ligo/gitlab-pages/docs/advanced/entrypoints-contracts.md

id	title
entrypoints-contracts	Access 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 called them access functions. An access 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 an access function is later called, only the parameter is provided, but the type of an access 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 an access 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

type storage = ...  // Any name, any type
type return = operation list * storage

type storage = ...;  // Any name, any type
type return = (list (operation), storage);

The contract storage can only be modified by activating an access function. It is important to understand what that means. What it does not mean is that some global variable holding the storage is modified by the access function. Instead, what it does mean is that, given the state of the storage on-chain, an access function specifies how to create another state for it, depending on a 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)

type parameter = nat
type storage = nat
type return = operation list * storage

let save (action, store: parameter * storage) : return =
  (([] : operation list), store)

type parameter = nat;
type storage = nat;
type return = (list (operation), storage);

let main = ((action, store): (parameter, storage)) : return =>
  (([] : list (operation)), store);

Entrypoints

In LIGO, the design pattern is to have one access function 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 access 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 access 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 access (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

type parameter =
  Action_A of nat
| Action_B of string

type storage = {
  counter : nat;
  name    : string
}

type return = operation list * storage

let entry_A (n, store : nat * storage) : return =
  ([] : operation list), {store with counter = n}

let entry_B (s, store : string * storage) : return =
  ([] : operation list), {store with name = s}

let access (action, store: parameter * storage) : return =
  match action with
    Action_A n -> entry_A (n, store)
  | Action_B s -> entry_B (s, store)

type parameter =
| Action_A (nat)
| Action_B (string);

type storage = {
  counter : nat,
  name    : string
};

type return = (list (operation), storage);

let entry_A = ((n, store): (nat, storage)) : return =>
  (([] : list (operation)), {...store, counter : n});

let entry_B = ((s, store): (string, storage)) : return =>
  (([] : list (operation)), {...store, name : s});

let access = ((action, store): (parameter, storage)) : return =>
  switch (action) {
  | Action_A (n) => entry_A ((n, store))
  | Action_B (s) => entry_B ((s, store))
  };

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 amount and failwith can be used to decline any transaction that sends more tez than 0mutez, 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 amount > 0mutez then
    (failwith ("This contract does not accept tokens.") : return)
  else ((nil : list (operation)), store)

type parameter = unit
type storage = unit
type return = operation list * storage

let deny (action, store : parameter * storage) : return =
  if amount > 0mutez then
    (failwith "This contract does not accept tokens.": return)
  else (([] : operation list), store)

type parameter = unit;
type storage = unit;
type return = (list (operation), storage);

let deny = ((action, store): (parameter, storage)) : return => {
  if (amount > 0mutez) {
    (failwith("This contract does not accept tokens."): return); }
  else { (([] : list (operation)), store); };
};

Access Control

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

const owner : address = ("tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx": address);

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

let owner : address = ("tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx": address)

let filter (action, store: parameter * storage) : return =
  if source <> owner then (failwith "Access denied." : return)
  else (([] : operation list), store)

let owner : address = ("tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx": address);

let access = ((action, store): (parameter, storage)) : storage => {
  if (source != owner) { (failwith ("Access denied.") : return); }
  else { (([] : list (operation)), store); };
};

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) = get_contract (dest);
    (* Reuse the parameter in the subsequent
       transaction or use another one, `mock_param`. *)
    const mock_param : parameter = Increment (5n);
    const op : operation = transaction (action, 0mutez, counter);
    const ops : list (operation) = list [op]
  } with (ops, store)

// counter.mligo

type parameter =
  Increment of nat
| Decrement of nat
| Reset

// ...

// proxy.mligo

type parameter =
  Increment of nat
| Decrement of nat
| Reset

type storage = unit

type return = operation list * storage

let dest : address = ("KT19wgxcuXG9VH4Af5Tpm1vqEKdaMFpznXT3" : address)

let proxy (action, store : parameter * storage) : return =
  let counter : parameter contract = Operation.get_contract dest in
  (* Reuse the parameter in the subsequent
     transaction or use another one, `mock_param`. *)
  let mock_param : parameter = Increment (5n) in
  let op : operation = Operation.transaction action 0mutez counter
  in [op], store

// counter.religo

type parameter =
| Increment (nat)
| Decrement (nat)
| Reset

// ...

// proxy.religo

type parameter =
| Increment (nat)
| Decrement (nat)
| Reset;

type storage = unit;

type return = (list (operation), storage);

let dest : address = ("KT19wgxcuXG9VH4Af5Tpm1vqEKdaMFpznXT3" : address);

let proxy = ((action, store): (parameter, storage)) : return => {
  let counter : contract (parameter) = Operation.get_contract (dest);
  (* Reuse the parameter in the subsequent
     transaction or use another one, `mock_param`. *)
  let mock_param : parameter = Increment (5n);
  let op : operation = Operation.transaction (action, 0mutez, counter);
  ([op], store)
};