The name freed up by the rename now means what C means by it: the members
overlay one storage, the size is the largest of them, the alignment the
strictest, and nothing anywhere records which one was written. It serves
two things that wanted it. Binding a C header means holding the union the
library holds and reading whichever member the library's own tag says is
live -- a tag Flan cannot see, because the rule relating them is prose in
a manual. Overlaying an f32 on a u32 to look at its bits is the other,
and it is the same read.
So that read is defined rather than refused. This is the one place in the
checker where bytes win over safety on purpose, and the alternative was
not a safer language, it was no feature: type punning *is* reading the
member that was not written. The promise is the one C's implementations
make and C's standard does not -- the layout is the target's, the bytes
are the bytes, a read is a reinterpretation of them -- and what is not
promised is anything about bytes nobody wrote, where a member wider than
the one last stored reads a tail that is indeterminate exactly as a
struct's padding is. ZII narrows that to almost nothing: a union starts
all-bytes-zero unless uninit says otherwise.
uninit on one is allowed, unlike on a defdata. The refusal there was
never about garbage; it is that a tag steers, and a tag no case names
falls past every comparison in a match into a block LLVM may treat as
unreachable. An untagged union steers nothing.
Which is also why three things are refused, each for a reason that does
not expire with a milestone. No move-only member: nothing knows which
member is live, so nothing can tear one down, and unlike the struct and
defdata refusals this is not waiting on recursive teardown -- there is no
fact for teardown to read. No bool at any depth: an i1 loaded from a byte
that is neither 0 nor 1 is a value the optimiser may assume cannot exist,
and a union is the only type that can produce one. No defdata at any
depth, for the reason uninit gives, arriving the other way round. An
Option member is fine and the walk says why: its match is a tag test and
a branch, not a chain with an unreachable tail.
Two members in one literal, a match on a union, a union map key and a
member written into a global initialiser are each refused by name.
A union is a field list whose every offset is zero, so it travels as a
Tast.structure and the checker, the emitter and the x86 backend each grow
one table rather than one shape. A value is a zeroed temporary and a
store -- Set over Pfield, which every backend already has -- so there is
no new IR node and no layout rule spelled out a second time per backend.
The LLVM type is the blob clang gives a union, the DWARF is
DW_TAG_union_type with every member at zero, and the printer names the
type and does not walk it: it cannot know which member is live, and one
of them may be a pointer.
cimport can now check what it could not. A C record holding a union
member was not recorded at all, so the defstruct beside it went unchecked
rather than checked wrongly; a named union member resolves to a defunion
now and the whole record is compared field by field. The defunion itself
is compared against the header's union as a set and not in order --
every member is at offset zero, so a permuted one is the same type and
reporting it would be a finding that is not one -- while a member the
header has and Flan lacks is reported, because that is what changes the
size. A defunion against a C struct, or a defstruct against a C union,
is reported in both directions. An anonymous union member is still
skipped, and the comment now says that the gap is on the Flan side:
there is nothing to declare.
A program that wants to load its data once and keep it could not say so. Every
move-only global was refused where it was declared, on an argument about the
dead set being per function: two functions each freeing the same global would
be a double free nothing could see. The argument was sound and the conclusion
was too strong. It assumed a global has an owner. It does not.
Reading a move-only global is now always a borrow. Nothing may take ownership
of one, so nothing may free one, and with no owner to hand over there is no
double free left to catch. This is not a general ownership model for globals
and is not meant to grow into one: it is sound precisely because the lifetime
question that model would exist to answer has a constant answer here, the
process's. The refusal lands at the read, which is where a move would have been
recorded for a local -- passing the global to something that owns its
parameter, binding it to a local, returning it and freeing it all reach the
same place, and each is told to borrow instead, or to clone if it really wants
something of its own.
Such a global is mutable where it stands. push, put, reserve and set already
take their target through the borrow path, so a global (Vec u8) is filled and
grown in place, and the aliasing that raises is the one every Vec has:
spec-memory.md's explicit Zig/Odin contract, where a push that reallocates
invalidates a slice taken before it and the dev build's generation word traps
on the stale one. Globals get no borrow rule locals do not have, because the
hazard is not new and the trap lives on the Vec rather than on the binding.
What a move-only global may not do is carry a computed initialiser. A global's
initialiser is a link-time constant -- there is no init-at-startup path in the
LLVM backend by design, and the x86 backend that has one deliberately leaves it
out of a reload module, because re-running an initialiser wipes the live state
reloading exists to preserve. So the global starts zeroed, which for a Vec is
an empty Vec and therefore a value rather than a placeholder, and the load is
an ordinary assignment in whichever function loads it. That is also what makes
the data survive: nothing runs between one entry to main and the next, so a
re-entered main finds the global as it left it. A defconst cannot be one at
all, since a constant is not an assignable place and nothing could ever load
it; both refusals name the (defvar g (Vec u8)) that works.
