icrc-ledger

What this skill does

# ICRC Ledger Standards

## What This Is
ICRC-1 is the fungible token standard on Internet Computer, defining transfer, balance, and metadata interfaces. ICRC-2 extends it with approve/transferFrom (allowance) mechanics, enabling third-party spending like ERC-20 on Ethereum.

## Prerequisites

- For Motoko: mops with `core = "2.3.1"` in mops.toml
- For Rust: `ic-cdk = "0.19"`, `candid = "0.10"`, `icrc-ledger-types = "0.1"` in Cargo.toml

## Canister IDs

| Token | Ledger Canister ID | Decimals |
|-------|-------------------|----------|
| ICP | `ryjl3-tyaaa-aaaaa-aaaba-cai` | 8 |
| ckBTC | `mxzaz-hqaaa-aaaar-qaada-cai` | 8 |
| ckETH | `ss2fx-dyaaa-aaaar-qacoq-cai` | 18 |

Index canisters (for transaction history):
- ICP Index: `qhbym-qaaaa-aaaaa-aaafq-cai`
- ckBTC Index: `n5wcd-faaaa-aaaar-qaaea-cai`
- ckETH Index: `s3zol-vqaaa-aaaar-qacpa-cai`

All ICRC-1 ledgers expose `icrc1_fee : () -> (nat) query` to return the current transfer fee. Fees are denominated in the token's smallest unit (e8s for ICP where 1 ICP = 10⁸ e8s, satoshis for ckBTC, wei for ckETH). Each ledger sets its own fee, and fees can change at runtime.

## Common Pitfalls

1. **Assuming all ledgers share the same fee** -- Each ledger sets its own fee (e.g., ICP = 10000 e8s, ckBTC = 10 satoshis). Never copy a fee value from one ledger and use it for another. Look up the fee via `icrc1_fee` (on-chain query or `icp canister call <ledger> icrc1_fee '()'` via CLI). Fees can also change at runtime, so always handle `BadFee { expected_fee }` — the ledger tells you the correct fee in the error response.

2. **Forgetting approve before transferFrom** -- ICRC-2 transferFrom will reject with `InsufficientAllowance` if the token owner has not called `icrc2_approve` first. This is a two-step flow: owner approves, then spender calls transferFrom.

3. **Not handling Err variants** -- `icrc1_transfer` returns `Result<Nat, TransferError>`, not just `Nat`. The error variants are: `BadFee`, `BadBurn`, `InsufficientFunds`, `TooOld`, `CreatedInFuture`, `Duplicate`, `TemporarilyUnavailable`, `GenericError`. You must match on every variant or at minimum propagate the error.

4. **Using wrong Account format** -- An ICRC-1 Account is `{ owner: Principal; subaccount: ?Blob }`, NOT just a Principal. The subaccount is a 32-byte blob. Passing null/None for subaccount uses the default subaccount (all zeros).

5. **Omitting created_at_time** -- Without `created_at_time`, you lose deduplication protection. Two identical transfers submitted within 24h will both execute. Set `created_at_time` to `Time.now()` (Motoko) or `ic_cdk::api::time()` (Rust) for dedup.

6. **Hardcoding canister IDs as text** -- Always use `Principal.fromText("ryjl3-tyaaa-aaaaa-aaaba-cai")` (Motoko) or `Principal::from_text("ryjl3-tyaaa-aaaaa-aaaba-cai")` (Rust). Never pass raw strings where a Principal is expected.

7. **Calling ledger from frontend** -- ICRC-1 transfers should originate from a backend canister, not directly from the frontend. Frontend-initiated transfers expose the user to reentrancy and can bypass business logic. Use a backend canister as the intermediary.

8. **Shell substitution in `--argument-file` / `init_args.path`** -- Expressions like `$(icp identity principal)` do NOT expand inside files referenced by `init_args: { path: ... }` or `--argument-file`. The file is read as literal text. Either use `--argument` on the command line (where the shell expands variables), or pre-generate the file with `envsubst` / `sed` before deploying.

9. **Minting account cannot call `icrc2_approve`** -- If the ledger's `minting_account` and `initial_balances` use the same principal, that principal cannot call `icrc2_approve` — the ledger traps with "the minting account cannot delegate mints." Always use a **separate** principal for `minting_account` and different ones for `initial_balances`. In production, the minting account is typically a dedicated minter canister (e.g., the ckBTC minter); for local development, any principal that differs from your funded accounts works.

