At the highest level, the smart contract is just a piece of state and code with defined functions and logic behind its selection. TON makes it even easier! It chose function selector depends on network event.

As soon as the account receives the message, it processes some actions. If the message is internal function selector is 0, if it is external function is -1.

Note: there are other cases when another continuation can be chosen. It will be discussed in future lessons.

FunC rides to the rescue!

The good knight FunC wraps this logic and lets you just write the code. The function for external messages (for example from you) should be named recv_external and for internal (for example from other space objects) it should be recv_internal.

Function's declaration has the following syntax:

([type, type, ...]) fun_name ([type arg_name, type arg_name, ...]) [modifier] {
	;; function body
}

Then the simplest contract that accepts only external messages would be:

() recv_external(slice in_msg) impure {
	;; function body
}

So recv_external doesn't return any value but expects one argument in_msg with type slice. impure says the function may modify contract storage. The brackets limits function block. ;; is used for comments. recv_internal has the same structure.

Just do it

Your spaceship is going to process your external messages and do nothing but accept internal ones.

Create the smart contract with two functions recv_external and recv_internal. Both functions have the same definition.

Result

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() recv_internal(slice in_msg) impure {
  ;; body
}

() recv_external(slice in_msg) impure {
	;; body
}