# Checks

When executing a step that send a blockchain transaction, you may want to assert that everything went as you desired.
MxOps provides you a way to do so: checks are additional information you can provide when
declaring a step.

If any of the checks you specified is not successful, it will stop the execution of MxOps
and raise an error.

At the moment, only two types of checks exists: `SuccessCheck` and `TransfersCheck`. We plan
on adding more types in the future such as `BalanceCheck`, `ErrorCheck`,
`StorageCheck` and much more.

Currently, checks can only be applied to contract call step.

## SuccessCheck

This is the most simple check and is included by default on every transaction step. This will verify
that the transaction went without any error.

If you use the checks keywords, make sure to add the `SuccessCheck` like this:

```yaml
type: ContractCall
sender: alice
contract: my_first_sc
endpoint: myEndpoint
gas_limit: 60000000
arguments:
  - arg1
value: 0
checks:
  - type: Success
```

In some cases, you may want to send many transactions quickly, without checking their results.
If you declare no checks, MxOps will send the transaction without waiting for the result,
gaining a significant time.

```yaml
type: ContractCall
sender: alice
contract: my_first_sc
endpoint: myEndpoint
gas_limit: 60000000
arguments:
  - arg1
value: 0
checks: []
```

## TransfersCheck

This class allows you to verify the token transfers that have been made during a transaction. This
is very useful to assert the behavior of any smart-contract that is dealing with tokens.

For example, let's say the user `alice` call a smart-contract named `super-swap-sc` to sell in one
transaction some classic ESDT and some Meta ESDT. She expects the contract to send her eGLD
in exchange.

```yaml
type: ContractCall
sender: alice
contract: super-swap-sc
endpoint: superSell
gas_limit: 60000000
esdt_transfers:
  - identifier: ALICE-123456
    amount: 58411548
    nonce: 0
  - identifier: XMEX-e45d41
    amount: 848491898
    nonce: 721
checks:
  - type: Success

  - type: Transfers
    condition: exact
    include_gas_refund: false # optional, false by default
    expected_transfers:
      - sender: "alice"
        receiver: "super-swap-sc"
        identifier: ALICE-123456
        amount: 58411548
      - sender: "alice"
        receiver: "super-swap-sc"
        identifier: XMEX-e45d41
        amount: 848491898
        nonce: 721 # can write 721 as integer or "0d21" for its hex representation
      - sender: "super-swap-sc"
        receiver: "alice"
        identifier: EGLD
        amount: 18541
```

If we only want to check that we received back the EGLD, we can use the value `included` for the
`condition` attribute. This tells MxOps to only look if the specified expected transfers are
included in the on-chain transaction.

```yaml
type: ContractCall
sender: alice
contract: super-swap-sc
endpoint: superSell
gas_limit: 60000000
esdt_transfers:
  - identifier: ALICE-123456
    amount: 58411548
    nonce: 0
  - identifier: XMEX-e45d41
    amount: 848491898
    nonce: 721
checks:
  - type: Success

  - type: Transfers
    condition: included
    expected_transfers:
      - sender: "%super-swap-sc.address"
        receiver: "%alice.address"
        identifier: EGLD
        amount: 18541
```