10. **Transfers to/from the minting account have zero fee** -- A transfer TO the minting account is a **burn**, and a transfer FROM the minting account is a **mint**. Both require `fee = null` (or `fee = ?0`). Passing the regular transfer fee (e.g., `fee = ?10000` for ICP) will fail with `BadFee { expected_fee = 0 }`. The error message gives no indication that burn/mint semantics are involved — it just says the expected fee is 0.


## Implementation

### Motoko

#### Imports and Types

```motoko
import Principal "mo:core/Principal";
import Nat "mo:core/Nat";
import Nat8 "mo:core/Nat8";
import Nat64 "mo:core/Nat64";
import Blob "mo:core/Blob";
import Time "mo:core/Time";
import Int "mo:core/Int";
import Runtime "mo:core/Runtime";
```

#### Define the ICRC-1 Actor Interface

```motoko
persistent actor {

  type Account = {
    owner : Principal;
    subaccount : ?Blob;
  };

  type TransferArg = {
    from_subaccount : ?Blob;
    to : Account;
    amount : Nat;
    fee : ?Nat;
    memo : ?Blob;
    created_at_time : ?Nat64;
  };

  type TransferError = {
    #BadFee : { expected_fee : Nat };
    #BadBurn : { min_burn_amount : Nat };
    #InsufficientFunds : { balance : Nat };
    #TooOld;
    #CreatedInFuture : { ledger_time : Nat64 };
    #Duplicate : { duplicate_of : Nat };
    #TemporarilyUnavailable;
    #GenericError : { error_code : Nat; message : Text };
  };

  type ApproveArg = {
    from_subaccount : ?Blob;
    spender : Account;
    amount : Nat;
    expected_allowance : ?Nat;
    expires_at : ?Nat64;
    fee : ?Nat;
    memo : ?Blob;
    created_at_time : ?Nat64;
  };

  type ApproveError = {
    #BadFee : { expected_fee : Nat };
    #InsufficientFunds : { balance : Nat };
    #AllowanceChanged : { current_allowance : Nat };
    #Expired : { ledger_time : Nat64 };
    #TooOld;
    #CreatedInFuture : { ledger_time : Nat64 };
    #Duplicate : { duplicate_of : Nat };
    #TemporarilyUnavailable;
    #GenericError : { error_code : Nat; message : Text };
  };

  type TransferFromArg = {
    spender_subaccount : ?Blob;
    from : Account;
    to : Account;
    amount : Nat;
    fee : ?Nat;
    memo : ?Blob;
    created_at_time : ?Nat64;
  };

  type TransferFromError = {
    #BadFee : { expected_fee : Nat };
    #BadBurn : { min_burn_amount : Nat };
    #InsufficientFunds : { balance : Nat };
    #InsufficientAllowance : { allowance : Nat };
    #TooOld;
    #CreatedInFuture : { ledger_time : Nat64 };
    #Duplicate : { duplicate_of : Nat };
    #TemporarilyUnavailable;
    #GenericError : { error_code : Nat; message : Text };
  };

  type Value = {
    #Nat : Nat;
    #Int : Int;
    #Text : Text;
    #Blob : Blob;
  };

  // Remote ledger actor reference (ICP ledger shown; swap canister ID for other tokens)
  transient let icpLedger = actor ("ryjl3-tyaaa-aaaaa-aaaba-cai") : actor {
    icrc1_balance_of : shared query (Account) -> async Nat;
    icrc1_transfer : shared (TransferArg) -> async { #Ok : Nat; #Err : TransferError };
    icrc2_approve : shared (ApproveArg) -> async { #Ok : Nat; #Err : ApproveError };
    icrc2_transfer_from : shared (TransferFromArg) -> async { #Ok : Nat; #Err : TransferFromError };
    icrc1_fee : shared query () -> async Nat;
    icrc1_decimals : shared query () -> async Nat8;
    icrc1_metadata : shared query () -> async [(Text, Value)];
    icrc1_supported_standards : shared query () -> async [{ name : Text; url : Text }];
  };

  // Fee for the ICP ledger — look up via icrc1_fee if targeting a different ledger
  transient let icpFee : Nat = 10000;

  // Check balance
  public func getBalance(who : Principal) : async Nat {
    await icpLedger.icrc1_balance_of({
      owner = who;
      subaccount = null;
    })
  };

  // Transfer tokens (this canister sends from its own account)
  // WARNING: Add access control in production — this allows any caller to transfer tokens
  public func sendTokens(to : Principal, amount : Nat) : async