Programming Guide

Lua provides useful functions and libraries. So you can easily create smart contracts using these functions.

Please refer to the Lua Reference Manual for detailed syntax, explanation, basic built-in functions and libraries.

Aergo currently uses Lua version 5.1.

Restrictions

Because Lua smart contract is performed by the aergo system, OS related functions including input/output are not provided for stability and security.

These are the base functions that are not available:

print, dofile, loadfile, module, require

You can replace print with system.print.

And you can use import instead of require. import is not a Lua syntax.

Use SHIP to build and deploy smart contracts using multiple files.

These are the default libraries that are not available:

coroutine, io, os, debug, jit

The string, math, bit and table packages are available. All functions from these packages are available, except for the math package. These are the functions available in the math package:

abs, ceil, floor, pow, max, min

There are no restrictions on literals, expressions and statements.

After the version 2.0, the Lua virtual machine can internally check the block execution timeout (about 0.25s ~ 0.75s per block) and return an error upon block execution timeout. The transaction which causes timeout is not included into the block like the pre-2.0 version. Additionally, if it is the only transaction in the block, it will be evicted from the mempool for the stable operation of the block producer.

Libraries

We provide libraries for smart contract as follows:

  • Blockchain API
    • system module
    • contract module
    • state module
    • db module
    • abi module
  • Utils
    • json module
    • crypto module
    • bignum module

You can find detailed descriptions for libraries on this page

Smart Contract

Layout

import "./path/to/library"

state.var {
  Value = state.value(),
  Map = state.map(),
  Array = state.array(10)
}

function constructor(init_value)
  Value:set(init_value)
end

function contract1(name, id)
  Map["name"] = name
  Map["ID"] = id
end

function contract2()
  local sum = 0
  for i, v in state.array_pairs(Array) do
    if v ~= nil then
      sum = sum + v
    end
  end
  return sum
end

abi.register(contract1, contract2) -- , contract3, ...

import

This replaces the require function.

It allows you to divide and develop one smart contract into multiple modules (files).

This is not a Lua feature. You should use SHIP to build and deploy smart contracts using multiple files.

state variable

The state.var function defines global state variables.

Three types of state variables can be defined:

value

This type stores any Lua value.

You can define a state value with the syntax var_name = state.value(). It has get and set methods for reading and writing data.

var_name:set("data")
local data = var_name:get()

map

The type map implements associative arrays.

It can be indexed only with string, but the value of a map element can be of any type.

You can define a state map with the syntax var_name = state.map(). The index operator is used for reading and inserting elements.

You can delete an element of a map by using the delete method, with the syntax var_name:delete(key)

state.var {
  Map_var = state.map()
}

function contract_func()
  Map_var["name"] = "kslee"
  Map_var["age"] = 38
  -- ...
  local age = Map_var["age"]
end

function delete_elem(key)
  Map_var:delete(key)
end

The state map supports multiple dimensions, with syntax var_name = state.map(dimension)

The multi index operator is used for reading and inserting elements.

state.var {
  Map_var = state.map(2)
}

function contract_func()
  Map_var["kslee"]["age"] = 38
  Map_var["kslee"]["birth"] = "1999/09/09"
  -- ...
  local kslee = Map_var["kslee"]
  local age = kslee["age"]
  local birth = kslee["birth"]
  
  kslee["birth"] = "1970/10/9"
end
Restrictions
  • max dimensions: 5
  • it does not support setting intermediate dimension element, like this:
state.var {
  Map_var = state.map(2)
  person_var = state.map()
}

function contract_func()
  person_var["age"] = 38
  person_var["birth"] = "1999/09/09"
  Map_var["kslee"] = person_var   -- NOT SUPPORTED!
end

array

The type array is a fixed-length ordinary array.

It can be indexed only with integer, but the value of an array element can be of any type. The index starts at 1.

You can define a state array with the syntax var_name = state.array(size)

The index operator is used for reading and inserting elements.

state.var {
  Arr_var = state.array(3)
}

function contract()
  Arr_var[1] = 1
  Arr_var[2] = 2
  Arr_var[3] = 3

  local sum1 = 0
  for i, v in Arr_var:ipairs() do
    if v ~= nil then
      sum1 = sum1 + v
    end
  end

  local sum2 = 0
  for i = 1, #Arr_var do
    if Arr_var[i] ~= nil then
      sum2 = sum2 + Arr_var[i]
    end
  end

  if sum1 == sum2 then
  -- ...
  end

end

The state array supports multiple dimensions, with syntax var_name = state.array(1st_ncount, 2nd_ncount,....)

