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authorJohn DeNero <denero@google.com>2011-04-07 18:05:15 -0700
committerJohn DeNero <denero@google.com>2011-04-07 18:05:15 -0700
commite99bef2c1649ef2d3d3e0b36b9bdd8c4a102289f (patch)
treed284d8f0618e677ee45db0ada068256889b4a3ea /doc/codewalk/functions.xml
parentc0b9fc9d9c32d10f8b636a9a65c81cbc3fc45b8b (diff)
downloadgo-e99bef2c1649ef2d3d3e0b36b9bdd8c4a102289f.tar.gz
A codewalk through a simple program that illustrates several aspects of Go functions: function objects, higher-order functions, variadic functions, tail recursion, etc. The example program simulates the game of Pig, a dice game with simple rules but a nontrivial solution.
R=adg, rsc, iant2, r CC=golang-dev http://codereview.appspot.com/4306045 Committer: Rob Pike <r@golang.org>
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+<codewalk title="First-Class Functions in Go">
+
+<step title="Introduction" src="doc/codewalk/pig.go">
+ Go supports first class functions, higher-order functions, user-defined
+ function types, function literals, closures, and multiple return values.
+ <br/><br/>
+
+ This rich feature set supports a functional programming style in a strongly
+ typed language.
+ <br/><br/>
+
+ In this codewalk we will look at a simple program that simulates a dice game
+ called <a href="http://en.wikipedia.org/wiki/Pig_(dice)">Pig</a> and evaluates
+ basic strategies.
+</step>
+
+<step title="Game overview" src="doc/codewalk/pig.go:/\/\/ A score/,/thisTurn int\n}/">
+ Pig is a two-player game played with a 6-sided die. Each turn, you may roll or stay.
+ <ul>
+ <li> 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. </li>
+ <li> If you stay, your turn score is added to your total score, and play passes
+ to your opponent. </li>
+ </ul>
+
+ The first person to reach 100 total points wins.
+ <br/><br/>
+
+ The <code>score</code> type stores the scores of the current and opposing
+ players, in addition to the points accumulated during the current turn.
+</step>
+
+<step title="User-defined function types" src="doc/codewalk/pig.go:/\/\/ An action/,/bool\)/">
+ 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.
+ <br/><br/>
+
+ The <code>action</code> type is a function that takes a <code>score</code>
+ and returns the resulting <code>score</code> and whether the current turn is
+ over.
+ <br/><br/>
+
+ If the turn is over, the <code>player</code> and <code>opponent</code> fields
+ in the resulting <code>score</code> should be swapped, as it is now the other player's
+ turn.
+</step>
+
+<step title="Multiple return values" src="doc/codewalk/pig.go:/\/\/ roll returns/,/stay.*true\n}/">
+ Go functions can return multiple values.
+ <br/><br/>
+
+ The functions <code>roll</code> and <code>stay</code> each return a pair of
+ values. They also match the <code>action</code> type signature. These
+ <code>action</code> functions define the rules of Pig.
+</step>
+
+<step title="Higher-order functions" src="doc/codewalk/pig.go:/\/\/ A strategy/,/action\n/">
+ A function can use other functions as arguments and return values.
+ <br/><br/>
+
+ A <code>strategy</code> is a function that takes a <code>score</code> as input
+ and returns an <code>action</code> to perform. <br/>
+ (Remember, an <code>action</code> is itself a function.)
+</step>
+
+<step title="Function literals and closures" src="doc/codewalk/pig.go:/return func/,/return roll\n\t}/">
+ 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.
+ <br/><br/>
+
+ 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 <code>k</code> is
+ enclosed by this function literal, which matches the <code>strategy</code> type
+ signature.
+</step>
+
+<step title="Simulating games" src="doc/codewalk/pig.go:/\/\/ play/,/currentPlayer\n}/">
+ We simulate a game of Pig by calling an <code>action</code> to update the
+ <code>score</code> until one player reaches 100 points. Each
+ <code>action</code> is selected by calling the <code>strategy</code> function
+ associated with the current player.
+</step>
+
+<step title="Comparing functions" src="doc/codewalk/pig.go:/if action/,/currentPlayer\)\)\n\t\t}/">
+ Functions can be compared for equality in Go. From the
+ <a href="http://golang.org/doc/go_spec.html#Comparison_operators">language specification</a>:
+ Function values are equal if they refer to the same function or if both are <code>nil</code>.
+ <br/><br/>
+
+ We enforce that a <code>strategy</code> function can only return a legal
+ <code>action</code>: either <code>roll</code> or <code>stay</code>.
+</step>
+
+<step title="Simulating a tournament" src="doc/codewalk/pig.go:/\/\/ roundRobin/,/gamesPerStrategy\n}/">
+ The <code>roundRobin</code> function simulates a tournament and tallies wins.
+ Each strategy plays each other strategy <code>gamesPerSeries</code> times.
+</step>
+
+<step title="Variadic function declarations" src="doc/codewalk/pig.go:/\/\/ ratioS/,/string {/">
+ Variadic functions like <code>ratioString</code> take a variable number of
+ arguments. These arguments are available as a slice inside the function.
+</step>
+
+<step title="Simulation results" src="doc/codewalk/pig.go:/func main/,/\n}/">
+ The <code>main</code> function defines 100 basic strategies, simulates a round
+ robin tournament, and then prints the win/loss record of each strategy.
+ <br/><br/>
+
+ Among these strategies, staying at 25 is best, but the <a
+ href="http://www.google.com/search?q=optimal+play+pig">optimal strategy for
+ Pig</a> is much more complex.
+</step>
+
+</codewalk>