Operations

The final stack after a contract execution is a pair containing a new storage and a list of operations. An operation is either a transfer, an account creation (a.k.a. origination), a delegation or the emission of an event.

There are no literal values of type operation. Instead, such values are created using the following functions from the standard library: Tezos.create_contract (origination), Tezos.transaction (transfer), Tezos.set_delegate (delegation), and Tezos.emit (emission of event).

Origination

The call Tezos.create_contract(e,d,a,s) returns a contract creation operation (origination) for the entrypoint e (as a function) with optional delegate d, initial amount a and initial storage s, together with the address of the created contract. Note that the created contract cannot be called immediately afterwards (that is, Tezos.get_contract_opt on that address would return None()), as the origination must be performed successfully first, for example by calling a proxy contract or itself.

type @return = [list<operation>, string];

@entry

const main = (_: string, storage: string) : @return => {

const entrypoint = (_param: nat, storage: string) =>

[list([]), storage];

const [op, _addr]: [operation, address] =

Tezos.create_contract(entrypoint,

(None() as option<key_hash>),

300000000mutez,

"one");

return [[op], storage];

}

Transaction

The call Tezos.transaction(param, amount, contract_addr) evaluates in an operation that will send the amount amount in mutez to the contract at the valid address contract_addr, with parameter param. If the contract is an implicit account, the parameter must be unit.

The following example shows a transaction sent from one contract to another. The former is derived from a namespace B with an entrypoint increment; the latter is derived from a namespace A with an entrypoint add. The entrypoint increment calls add with 1. We assume that the contract associated with namespace A is deployed under the address "KT1TxqZ8QtKvLu3V3JH7Gx58n7Co8pgtpQU5".

Contract (derived from) B needs to provide the parameter of the called contract. The convention is to transform the entrypoint being called to a variant. Here, add in A becomes Add. The variant Add carries the value of the parameter of the entrypoint, here 1 because we show how to increment the storage (storage + delta in A). The type of the parameter of the contract A is obtained by A parameter_of, and it is the type of Add(1).

type @return<storage> = [list<operation>, storage];

namespace A {

type storage = int;

@entry

const add = (delta: int, storage: storage): @return<storage> =>

[[], storage + delta];

}

namespace B {

type storage = int;

@entry

const increment = (_param: unit, storage: storage): @return<storage> => {

const operation =

Tezos.transaction(Add(1) as parameter_of A,

0tez,

Tezos.get_contract(

"KT1TxqZ8QtKvLu3V3JH7Gx58n7Co8pgtpQU5" as address))

return [[operation], storage];

}

}

It is possible for a contract to have multiple entrypoints, which is implicitly translated in LIGO to a parameter with a variant type as shown below. The following contract:

type storage = int

@entry

const sub = (i: int, x: storage) : [list<operation>, storage] =>

[[], x - i]

@entry

const add = (i: int, x: storage) : [list<operation>, storage] =>

[[], x + i]

is translated internally to a contract similar to this one:

type storage = int;

type parameter = ["Sub", int] | ["Add", int];

let main = (p: parameter, x: storage): [list<operation>, storage] =>

[list ([]),

match(p) {

when(Sub(i)): x - i;

when(Add(i)): x + i

}];

This contract can be called by another contract, like this one:

type storage = int;

type parameter = int;

type remote_param = | ["Sub", int]; // Note the leading vertical bar.

@entry

const main = (_p: parameter, s: storage): [list<operation>, storage] => {

let contract_addr =

Tezos.get_entrypoint(

"%sub", // Corresponds to the `Sub` variant of `remote_param`.

"tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx" as address);

return [[Tezos.transaction(Sub(2), 2mutez, contract_addr)], s];

};

Notice how we directly use the %sub entrypoint without mentioning the %add entrypoint. This is done with the help of annotations. Those annotations correspond to the name of the function defining the entrypoints, prefixed by a %, so sub becomes %sub. Also, as shown above, each entrypoint corresponds to an implicit variant type whose name is the entrypoint function whose first character is capitalised, so sub becomes Sub, as far as parameters are concerned.

Delegation

The call Tezos.set_delegate(d) evaluates in an operation that sets the delegate of the current smart contract to be d, where d is an optional key hash. If None(), the delegation is withdrawn. If the contract has no delegation, then no change occurs. If d is Some(kh), where kh is the key hash of a registered delegate that is not the current delegate of the contract, then this operation sets the delegate of the contract to this registered delegate. A failure occurs if kh is the current delegate of the contract or if kh is not a registered delegate. However, the instruction in itself does not fail; it produces an operation that will fail when applied.

const check = (kh: key_hash) : list<operation> =>

[Tezos.set_delegate (Some(kh))];

Event emission

The call Tezos.emit(event_tag, event_type) evaluates in an operation that will write an event into the transaction receipt after the successful execution of this contract. The event is annotated by the string event_tag if it is not empty. The argument event_type is used only to specify the type of data attachment.

type storage = unit;

@entry

const main = (param: [int, int], storage: unit) : [list<operation>, storage] =>

[[Tezos.emit("%foo", param), Tezos.emit("%bar", param[0])], storage];