Add records: TYPE, DIM ... AS, field access, copy on assign
BASIC 7.0 has no records at all, so none of this is a port. The `@` suffix was
not invented either: the Go reference reserved IDENTIFIER_STRUCT and never used
it, and src/grammar.c rendered such a leaf as "NOT IMPLEMENTED" until now.
Declaring the type is what buys the storage model. A TYPE states its fields, so
an instance has a known slot count and is laid out exactly as an array is -- one
contiguous run from the same value pool DIM already draws from, with field access
as offset arithmetic. No new pool holds data; the only new table holds
descriptors. A nested value flattens into its container's run, which is why
LINE with two POINTs and a string is five slots rather than three.
Each field takes its type from its own suffix, the same rule every other name
here follows, so a field list needs no type column. An `@` field is the
exception and has to name its type, because three primitive types fit in three
suffix characters and N declared types do not fit in one.
The declaration is prescanned before the program runs, like labels and DATA and
for the same reason: it has to be in effect wherever control goes. Three passes,
each for a case the one before cannot do -- names first so a field can refer to
a type declared later, then field lists, then sizes by repeated resolution. What
never resolves is a cycle of by-value containment, so "a TYPE cannot contain
itself by value" is a diagnosis rather than an assumption, and the message says
to use PTR TO instead.
Assignment copies. That interception is the whole feature and it cannot live in
akbasic_value_clone(), which copies one slot -- and one slot holds a *reference*
to an instance rather than the instance, so going through it would alias. A
structure is intercepted before that path and its slots are copied one at a
time, walking the descriptor rather than memcpy-ing the run, because a pointer
field must copy its reference where a value field must copy its slots.
Two smaller things the work required. All three prescans now sit inside one
ATTEMPT: a malformed declaration is the program's mistake, and it was printing a
stack trace and taking the driver with it, which is the boundary goal 3 exists
to draw. And a fresh variable's structtype is -1 rather than the 0 a memset
leaves, because 0 is a valid type index and every new variable was claiming to
be the first type declared.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
2026-08-01 11:39:47 -04:00
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10 REM A field name is checked against a closed set the program declared, which
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20 REM is the one thing in this language whose valid spellings are written down.
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30 REM A misspelled *variable* is still silent -- see the last two lines.
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40 TYPE RECT
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50 W#
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60 H#
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70 END TYPE
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80 DIM R@ AS RECT
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90 R@.W# = 3
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100 PRINT R@.W#
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110 REM A variable nobody assigned is zero, with no complaint at all.
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120 PRINT TOTLA#
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130 REM A field the type does not declare is refused, and lists the real ones.
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140 PRINT R@.NOPE#
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