Affine Builder Types

Phase 6 adds a narrow, runtime-checked form of affine builder type to Plato:

unique type List<T> { }
unique type Buffer<T> { }

The unique modifier is declaration-site only. A use of List<T> or Buffer<T> is not annotated; the type itself carries the affine convention. For this first phase only the intrinsic builder names List and Buffer may be declared unique. The compiler rejects the modifier on every other type.

Runtime Model

List<T> is backed by handwritten PlatoList<T> in Plato.Intrinsics. It is a growable append builder with:

• Count and At observe the builder.
• Add, AddRange, and Set mutate in place and return the builder.
• Freeze() consumes the builder and returns IArray<T> without copying.

Buffer<T> is backed by handwritten PlatoBuffer<T>. It allocates a fixed number of slots, writes by index, then freezes:

• Count and At observe the builder.
• Set mutates in place and returns the builder.
• Freeze() consumes the builder and returns IArray<T> without copying.

Both runtime builders carry a frozen flag. Any use after Freeze() throws InvalidOperationException. This gives the safety backstop now, before the future type-checker can enforce affine occurrence counts statically.

Source Conventions

Call sites should rebind or chain mutators:

new List<Integer>().Add(1).Add(2).Freeze()

For generic algorithms, use an existing array value to create an empty builder whose element type is known:

xs.Reduce(xs.EmptyList(), (acc, x) => predicate(x) ? acc.Add(x) : acc).Freeze()

Write Freeze() with parentheses. In classic extension-method output, no-arg library functions that move out of structs are methods, and the explicit call syntax keeps this API shape clear.

Compiler Rules In This Phase

Implemented now:

• Parser and AST recognize unique type.
• Symbols carry TypeDef.IsUnique.
• Only List and Buffer may be unique.
• C# generation maps List<T> to PlatoList<T> and Buffer<T> to PlatoBuffer<T>.
• Unique types are not emitted as generated structs.
• LINT006 reports fields whose direct type is unique.
• LINT007 reports fields, parameters, or return values that put a unique type inside another generic type.

Deferred to the future type-checker:

• Occurrence counting along control-flow paths.
• Rejecting use after consume statically.
• Rejecting builder capture in lambdas.
• Stronger effect checking from the compiler's UniqueTypes table.

Validation

The conformance-only file stdlib-legacy-tests/unique.algorithms.plato contains the validation slice. It keeps production output additive while exercising:

• append/freeze with List<T>;
• generic Filter;
• convex quad triangulation as the ear-clipping proof slice;
• closed-triangle extrusion side indices;
• fixed-size indexed writes with Buffer<T>;
• AddRange plus Set.