Things to Focus on while Working with TON Blockchain
TON or The Open Network is a layer-1 blockchain developed by Telegram. The developers state that the network is fully decentralized, ultra-fast, cheap, and environmentally friendly.

Today we take a closer look at TON and discuss the elements to focus on for those who want to develop TON apps.
Working with TON, you will interact with two programming languages: Func and Fift. Func is a C-like language developed for writing smart contracts. Although it might look a bit confusing, once you get used to it, the language makes sense and in fact, seems quite comprehensive. Fift is a stack-based language that serves as a language of interaction with TON. It contains TVM and Fift assembly instructions that TON "understands".

Both languages are relatively new, which results in multiple potential errors, especially from newby developers. Here’s a checklist for TON.
Name collisions
Func variables and functions may contain almost any legit character. I.e. var++, ~bits, foo-bar+baz including commas are valid variables' and functions' names.

When writing and inspecting a Func code, Linter should be used
Check the throw values
Each time the TVM execution stops normally, it stops with exit codes 0 or 1. Although it is done automatically, TVM execution can be interrupted directly in an unexpected way if exit codes 0 and 1 are thrown directly by either throw(0) or throw(1) command.
Func is a strictly typed language with data structures holding exactly what they are supposed to store
It is crucial to keep track of what the code does and what it may return. Keep in mind that the compiler cares only about the code and only in its initial state. After certain operations stored values of some variables can change.

Reading unexpected variables' values and calling methods on data types that are not supposed to have such methods (or their return values are not stored properly) are errors and are not skipped as "warnings" or "notices" but lead to unreachable code. Keep in mind that storing an unexpected value may be okay, however, reading it may cause problems e.g. error code 5 (integer out of expected range) may be thrown for an integer variable.
You should have a notion about Fift and TVM assembly instructions
Func functions are frequently built with direct Fift "asm" directives. The auditor must know what they do and observe keenly what the stack contains.
Unlike other blockchains, TON does not contain revert messages, only exit codes
It is helpful to think through the roadmap of exit codes for the code flow (and have it documented) before starting programming your TON smart contract.
Messages have modes
(a collection of flags represented as a single number), one of them may serve as self-destruct and one serves to send all remaining coins on balance. It is essential for the TON blockchain auditor to check the message mode.
TON fully implements the actor model
It means the code of the contract can be changed. It can either be changed permanently, using SETCODE TVM directive, or in runtime, setting the TVM code registry to a new cell value until the end of execution.
TON Blockchain has several transaction phases: computational phase, actions phase, and a bounce phase among them
The computational phase executes the code of smart contracts and only then the actions are performed (sending messages, code modification, changing libraries, and others). So, unlike on Ethereum-based blockchains, you won't see the computational phase exit code if you expected the sent message to fail, as it was performed not in the computational phase, but later, during the action phase
Func functions that have medhod_id identifiers have method IDs
They can be either set explicitly "method_id(5)", or implicitly by a func compiler. In this case, they can be found among methods declarations in the .fift assembly file. Two of them are predefined: one for receiving messages inside of blockchain (0), commonly named recv_internal, and one for receiving messages from outside (-1), recv_external.
TON Crypto address may not have any coins or code
Smart contracts' addresses in TON blockchain are deterministic and can be precomputed. Ton Accounts, associated with addresses may even contain no code which means they are uninitialized (if not deployed) or frozen while having no more storage or TON coins if the message with special flags was sent.
TON addresses may have three representations
A full representation can either be "raw" (workchain:address) or "user-friendly". The last one is the one users encounter most often. It contains a tag byte, indicating whether the address is bounceable or not bounceable, and a workchain id byte. This information should be noted.
Keep track of the flaws in code execution
Unlike Solidity where it's up to you to set methods visibility, in the case of Func, the visibility is restricted in a more intricate way either by showing errors or by "if" statements.
Keep an eye on gas before sending bounced messages
In case the smart contract sends the bounced messages with the value, provided by a user, make sure that the corresponding gas fees are subtracted from the returned amount not to be drained.
Monitor the callbacks and their failures
TON blockchain is asynchronous. That means the messages do not have to arrive successively. e.g. when a fail notification of an action arrives, it should be handled properly.
Check if the bounced flag was sent receiving internal messages
You may receive bounced messages (error notifications), which should be handled.
Write replay protection for external messages:
there are two custom solutions for wallets (smart contracts, storing users' money): seqno-based (check the counter not to process message twice) and high-load(storing processes' identifiers and its expirations).

TON might seem like a complicated chain to an unprepared auditor or developer, but knowing what to expect gives you a clear advantage. The 16 tips above can make a huge difference for any smart contract deployed to TON.