The multi index operator is used for reading and inserting elements.

state.var {
  -- declare 2 dimensional array 
  Arr_var = state.array(2, 3)
}

function contract_func()
  Arr_var[1][1] = 1
  Arr_var[1][2] = 2
  Arr_var[1][3] = 3

  -- ...
  local Arr2 = Arr_var[2]
  Arr2[1] = 4
  Arr2[2] = 5
  Arr2[3] = 6

  local sum1 = 0
  for i, v in Arr_var:ipairs() do
   for j, k in v:ipairs() do
    if k ~= nil then
      sum1 = sum1 + k
    end
   end
  end

  local sum2 = 0
  for i = 1, #Arr_var do
   for j = 1, #Arr_var[i] do
    if Arr_var[i][j] ~= nil then
      sum2 = sum2 + Arr_var[i][j]
    end
   end
  end

  if sum1 == sum2 then
  -- ...
  end
end
Restrictions
  • max dimensions: 5
  • it does not support setting intermediate dimension element, like this:
state.var {
  Marr_var = state.map(2,3)
  Arr_var = state.map(3)
}

function contract_func()
  Arr_var[1] = 1
  Arr_var[2] = 2
  Arr_var[3] = 3

  Marr_var[1] = Arr_var    -- NOT SUPPORTED!
end

Note: The state variables are just syntax sugar that replace system.getItem(), system.setItem() functions.

The fields of state variables that are directly modified cannot update the state db.

See InvalidUpdateAge() and ValidUpdateAge() functions in the example.

state.var{
   Person = state.value()
}

function constructor()
  Person:set({ name = "kslee", age = 38, address = "blahblah..." })
end

function InvalidUpdateAge(age)
  Person:get().age = age
end

function ValidUpdateAge(age)
  local p = Person:get()
  p.age = age
  Person:set(p)
end

function GetPerson()
  return Person:get()
end

abi.register(InvalidUpdateAge, ValidUpdateAge, GetPerson)

Private functions

These functions cannot be accessed from outside the smart contract. You can use them as helper functions.

Exported functions

These are the functions that can be called from contract call and query commands. They should be registered using abi.register().

You can restrict who can call these functions by checking the caller using system.getOrigin() or system.getSender(). Example:

function my_restricted_function()
  assert(system.getOrigin() == system.getCreator(), "permission denied")
  ...
end

abi.register(my_restricted_function)

Special functions

constructor

The constructor function is executed only once during the contract deployment. It does not need to be exported with abi.register().

It can have arguments, that should be passed at the contract deployment.

It can also write to the contract state, generally initializing it. Example:

state.var {
  my_number = state.value()
}

function constructor(value)
  my_number:set(value)
end

...

default

The default function is called when the function name cannot be found or when the transaction has no call information. It does not need to be exported through abi.register(). default is used internally by the VM. You should not use default for any other purpose.

You can define a default function as follows:

...
function default()
  ...
end
...

You can call this default function. This happens when a transaction of type call is sent to the contract without information about which function to call.

./aergocli contract call <sender> <contract>

payable

The payable is a property of a function. Only payable functions can receive Aergos sent from a sender. We can make a payable function using abi.payable(). payable functions are automatically exported. Therefore, you do not need to register them using abi.register(). constructor and default are not payable functions by default. They can be made payable functions by using abi.payable().

On the example below we can call the ReceiveAergo function with aergo, but we cannot call the NotReceiveAergo function with aergo:

...
function ReceiveAergo()
  ...
end

function NotReceiveAergo()
  ...
end

abi.register(NotReceiveAergo)
abi.payable(ReceiveAergo)
./aergocli contract call --amount=10 <sender> <contract> ReceiveAergo     # success
./aergocli contract call --amount=10 <sender> <contract> NotReceiveAergo  # fail

view

The view is a property of a function. Functions can be declared view in which case they promise not to modify the state (send aergo, emit event, set state, etc…). We can make a view function using abi.register_view(). register_view functions are automatically exported. Therefore, you do not need to register them using abi.register().

