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Main function and Entrypoints

Access Functions

A LIGO contract is made of a series of constant and function declarations. Only functions having a special type can be called when the contract is activated: we call them main functions. A main function takes two parameters, the contract parameter and the on-chain storage, and returns a pair made of a list of operations and a (new) storage.

When the contract is originated, the initial value of the storage is provided. When a main function is later called, only the parameter is provided, but the type of a main function contains both.

The type of the contract parameter and the storage are up to the contract designer, but the type for list operations is not. The return type of a main function is as follows, assuming that the type storage has been defined elsewhere. (Note that you can use any type with any name for the storage.)

type storage is ... // Any name, any type
type return is list (operation) * storage

The contract storage can only be modified by activating a main function: given the state of the storage on-chain, a main function specifies how to create another state for it, depending on the contract's parameter.

Here is an example where the storage is a single natural number that is updated by the parameter.

type parameter is nat
type storage is nat
type return is list (operation) * storage
function save (const action : parameter; const store : storage) : return is
((nil : list (operation)), store)

Entrypoints

In LIGO, the design pattern is to have one main function called main, that dispatches the control flow according to its parameter. Those functions called for those actions are called entrypoints.

As an analogy, in the C programming language, the main function is the unique main function and any function called from it would be an entrypoint.

The parameter of the contract is then a variant type, and, depending on the constructors of that type, different functions in the contract are called. In other terms, the unique main function dispatches the control flow depending on a pattern matching on the contract parameter.

In the following example, the storage contains a counter of type nat and a name of type string. Depending on the parameter of the contract, either the counter or the name is updated.

type parameter is
Action_A of nat
| Action_B of string
type storage is record [
counter : nat;
name : string
]
type return is list (operation) * storage
function entry_A (const n : nat; const store : storage) : return is
((nil : list (operation)), store with record [counter = n])
function entry_B (const s : string; const store : storage) : return is
((nil : list (operation)), store with record [name = s])
function main (const action : parameter; const store : storage): return is
case action of
Action_A (n) -> entry_A (n, store)
| Action_B (s) -> entry_B (s, store)
end

Tezos-specific Built-ins

A LIGO smart contract can query part of the state of the Tezos blockchain by means of built-in values. In this section you will find how those built-ins can be utilized.

Accepting or Declining Tokens in a Smart Contract

This example shows how Tezos.amount and failwith can be used to decline any transaction that sends more tez than 0tez, that is, no incoming tokens are accepted.

type parameter is unit
type storage is unit
type return is list (operation) * storage
function deny (const action : parameter; const store : storage) : return is
if Tezos.amount > 0tez then
(failwith ("This contract does not accept tokens.") : return)
else ((nil : list (operation)), store)

Note that amount is deprecated. Please use Tezos.amount.

Access Control

This example shows how Tezos.source can be used to deny access to an entrypoint.

const owner : address = ("tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx": address);
function main (const action : parameter; const store : storage) : return is
if Tezos.source =/= owner then (failwith ("Access denied.") : return)
else ((nil : list (operation)), store)

Note that source is deprecated. Please use Tezos.source.

Inter-Contract Invocations

It would be somewhat misleading to speak of "contract calls", as this wording may wrongly suggest an analogy between contract "calls" and function "calls". Indeed, the control flow returns to the site of a function call, and composed function calls therefore are stacked, that is, they follow a last in, first out ordering. This is not what happens when a contract invokes another: the invocation is queued, that is, follows a first in, first our ordering, and the dequeuing only starts at the normal end of a contract (no failure). That is why we speak of "contract invocations" instead of "calls".

The following example shows how a contract can invoke another by emiting a transaction operation at the end of an entrypoint.

The same technique can be used to transfer tokens to an implicit account (tz1, ...): all you have to do is use a unit value as the parameter of the smart contract.

In our case, we have a counter.ligo contract that accepts an action of type parameter, and we have a proxy.ligo contract that accepts the same parameter type, and forwards the call to the deployed counter contract.

// counter.ligo
type parameter is
Increment of nat
| Decrement of nat
| Reset
type storage is unit
type return is list (operation) * storage
// proxy.ligo
type parameter is
Increment of nat
| Decrement of nat
| Reset
type storage is unit
type return is list (operation) * storage
const dest : address = ("KT19wgxcuXG9VH4Af5Tpm1vqEKdaMFpznXT3" : address)
function proxy (const action : parameter; const store : storage): return is
block {
const counter : contract (parameter) =
case (Tezos.get_contract_opt (dest) : option (contract (parameter))) of
Some (contract) -> contract
| None -> (failwith ("Contract not found.") : contract (parameter))
end;
(* Reuse the parameter in the subsequent
transaction or use another one, `mock_param`. *)
const mock_param : parameter = Increment (5n);
const op : operation = Tezos.transaction (action, 0tez, counter);
const ops : list (operation) = list [op]
} with (ops, store)