core/.trellis/tasks/09-04-dever-language-mvp/research/runtime-dependencies.md

Core 0.1 Runtime Dependency Research

• Checked: 2026-09-03
• Scope: dependencies required by already-approved Decimal and Map semantics

Decimal128

Selected for the MVP: dec = "=0.4.11".

Evidence:

• The dec crate documentation describes Decimal128 as a 128-bit decimal representation with 34 digits of precision and exposes a configurable Context with rounding and exceptional-condition status.
• The Decimal128 API supports the required arithmetic and classification operations.

Reason:

• It directly implements the approved decimal128 contract instead of approximating it with a lower-precision financial decimal.
• Context status lets Dever convert overflow, division by zero and invalid Decimal states into language-level runtime faults.
• A reusable Context avoids the per-operation default-context overhead documented by the crate.

Constraint:

• dec is a safe Rust wrapper over libdecnumber and therefore introduces a native C build dependency. It must remain isolated behind dever_runtime::number::DecimalValue.
• Dever conformance tests, not the crate's operator defaults or special-value surface, define public behavior.
• A future pure-Rust replacement is allowed only when it passes the same conformance suite; this does not change .dever semantics.

Rejected for Core 0.1:

• rust_decimal: pure Rust, but its 96-bit coefficient does not provide the approved 34-digit decimal128 contract.
• Hand-written decimal arithmetic: correct rounding, exponent handling and status behavior are too risky for the language MVP.
• Newly published decimal128 implementations: potentially useful later, but less established than the selected reference implementation.

Insertion-Ordered Map

Selected for the MVP: indexmap = "=2.14.0".

Evidence:

• The IndexMap project documents hash-based lookup with iteration independent of hash order.
• The IndexMap API documents that insertion appends new keys, overwriting preserves the existing position, and order-preserving removal is available.

Reason:

• Those operations match the approved Dever Map contract without maintaining a duplicate vector-plus-index implementation.
• Lookup remains hash based while iteration is deterministic.

Constraint:

• Dever wraps IndexMap and exposes only get, put, remove and entries.
• Removal uses the order-preserving operation rather than swap removal.
• Dependency-specific index access never appears in .dever or public diagnostics.

Dependency Discipline

These are planned pins; the initial parser milestone has no external dependencies. Verify availability and pin each version when the runtime first consumes it. No dependency becomes a public language contract. Upgrading either dependency requires running the focused Decimal/Map conformance tests and confirming that serialized/displayed Dever values remain unchanged.