On the example below we cannot call the sendAergo function:

...
function sendAergo()
  contract.send(addr, "1 aergo")
end

abi.register_view(sendAergo)
./aergocli contract call <sender> <contract> sendAergo  # fail

SQL

Aergo smart contract has db library that supports SQL features.

Note: The db package is currently only available on private networks and publicly on SQL TestNet.

The code below is an example of creating a table and inserting a row using db.exec()

-- creates a customer table
function createTable()
  db.exec([[create table if not exists customer(
        id text,
        passwd text,
        name text,
        birth text,
        mobile text
    )]])
end

-- insert a row to the customer table
function insert(id, passwd, name, birth, mobile)
  db.exec("insert into customer values (?, ?, ?, ?, ?)",
          id, passwd, name, birth, mobile)
end

The db.query() function returns a result set. You can fetch rows from the result set.

function query(name)
  local rt = {}
  local rs = db.query("select * from customer where name like '%' || ? || '%'", name)
  while rs:next() do 
    local col1, col2, col3, col4, col5 = rs:get()
    local item = {
        id = col1,
        passwd = col2,
        name = col3,
        birth = col4,
        mobile = col5
    }
    table.insert(rt, item)
  end
  return rt
end

You can also use prepared statements:

function insert(id , passwd, name, birth, mobile)
  stmt = db.prepare("insert into customer values (?, ?, ?, ?, ?)")
  stmt.exec(id, passwd, name, birth, mobile)
end

function insert_contacts(contacts)
  local stmt = db.prepare("insert into contacts (name, email) values (?, ?)")
  for _,contact in ipairs(contacts) do
    stmt.exec(contact.name, contact.email)
  end
end

function query_names(ids)
  local names = {}
  local stmt = db.query("select name from customer where id=?")
  for _,id in ipairs(ids) do
    local rs = stmt:query(id)
    if rs:next() then
      table.insert(names, rs:get())
    else
      table.insert(names, "")
    end
  end
  return names
end

Security

DO NOT concatenate values when building SQL commands!

This would make your smart contract vulnerable to SQL injection attacks.

These are bad examples that should NOT be used:

  -- WRONG! AVOID THIS:
  db.exec("insert into customer values ('" .. id .. "', '"
      .. passwd .. "', '"
      .. name .. "', '"
      .. birth .. "', '"
      .. mobile .. "')")

  -- WRONG! AVOID THIS:
  local rs = db.query("select * from customer where id like '%'" .. id .. "'%'")

Restrictions

LiteTree is used as the SQL processing engine for Aergo smart contracts. LiteTree is implemented based on SQLite.

Detailed SQL usage can be found at https://sqlite.org/lang.html and https://sqlite.org/lang_corefunc.html

However, we do not provide full SQL functionality. There are some limitations due to stability and security.

Data types

Allow only SQL datatypes corresponding to Lua strings and numbers(int, float).

  • text
  • integer
  • real
  • null
  • date, datetime

SQL statements

You can execute the following SQL statements. However, DDL and DML can not be run on smart contract queries.

  • DDL
    • TABLE: ALTER, CREATE, DROP
    • INDEX: CREATE, DROP
  • DML
    • INSERT, UPDATE, DELETE, REPLACE
  • Query
    • SELECT

functions

Here is a list of functions that are not available:

  • load_XXX functions
  • random function
  • sqlite_XXX functions
  • data and time related functions can be used, except now timestring and localtime modifier.

A list of other functions and descriptions is available via the links below.

  • basic : https://www.sqlite.org/lang_corefunc.html
  • data and time : https://www.sqlite.org/lang_datefunc.html
  • aggregation : https://www.sqlite.org/lang_aggfunc.html

contraints

You can use the following contraints:

  • NOT NULL
  • DEFAULT
  • UNIQUE
  • PRIMARY KEY (FOREIGN KEY)
  • CHECK

Tools

aergoluac

aergoluac is a compiler for Lua smart contracts.

aergocli

aergocli is a command line tool that interfaces with the GRPC exposed by aergosvr.

It provides smart contract-related commands as follows:

  • contract deploy/call/query/abi/statequery
  • receipt get
  • event list/stream
  • Reference

brick

Toy for Contract Developers. You can use it to test smart contracts.

Style conventions

It is good to adopt a consistent coding style for readability. We recommend the Lua style guide.