Expects¶
XCM contains instructions to check for specific conditions during the execution of the message. These 'expect' instructions check for a specific condition and if it's not fulfilled, an error is then thrown. These instructions are used for things like checking the state of the registers before executing specific instructions. XCM contains the following expect instructions:
ExpectAssetExpectOriginExpectPalletExpectErrorExpectTransactStatus
ExpectAsset¶
The ExpectAsset instruction throws an ExpectationFalse error if the holding register does not
contain at least the given assets.
ExpectAsset(MultiAssets)
Example¶
For the full example, check here.
```rust, noplayground WithdrawAsset((Here, AMOUNT).into()), BuyExecution { fees: (Here, AMOUNT).into(), weight_limit: WeightLimit::Unlimited }, // Set the instructions that are executed when ExpectAsset does not pass. // In this case, reporting back an error to the Parachain. SetErrorHandler(Xcm(vec![ ReportError(QueryResponseInfo { destination: Parachain(1).into(), query_id: QUERY_ID, max_weight: Weight::from_all(0), }) ])), ExpectAsset((Here, AMOUNT + 10).into()), // Add Instructions that do something with assets in holding when ExpectAsset passes.
## ExpectOrigin
The `ExpectOrigin` instruction throws an `ExpectationFalse` error if the origin register does not
equal the expected origin.
```rust
ExpectOrigin(Option<MultiLocation>)
Example¶
For the full example, check here. The
ExpectOrigin instruction errors because the ClearOrigin clears the origin register and we expect
it to be equal to Parachain(1).
// Set the instructions that are executed when ExpectOrigin does not pass.
// In this case, reporting back an error to the Parachain.
SetErrorHandler(Xcm(vec![ReportError(QueryResponseInfo {
destination: Parachain(1).into(),
query_id: QUERY_ID,
max_weight: Weight::from_all(0),
})])),
ClearOrigin,
// Checks if the XcmContext origin is equal to `Parachain(1)`.
ExpectOrigin(Some(Parachain(1).into())),
ExpectPallet¶
The ExpectPallet instruction ensures that a particular pallet with a particular version exists in
the destination's runtime. It throws a PalletNotFound error if there is no pallet at the given
index. It throws a NameMismatch error is the name or module_name mismatch and a
VersionIncompatible error if the crate_major or crate_minor mismatch. The name and
module_name represent a byte representation of the pallet's name and module name (e.g. 'Balances'
and 'pallet_balances'). Consensus systems that are not substrate-based may throw an Unimplemented
error for this instruction.
ExpectPallet {
#[codec(compact)]
index: u32,
name: Vec<u8>,
module_name: Vec<u8>,
#[codec(compact)]
crate_major: u32,
#[codec(compact)]
min_crate_minor: u32,
},
Example¶
For the full example, check here.
``rust, noplayground
// Set the instructions that are executed when ExpectPallet does not pass.
// In this case, reporting back an error to the Parachain.
SetErrorHandler(Xcm(vec![
ReportError(QueryResponseInfo {
destination: Parachain(1).into(),
query_id: QUERY_ID,
max_weight: Weight::from_all(0),
})
])),
// Configured pallet has differentcrate_majorsoVersionIncompatible` error is thrown.
ExpectPallet {
index: 1,
name: "Balances".into(),
module_name: "pallet_balances".into(),
crate_major: 3,
min_crate_minor: 0,
}
## ExpectError
The `ExpectError` instruction throws an `ExpectationFalse` error if the error register does not
equal the expected error at that point in the execution. This instruction is useful during the error
handler execution to halt the error handler if the error that started the execution of the error
handler is not as expected. The `ExpectError` instruction allows to only execute the instructions in
the error handler, when a specific error is thrown.
```rust
ExpectError(Option<(u32, Error)>)
Example¶
For the full example, check here.
SetErrorHandler(Xcm(vec![
ExpectError(Some((1, XcmError::VersionIncompatible))),
ReportError(QueryResponseInfo {
destination: Parachain(1).into(),
query_id: QUERY_ID,
max_weight: Weight::from_all(0),
}),
])),
// Pallet index is wrong, so throws `PalletNotFound` error.
ExpectPallet {
index: 100,
name: "Balances".into(),
module_name: "pallet_balances".into(),
crate_major: 4,
min_crate_minor: 0,
},
ExpectTransactStatus¶
The ExpectTransactStatus instruction throws an ExpectationFalse error if the transact status
register does not equal the expected transact status.
Example¶
For the full example, check here. The
transact status is reported to Parachain(1) if the call in the Transact errors.
SetErrorHandler(Xcm(vec![ReportTransactStatus(QueryResponseInfo {
destination: Parachain(1).into(),
query_id: QUERY_ID,
max_weight: Weight::from_all(0),
})])),
Transact {
origin_kind: OriginKind::SovereignAccount,
require_weight_at_most: Weight::from_parts(INITIAL_BALANCE as u64, 1024 * 1024),
call: call.encode().into(),
},
ExpectTransactStatus(MaybeErrorCode::Success),