocols) since the 80s. Sorry for the smugness, my argument boils down to "Don't reinvent the wheel too much".

Those spell checkers are easier to make if the language already includes tooling to support that functionality.