The reload fixture gains a global Vec in the host and another that arrives at
run time, because that is where declaring instead of defining has teeth: a
module that defined the host's Vec would take a zeroed header of its own and
strand the block the process is still using, which a re-zeroed i64 cannot
demonstrate.
handler-bind takes (Type [name] body ...) and has since it was written; the
sketch paired a type with an fn, which is the shape parse.ml names in its own
refusal message. load-level had two return types. And there is no defcondition
anywhere in the tree -- a condition type is an ordinary struct, which is what
both spec-conditions.md and conditions.org say, so the one form in this file that
introduced one was inventing it.
The header's rules went with them: lowercase-is-a-type-variable and "no sigils"
are both the pre-$t spelling, and let never took an annotation.
spec-memory.md's generics section still said there were no constraints and wrote
its type variable as a bare lowercase name, which is the spelling the $t sigil
replaced -- the largest example in it would not have compiled. The where clause
and the five predicates are plan.org's account and this now agrees with it rather
than contradicting it. The empty-map example named no types and used defvar,
neither of which is how map-new is called.
spec-conditions.md named BoundsError once and nothing else. The four conditions
the runtime signals, and the split between the two that establish a retry and the
two that deliberately establish nothing, belong in \xc2\xa75 because that is the section
whose account of restarts the split could have falsified. It does not: the rule is
that the restart a bad index wants is the one the program already had.
The slot after a defn's parameters is unconditionally a type. Parse.decl no
longer takes a set of type names, and is_type_form, qualified_type, types_in,
declared_types and prelude_types are gone with the pre-pass that fed them.
What they were for: (Option f64) and (Some 1) are the same s-expression, so the
parser decided which it had by looking the head up in a set of the file's own
type names. Sound -- one top-level namespace means a name cannot be both a type
and a value -- and brittle, because the set had to be complete. It was wrong
twice in one day, the second time parsing (defn f [] (Rune {.code 65}) (bar))
as a function returning a Rune with a one-form body, silently, in every file in
the language.
Two things fall out. A type the parser could not have known -- a struct
declared further down the file, rl/Vector2 behind an unresolved alias, a
prelude type -- never needed recognising, only placing. And a mistyped type is
a mistyped type: (defn f [] f65 0.0) reaches the resolver's near-miss check and
says did you mean f64, where it used to be read as the first form of the body
and reported as an unknown name.
Unit is written (). The old spelling is refused with a message naming the new
one, the rule the colon-to-dot change followed. Internally it is still
Tname "Unit" and Types.Unit, so the resolver, the shim and the emitter did not
change; Cimport still builds Tname "Unit" for C's void without going through
the parser. Types.to_string prints () though -- that printer prints what a
person would write for every other type it knows, [i32], {K V}, (Ptr T), and
Unit was the odd one out once the source spelling moved.
Dropping prelude_types removes one of the two reasons Macro.reduce may only
drop defns: the memoised set a bootstrap build could have poisoned is gone, so
the remaining reason is the plain one.
web/index.html's Flan blocks convert and its output blocks do not, which
is the same split render.ml makes: the printed form keeps the colon until
the Emacs inspector that reads it moves too. Same in BUILT.md.
plan.org, spec-conditions.md and spec-memory.md carried struct literals in
the old spelling and now do not.
NEXT.md decision 6 is struck, and batch item 2 with it, naming what to run
at merge. BUILT.md says why the colon belongs to keys -- mostly that a map
literal wants {:key value}, and two literals sharing one syntax would have
left the reader asking the checker which it was looking at.
The sweep was not idempotent and is now: {.k :hi} -- a field already
converted, holding an enum member -- read as a destructuring pair on a
second run and ate the member. A re-run over a lane's files would have
corrupted them silently, which is exactly what the tool exists to do
safely.
The struct/class split, written down before anything is built on it. A struct
stays a fixed-layout value with C's layout, which is what keeps the FFI, SoA and
wasm stories intact; a class is a separate kind with identity, metadata and an
implementation-defined representation, for the long-lived gameplay objects that
want to change shape while the program is running.
The tagline loses "no GC" for "no mandatory GC", because a small collector
confined to class instances is now an option rather than a contradiction. CLOS
goes from a flat non-goal to a bounded one: the metaclasses, method combination
and arbitrary change-class are out; exact-class single dispatch and an explicit
frame-boundary migration are in.
Migration is eager and explicit rather than CLOS's lazy-on-access, which would
put a check on every slot read. Class identity is stable and layouts are
numbered, the same shape as the function versions the hot reload section grew.
Nothing is frozen and nothing is to be built until struct, Handle and reload
semantics are working.
spec-memory.md drops (set (get m k) v) from the assignable forms: a map has an
upsert of its own, put, which either inserts or replaces, so there is no store
into a lookup - and an absent entry has no location to store into anyway.
The compiler still parsed it into an Ast.Pkey and refused it downstream as
unimplemented, milestone 6, which is the wrong reason for something that is
never arriving. The place form is gone from ast, tast, load, check and emit,
and the parser refuses the shape where it is written, with the reason and a
pointer to put.
