Chapter 5: Function and Require Statement


Functions are declared using the function keyword. The main purpose of a function is to encapsulate the internal logic of the contract and code reuse. When defining a function, you need to use a colon : to indicate the type of return value, such as:

contract Test { function valueOf(int x) : int { return x; } ... }

Public Function

The public function uses the public modifier to modify the function, which is the interface for externally calling the contract. The parameters of the public function represent the unlock parameters in the UTXO model (described in the next chapter). The public function does not have a clear return type declaration and the return statement at the end of the function, which implicitly returns the bool type, and the return value is true

Require Statement

The require statement contains the require keyword and a boolean expression:

require(a > 0);

This statement checks whether the Boolean expression is true. When certain conditions are not met, an error is thrown and execution is stopped. This is similar to the require of the solidity language. The last statement of the sCrypt public function must be a require statement, and each public function of the contract has at least one require statement. If and only if all require statements are checked, the contract can be successfully unlocked.

contract Test { public function unlock(int y) { require(y == 42); } ... }

Put it to the test

There are 3 functions in the TicTacToe contract:

move() : Alice and Bob each lock X bitcoins in a UTXO containing the above contract. Next, they alternately play the game by calling the public function move(). The public function move() has 4 parameters, which represent:

  • n : where on the board to play the chess
  • sig : the player's signature
  • amount : the contract balance after subtracting transaction fees
  • txPreimage: will be introduced in Chapter 7

won() : Check if a player has won the game, he will be able to take away all contract locked bets

full() : Check if all squares of the board have pieces, if no one wins the game, the two players will split the bet evenly

  1. Add return type bool for won() and full() function and change move() to a public function

  2. Add a require statement to the public function move(), requiring that the function parameter n must be greater than or equal to 0 and less than the contract's static property N

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