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 w

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 http://en.wikipedia.org/wiki/Nominal_type_system