refactor(motoko/icrc2-swap): generate ICRC bindings from a committed .did

[marc0olo]

What

Replaces the hand-written backend/ICRC.mo type module in icrc2-swap with Motoko bindings generated by didc from a committed ICRC-1/ICRC-2 .did. This example is the deliberate counterpart to the single-fixed-target --actor-env-alias examples (ic-pos, evm_block_explorer, llm_chatbot): a backend that talks to many ledgers, with the ledger chosen at runtime.

Why this pattern for ICRC

icrc2-swap holds two ledgers at once (token_a, token_b) and picks the target per call (actor(args.token.toText()) : ICRC.Self). That's the "one interface, many ledgers, runtime-chosen principal" case — which --actor-env-alias structurally cannot express (it binds one import to one principal). The right tool is a shared generated service type + actor(id). This generalizes to any DeFi-shaped backend (swaps, routers, wallets) that must interact with many ICRC ledgers (ICP, ckBTC/ckETH, SNS tokens, …).

Contrast with ic-pos, which talks to a single configured ledger and also calls a ledger-specific method (get_transactions) — there --actor-env-alias with the full ledger .did is the right fit. The two examples now demonstrate both axes on purpose.

Changes

• candid/icrc.did — the ICRC-1/ICRC-2 interface the backend calls, as a focused plain service. Type definitions are taken from the ic-icrc1-ledger reference wasm (the exact artifact this example pins for token_a/token_b); the service is the standard ICRC-1 + ICRC-2 method subset. This is the regeneration source.
• bindings/ICRC.mo — didc bind -t mo candid/icrc.did. Committed, so building needs no extra tooling.
• backend/app.mo — imports the generated bindings (ICRC.Self), keeps the runtime-principal pattern; hand-written ICRC.mo removed.
• Unrelated tidy-ups in the same example: Map swap (return always discarded) → add (which is literally ignore swap), 6 sites; and mo:core 2.5.0 → 2.6.0.

Testing

bash deploy.sh && bash test.sh — all 7 tests pass (deposit via icrc2_transfer_from, atomic swap, withdraw via icrc1_transfer with correct fee-adjusted balance deltas), confirming the generated bindings are wire-compatible with the real ledger.

Discussion: binding generation & tooling (cc @Kamirus)

The bindings here are idiomatic straight from didc today because ICRC-1/2 Candid type names are already PascalCase. For interfaces whose Candid type names are snake_case (e.g. an LLM canister's chat_message_v1), that isn't the case — which is exactly what dfinity/candid#729 fixes (PascalCase Motoko type aliases). Flagging it because it's the generator that makes this pattern broadly clean.

Bigger picture: we commit bindings/ICRC.mo here because the only generator today is didc, a separate tool outside the icp-cli/mops/ic-wasm toolchain — gitignoring it would force a didc install on everyone who builds. The ideal end state would mirror the JS/TS story (@icp-sdk/bindgen generates src/bindings/*.ts at build time, gitignored): mops generating .did→Motoko bindings as a pre-build step, with the .did committed and the generated .mo gitignored (ideally with committing as an opt-in for teams that prefer it). mops already does the forward direction (mops generate candid), so this is the symmetric missing half — and #729 is the generator that would power it. Until then, committing the bindings is the right call.

(didc releases: https://github.com/dfinity/candid/releases)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR refactors the motoko/icrc2-swap example to use a committed, didc-generated Motoko binding for the standard ICRC-1/ICRC-2 ledger interface, allowing the backend to type ledgers selected by principal at runtime (actor(<principal>) : ICRC.Self) without maintaining a hand-written interface module.

Changes:

• Adds a focused candid/icrc.did for the ICRC-1/ICRC-2 method subset used by the example and commits the generated Motoko binding in bindings/ICRC.mo.
• Updates the backend to import and use ICRC.Self from the generated bindings, while preserving the runtime-selected-ledger pattern.
• Bumps mo:core from 2.5.0 to 2.6.0 and applies minor map-update tidy-ups (swap → add).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
motoko/icrc2-swap/README.md	Updates architecture docs and adds binding-regeneration instructions using didc.
motoko/icrc2-swap/mops.toml	Bumps mo:core dependency version.
motoko/icrc2-swap/candid/icrc.did	Introduces the committed Candid interface used as the binding source-of-truth.
motoko/icrc2-swap/bindings/ICRC.mo	Adds committed, generated Motoko service/types binding (ICRC.Self, records, variants).
motoko/icrc2-swap/backend/app.mo	Switches backend to the generated binding import and updates types/usages accordingly.

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[Kamirus]

Yes, I think once dfinity/candid#729 lands it would make sense for mops to adopt this tool.
Mops would handle fetching it via mops toolchain and we could wrap it with mops generate ... command