Overview

This change adds support for serializing and hashing arbitrary-precision Integer and Natural values in the Unison runtime.

Problem: When a Unison value containing an Integer or Natural was serialized via reflectValue, the runtime would fail with the error: reflectValue: cannot prepare value for serialization: foreign value. This caused operations like storing values in event logs or transferring them across the network to fail.

Solution: Added BigInt and BigNat constructors to the BLit (boxed literal) type in the ANF representation, with corresponding serialization, deserialization, and hashing support.

User experience: Code that previously hung or failed silently when serializing values containing Integer or Decimal types now works correctly.

Implementation approach and notes

1. ANF.hs: Added BigInt Integer and BigNat Natural constructors to the BLit data type, alongside existing literals like Text, Bytes, Pos, Neg, etc.

2. Tags.hs: Added BigIntT (tag 15) and BigNatT (tag 16) to the BLTag enum for wire format identification.

3. ValueV5.hs & ANF/Serialize.hs: Added serialization functions that encode arbitrary-precision numbers as:

   • Natural: length-prefixed list of Word64 chunks (most significant first)
   • Integer: sign byte (0=positive, 1=negative) + Natural magnitude

4. MurmurHash/Untyped.hs: Added hashing support for BigInt and BigNat by hashing the sign and Word64 chunks.

5. Machine.hs: Updated reflectValue (goF) to convert WrapInteger/WrapNatural foreign values to ANF.BigInt/ANF.BigNat, and updated reifyValue (goL) to convert back.

Interesting/controversial decisions

• Chose BLit over alternatives: BLit already contains other serializable foreign values without literal syntax (Code, Quote, BArr, Arr), so adding BigInt/BigNat here is consistent with existing patterns.

• Serialization format: Used a simple length-prefixed Word64 chunk format rather than a more compact variable-length encoding. Happy to change this if controversial.

• Tail recursion: Made the naturalToWord64s and integerToWord64s helper functions tail-recursive and strict.

Test coverage

• Added property-based tests using Hedgehog in Unison/Test/Runtime/ANF/Serialization.hs
• Added genInteger generator that produces small integers (fit in Int64), large positive integers (2^64 to 2^256), and large negative integers
• Added genNatural generator that produces small naturals (fit in Word64) and large naturals (2^64 to 2^256)
• These generators are included in genBLit, which feeds into the existing valueRoundtrip property test
• Added transcript to exercise the serialization

All 276 runtime tests pass.

Loose ends

Serialization format can be debated.