Codewalk: First-Class Functions in Go

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Go supports first class functions, higher-order functions, user-defined function types, function literals, closures, and multiple return values.

This rich feature set supports a functional programming style in a strongly typed language.

In this codewalk we will look at a simple program that simulates a dice game called Pig and evaluates basic strategies.
Game overview
Pig is a two-player game played with a 6-sided die. Each turn, you may roll or stay.
  • If you roll a 1, you lose all points for your turn and play passes to your opponent. Any other roll adds its value to your turn score.
  • If you stay, your turn score is added to your total score, and play passes to your opponent.
The first person to reach 100 total points wins.

The score type stores the scores of the current and opposing players, in addition to the points accumulated during the current turn.
User-defined function types
In Go, functions can be passed around just like any other value. A function's type signature describes the types of its arguments and return values.

The action type is a function that takes a score and returns the resulting score and whether the current turn is over.

If the turn is over, the player and opponent fields in the resulting score should be swapped, as it is now the other player's turn.
Multiple return values
Go functions can return multiple values.

The functions roll and stay each return a pair of values. They also match the action type signature. These action functions define the rules of Pig.
Higher-order functions
A function can use other functions as arguments and return values.

A strategy is a function that takes a score as input and returns an action to perform.
(Remember, an action is itself a function.)
Function literals and closures
Anonymous functions can be declared in Go, as in this example. Function literals are closures: they inherit the scope of the function in which they are declared.

One basic strategy in Pig is to continue rolling until you have accumulated at least k points in a turn, and then stay. The argument k is enclosed by this function literal, which matches the strategy type signature.
Simulating games
We simulate a game of Pig by calling an action to update the score until one player reaches 100 points. Each action is selected by calling the strategy function associated with the current player.
Simulating a tournament
The roundRobin function simulates a tournament and tallies wins. Each strategy plays each other strategy gamesPerSeries times.
Variadic function declarations
Variadic functions like ratioString take a variable number of arguments. These arguments are available as a slice inside the function.
Simulation results
The main function defines 100 basic strategies, simulates a round robin tournament, and then prints the win/loss record of each strategy.

Among these strategies, staying at 25 is best, but the optimal strategy for Pig is much more complex.