Type-Aware SCALE-to-JSON Serializer

[TarikGul]

Background

Currently, the API uses heuristics to identify which SCALE-encoded values should be transformed during JSON
serialization:

Extrinsics: Field-name pattern matching (~20 patterns)

// Generic approach: "dest", "who", "target", "beneficiary", etc.
if is_account_field(field.name()) {
    // Convert AccountId32 → SS58 address
}

Events: Position-based mappings (~100+ event-specific mappings)
match (pallet, event) {
    ("balances", "Transfer") => vec![0, 1],  // positions 0 and 1 are accounts
    ("treasury", "Awarded") => vec![1],       // position 1 is beneficiary
    // ... 100+ more mappings
}

Problem

These heuristics have several limitations:

1. Maintenance burden: Every new event type requires manual position mapping updates
2. False positives: Field names like "data" or "hash" might accidentally match patterns
3. False negatives: Unusual field names for accounts might be missed
4. Type ambiguity: Can't distinguish AccountId32 from H256 (both 32 bytes)
5. Limited scope: Only handles account transformations, but other types need similar treatment (e.g., Balance →
   decimal formatting)

Desired Solution

A type-aware serializer that uses Substrate's type metadata to make transformation decisions:

// Pseudocode - what we want:
for field in value.fields() {
let type_id = field.type_id();

  match registry.resolve(type_id) {
      TypeDef::Composite(AccountId32) => serialize_as_ss58(field),
      TypeDef::Composite(Balance) => serialize_as_decimal(field),
      TypeDef::Composite(H256) => serialize_as_hex(field),
      // ... other type transformations
  }

}

This would work for any type transformation, not just accounts:

• AccountId32 → SS58 address
• Balance → decimal string with proper precision
• Hash → 0x-prefixed hex
• Custom types → domain-specific formatting

Blocker: Metadata Version Incompatibility

This approach is blocked until subxt-historic v0.50 ships.

Why: Historic blocks use different metadata versions with incompatible type identification systems:

• Legacy metadata (V8-V13): Uses LookupName for type IDs
• Modern metadata (V14+): Uses u32 for type IDs

There's no uniform way to resolve types across all metadata versions until subxt-historic v0.50, which will
translate all historic metadata to a modern, uniform format.

Current Status

The heuristic approach (field patterns + position mappings) is the pragmatic solution given these constraints. It
works well for common cases and can be incrementally improved by:

1. Adding more field name patterns as we discover them
2. Expanding event position mappings for new pallets
3. Documenting edge cases and limitations

Future Work

Once subxt-historic v0.50 ships with metadata translation:

1. Design a type-aware serialization system using PortableRegistry
2. Create transformation rules based on type definitions (not field names/positions)
3. Support arbitrary type transformations (not just accounts)
4. Remove the hardcoded heuristics
5. Reduce maintenance burden and improve correctness