Hi, please point your browser to
"http://nothingmuch.woobling.org/compilation_of_circular_prelude.";
~ any(<graffle vdx pdf png jpg>).pick;
My proposition:
1. *the* perl 6 compiler should ship a reference implementation of
the prelude, that is circular. For example
multi &*infix:<*> (Int $x, Int $y) { [+] $x xx $y }
is the reference implementation of multiplication on integers.
2. each block of code has a cryptographic digest, which is the hash
of it's body with the digests of all the functions it calls.
This digest is stable regardless of optimization semantics, and
is applied to the PIL structure.
3. a foreign function interface must exist, and have function
somewhat like the Inline:: modules function today (presumably
with a lower level but zero-copy interface an option).
4. FFI definitions are defined using a uniform interface, whose
most basic interface is a constructor for a code object that
takes a runtime and a string body, e.g.
Code::Foreign.new(:language<C>, :body("int foo () { return 10
}"));
(Some FFIs might only allow construction of foreign functions at
compile time.)
5. Functions have a notion of equivelence. This is managed based on
the digest. For example
my &c_int_mul = BEGIN { Code::Foreign.new(:language<C>, :body("
int foo (int x, int y) { return x * y }
") };
multi &infix:<*> (int $x, int $y --> int) {
[+] $x xx $y;
}
my $str = &infix:<*><int, int --> int>.digest; # must specify
the variant
&c_int_mul.set_equiv($str); # could be in another file
# or, if they are maintained together
&c_int_mul.set_equiv(&infix:<*><int, int --> int>);
This equivelence is with respect to the semantics of the input
and output. The test suite supposedly can assure that these are
really equivelent by running the same tests against either
version.
6. the runtime is just an FFI provider, which happens to be the
default one too
7. When applying code in the runtime, the runtime is free to use
any equivelent function
8. In order to run perl code at all, some native equivelent
functions must be provided by the runtime, for the basic
operations, and things that can't have reference implementations
like IO calls (which are implemented in the prelude as stubs).
9. static analysis may be leveraged to compile direct calls to
native functions when compile time resolution is possible. In
the example graph, for example, no eval or symbol table
assignments are made, so there is no way the code will ever
change. Hence the entire program can be pre-resolved. This
should be controlled via the 'optimize' pragma.
the reasons for this will now be discussed.
A unified prelude with reference implementations for even the
simplest operations gives us several things:
* a circular structure that doesn't make sense when we try to
run it, since the prelude depends on itself (bad)
* must break circularity by going to native operations
* a reference spec that alternative runtimes can compare to
* a way to kickstart implementations
* just provide a root set of native operation, enough to
break circularity in one single point
* demagicalization of the language
Since FFIs are going to be a core feature of perl 6, they can be
used to bootstrap the whole compilation process. In effect, the
"primitive" operations are now just FFI calls to the runtime we
happen to be executing on.
To promote code reuse and to simplify the model the notion of
equivelence is introduced, letting the runtime pick which version of
a function (FFI or native) it uses.
To make things safe, when the prelude is bug fixed and the runtime
is not yet updated, the cryptographic hash of the function changed,
so it is no longer equal to the native one based on the way they are
paired.
To make things modular, the paring of FFI and pure perl functions is
orthogonal to their location of definition based on the hashing
scheme.
This has some nice properties:
Modules like Template::Stash::XS are depracated. Instead, an FFI
based Template::Stash can be automatically loaded from the
runtime library prefix, and it will be set as equivalenet to the
pure perl Template::Stash.
Modules like DBD::your_db which rely on a certain library can be
stubbed in Perl 6 to tell the user that they must use a certain
runtime. The stubs are set as equal to the FFI calls into the
library.
WRT MMD, you can set the entire MM equivalent to
a certain foreign function, and you can also set any variant
individually. You can even set a single variant to be equivalent to
a multimethod to make the FFI implementation simpler. The compiler
simply presents the runtime with all the possible MMD choices, and
lets the runtime choose between conflicting ones.
For example, in this version the runtime is told that it has two
choices:
multi factorial (int $n) { $n * factorial($n - 1) }
multi factorial (0) { 1 }
my &c_fact = BEGIN {
Code::Foreign.new(:language<C>, :body("
int factorial (int n) {
switch (n) {
case 0:
return 1;
default
return n * factorial(n - 1);
}
}
")
}
# the C version is compatible with both behaviors
&factorial.set_equiv(&c_fact); # all variants
&c_fact.set_equiv(&factorial<int $n> | &factorial<0>); # or some
variants
The runtime needs to choose &factorial<int $n> or &c_fact for one
variant, and &factorial<0> or &c_fact for the other variant. If it
chooses the same implementation for both, then MMD is taken out of
the picture entirely.
Note that this choice might be delayed to runtime if the MMD
choices cannot be statically determined, and might be re-made if
unsafe statical analysis was made (all variants could be determined,
but new ones may be introduced in runtime). This is highly dependant
on the capabilities of the runtime.
--
() Yuval Kogman <[EMAIL PROTECTED]> 0xEBD27418 perl hacker &
/\ kung foo master: /me dodges cabbages like macalypse log N: neeyah!
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