A single exact base-10 decimal type for financial / exact-decimal use,
replacing the historical Currency/Comp/Extended proliferation with one type.
- Construction: DecFromInt/Int64/Str, plus a safe/exact float split
(DecFromFloat takes the shortest decimal — 0.1 stays 0.1 — while the
dangerous binary-exact path is the explicitly-named DecFromFloatExact).
- Value semantics + value-based equality: 2.0 = 2.00 with a consistent hash,
so it is safe as a dictionary key (unlike Java BigDecimal).
- Arithmetic: Add/Subtract (scale = max), Multiply (scale = sum), Negate, Abs,
arbitrary precision via a decimal-digit magnitude (compact Int64 fast path
inflating on overflow).
- Division + rounding: a layered design — a TRoundingMode enum (8 modes,
banker's default) over an IRoundingStrategy interface users can implement
for custom rounding. Division always carries an explicit scale + mode.
- Formatting: ToString/ToPlainString never use scientific notation;
StripTrailingZeros keeps integer zeros (600 stays 600, never 6E+2).
- Conversions out: ToDouble (lossy), ToInt64 (truncating).
IR tests (32) + e2e tests (46). Add/Subtract/Multiply and the value-semantics
run dual-backend; Divide/RoundTo and float conversion run QBE-only pending
native codegen fixes (logic verified on QBE; see bugs.txt).
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