Symbols and the round-trip invariant

Canonical form is BASE-QUOTE, uppercase, dash-separated: BTC-USD.

Your SymbolFormat declares the contract

defmodule DpExchange.YourVenue.SymbolFormat do
  @behaviour DpExchange.Core.SymbolNormalizer

  @mapping %{sep: "", quotes: ~w(BUSD USDC USDT USD EUR BTC ETH)}

  @impl true
  def to_canonical_symbol(native),
    do: DpExchange.Core.CanonicalPair.to_canonical(@mapping, native)

  @impl true
  def to_exchange_symbol(canonical),
    do: DpExchange.Core.CanonicalPair.to_exchange(@mapping, canonical)
end

Both directions are required. A package shipping one fails the compiler's missing-callback check, which is deliberate: one-directional normalisation round-trips wrong silently.

Quotes must be ordered longest-first — and here is what that actually means

The rule is usually stated as "longest first", which is right but under-explains itself.

The collision is a quote that contains a shorter quote. BUSD ends with USD. So BTCBUSD matches USD before it matches BUSD, and a shortest-first list splits the base as BTCB — a pair that does not exist, carrying values that all look plausible.

Worth knowing what does not collide: USD, USDT and USDC round-trip correctly in either order, because none is a suffix of another. A fixture built only on those looks like it is testing the ordering rule while testing nothing.

The invariant

to_canonical_symbol(to_exchange_symbol(pair)) == pair

For every pair, not just the ones you thought of. The conformance suite generates pairs over your own declared supported_quotes for exactly that reason.

Both directions are total

Malformed input must never raise. Unparseable input is uppercased and passed through, never dropped — a dropped symbol is invisible, a strange one is reviewable.

Asset aliases

Where your venue publishes legacy codes, map them:

@mapping %{sep: "", quotes: ~w(USD EUR), asset_aliases: %{"XBT" => "BTC"}}

The alias is reversed on the way out, so the round trip still holds.