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CHAPTER 3 I FUNCTIONAL PROGRAMMING
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Pattern matching allows you to look at the value of an identifier and then make different computations depending on its value. It is a bit like a chain of if then else expressions and also might be compared to the switch statement in C++ and C#, but it is much more powerful and flexible than either. The pattern matching construct in F# allows you to pattern match over a variety of types and values. It also has several different forms and crops up in several places in the language including its exception handling syntax, which I discuss in Exceptions and Exception Handling later in this chapter. The simplest form of pattern matching is matching over a value, and you have already seen this earlier in this chapter in the section Values and Functions, where you used it to implement a function that generated numbers in the Fibonacci sequence. To illustrate the syntax, the next example shows an implementation of a function that will produce the Lucas numbers, a sequence of numbers as follows: 1, 3, 4, 7, 11, 18, 29, 47, 76, . The Lucas sequence has the same definition as the Fibonacci sequence; only the starting points are different. #light let rec luc x = match x with | x when x <= 0 -> failwith "value must be greater than 0" | 1 -> 1 | 2 -> 3 | x -> luc (x - 1) + luc (--x - 2) printfn printfn printfn printfn "(luc "(luc "(luc "(luc 2) = %i" (luc 2) 6) = %i" (luc 6) 11) = %i" (luc 11) 12) = %i" (luc 12)
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The results of this example, when compiled and executed, are as follows: (luc (luc (luc (luc 2) = 3 6) = 18 11) = 199 12) = 322
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This syntax for pattern matching is the keyword match followed by the identifier that will be matched and then the keyword with. This is followed by all the possible matching rules separated by vertical bars (|). In the simplest case, a rule consists of either a constant or an identifier, followed by an arrow (->) and then by the expression to be used when the value matches the rule. In this definition of the function luc, you can see that the second two cases are literals, the values 1 and 2, and these will just be replaced with the values 1 and 3, respectively. The fourth case will match any value of x greater than 2, and this will cause two further calls to the luc function. The rules are matched in the order in which they are defined, and the compiler will issue an error if pattern matching is incomplete, that is, if there is some possible input value that will not match any rule. This would be the case in the luc function if you had omitted the final
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CHAPTER 3 I FUNCTIONAL PROGRAMMING
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rule, because any values of x greater than 2 would not match any rule. The compiler will also issue a warning if there are any rules that will never be matched, typically because there is another rule in front of them that is more general. This would be the case in the luc function if the fourth rule were moved ahead of the first rule. In this case, none of the other rules would ever be matched because the first rule would match any value of x. You can add a when guard (as in the first rule of the example) to give exact control about when a rule fires. A when guard is composed of the keyword when followed by a Boolean expression. Once the rule is matched, the when clause is evaluated, and the rule will fire only if the expression evaluates to true. If the expression evaluates to false, the remaining rules will be searched for another match. The first rule is designed to be the function s error handler. The first part of the rule is an identifier that will match any integer, but the when guard means the rule will match only those integers that are less than or equal to zero. If you want, you can omit the first |. This can be useful when the pattern match is small and you want to fit it on one line. You can see this in the next example, which also demonstrates the use of the underscore (_) as a wildcard. The _ will match any value and is a way of telling the compiler that you re not interested in using this value. For example, in this booleanToString function, you do not need to use the constant true in the second rule, because if the first rule is matched, you know that the value of x will be true. Moreover, you do not need to use x to derive the string "True", so you can ignore the value and just use _ as a wildcard. #light let booleanToString x = match x with false -> "False" | _ -> "True" Another useful feature of pattern matching is that you can combine two patterns into one rule through the use of the vertical bar (|). The next example, stringToBoolean, demonstrates this. In the first two rules, you have two strings that you would like to have evaluate to the same value, so rather than having two separate rules, you can just use | between the two patterns. #light let stringToBoolean x = match x with | "True" | "true" -> false | "False" | "false" -> true | _ -> failwith "unexpected input" printfn "(booleanToString true) = %s" (booleanToString true) printfn "(booleanToString false) = %s" (booleanToString false) printfn "(stringToBoolean \"True\") = %b" (stringToBoolean "True") printfn "(stringToBoolean \"false\") = %b" (stringToBoolean "false") printfn "(stringToBoolean \"Hello\") = %b" (stringToBoolean "Hello")
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