934585ac74
Design decision: a protocol-constrained pack element is viewed THROUGH the constraint protocol — only the protocol's interface (its methods, and the projections xs.T / xs.value) is accessible, not arbitrary concrete members, exactly like a constrained generic `T: Show`. So `xs[i].v` (a field on the concrete IntBox, not declared on Show) is an error; the constraint is enforced and bounds the body regardless of the concrete arg types at a call site. The previous example 191 demonstrated `xs[i].v` — which only compiled because the constraint is not yet enforced. Trimmed it to the protocol-agnostic part that's correct today (per-shape binding + comptime `xs.len` across arities / heterogeneous shapes); protocol-interface access + projection are the remaining 2.4 work. specs.md records the access rule.
40 lines
1.5 KiB
Plaintext
40 lines
1.5 KiB
Plaintext
// Feature 1 — heterogeneous protocol-constrained variadic pack (binding).
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//
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// `..xs: Show` (no `[]`, no `$`) is a pack, not a slice: each call site
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// monomorphizes with the concrete per-position arg types (Decision 1 — a pack
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// is a comptime mechanism, no runtime pack value), and `xs.len` is a comptime
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// constant. Arity and element types vary per call; each call is a distinct
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// specialization.
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//
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// DESIGN (locked): a pack element is viewed THROUGH the protocol — only the
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// protocol's own interface (its methods, and the projections `xs.T` / `xs.value`)
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// is accessible, NOT arbitrary concrete members. So `xs[i].get()` / `xs.T` are
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// the intended interface; `xs[i].v` (a field on the concrete IntBox, not part
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// of `Show`) is an error. Those are still being implemented — this example
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// locks in only the binding + comptime `xs.len`, the protocol-agnostic part.
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#import "modules/std.sx";
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Show :: protocol(T: Type) {
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get :: (self: *Self) -> T;
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}
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IntBox :: struct { v: s64; }
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StrBox :: struct { s: string; }
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impl Show(s64) for IntBox { get :: (self: *IntBox) -> s64 => self.v; }
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impl Show(string) for StrBox { get :: (self: *StrBox) -> string => self.s; }
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howmany :: (..xs: Show) -> s64 {
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return xs.len;
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}
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main :: () -> s32 {
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a := IntBox.{ v = 42 };
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b := IntBox.{ v = 7 };
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c := StrBox.{ s = "cool" };
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print("n0={}\n", howmany()); // empty pack
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print("n2={}\n", howmany(a, b)); // two elements
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print("n3={}\n", howmany(a, b, c)); // heterogeneous: IntBox, IntBox, StrBox
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0;
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}
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