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+# Step: Clarion CLASS lowering — design (not yet implemented)
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+
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+Status: **not started** (scoped here). Suite **137/137**; fixpoint OK.
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+
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+## Current state
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+
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+Classes *parse, scope and check* — fields, methods, `=` consts,
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+single/multiple parent lists, `VIRTUAL`, and `CLASS IMPLEMENTATION`
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+blocks with nested procedures — but every class/method ends in error
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+**230** and method bodies are suppressed (`QbeGen.SetNoEmit(TRUE)` +
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+`AbortFunc` in `MethodImpl`). See `docs/OOP.txt` for the grammar.
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+
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+## Goal
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+
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+Make a class a usable **value** with callable methods:
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+
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+```modula-2
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+TYPE
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+ CLASS Point;
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+ x, y : INTEGER;
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+ PROCEDURE Set(a, b : INTEGER);
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+ PROCEDURE Get() : INTEGER;
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+ END Point;
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+CLASS IMPLEMENTATION Point;
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+ PROCEDURE Set(a, b : INTEGER);
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+ BEGIN x := a; y := b END Set;
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+ PROCEDURE Get() : INTEGER;
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+ BEGIN RETURN x + y END Get;
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+END Point;
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+VAR p : Point;
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+BEGIN p.Set(3, 4); ExitCode := p.Get() END
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+```
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+
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+## Design sketch
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+
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+1. **Layout** — a class is a record: fields get declaration-order
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+ offsets via the existing `ComputeOffsets`/`FieldOffset` machinery
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+ (already shared with `RECORD`; `PushRecord` already accepts
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+ `FClass`). `obj.field` then works through the record field path.
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+ Single inheritance prepends the parent's fields (offset by the
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+ parent's size); multiple parents stay 230.
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+
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+2. **Receiver** — each method gets a **hidden first parameter** `THIS`
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+ of type `POINTER TO <class>`. Inside a method body, a bare field
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+ name `f` is an implicit `THIS^.f`; the class scope already holds the
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+ fields as `KindField`, so `Design` needs a "current receiver"
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+ addressing mode (like `WITH`): push the receiver register at method
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+ entry, add `FieldOffset` to it, load/store.
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+
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+3. **Calls** — `obj.M(args)` binds statically to the method's mangled
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+ symbol and passes `ADR obj` as the hidden first argument, then the
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+ user arguments. `QbeGen.CallBegin`/`ArgList` gain a flag for the
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+ implicit receiver. Non-virtual first; `VIRTUAL` accepts but
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+ dispatches statically with a `230`-free path.
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+
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+4. **Dispatch (later)** — a per-class vtable (one static array of code
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+ pointers, parent entries first) stored as a hidden first field;
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+ `VIRTUAL` calls load the slot and call indirectly (procedure types
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+ + indirect calls already exist from step 8.5). This can be a
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+ follow-on increment.
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+
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+5. **`WITH`-style field binding** — reuse `PushRecord`/`PushWith`; the
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+ receiver is the `WITH` base, so `WITH p DO x := 1 END` works for
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+ free once (2) lands.
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+
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+## Acceptance (planned)
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+
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+- The `Point` program above compiles and exits 7.
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+- `obj.field` read/write, method calls with value and `VAR` params,
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+ methods returning values.
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+- Single inheritance: child methods see parent fields.
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+- Suite green; fixpoint still byte-identical.
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+
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+## Risks
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+
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+- `Design`'s addressing is threaded through several productions;
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+ adding a receiver mode must not change existing `RECORD`/`WITH`
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+ codegen (fixpoint is the guard).
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+- `VIRTUAL`/vtable must not perturb layout for non-virtual classes.
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+
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+## Files (planned)
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+
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+`compiler/src/M2.atg` (`ClassRest`/`ClassImplRest`/`MethodImpl`
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+lowering, `Design` receiver mode, call-site receiver),
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+`compiler/src/SymTab.def`/`.mod` (receiver scope, parent field
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+offsets, vtable), `compiler/src/QbeGen.def`/`.mod` (receiver param,
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+vtable emission, indirect `VIRTUAL` calls),
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+`compiler/tests/t_class*.mod`, `docs/OOP.txt`.
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