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in the list to see whether they are the sequence of integers 1, 2, 3, and if they are, it prints a message to the console. The final two rules are the standard head/tail treatment of a list, designed to work their way through the list doing nothing, if there is no match with the first two rules. #light let rec findSequence l = match l with | [x; y; z] -> printfn "Last 3 numbers in the list were %i %i %i" x y z | 1 :: 2 :: 3 :: tail -> printfn "Found sequence 1, 2, 3 within the list" findSequence tail | head :: tail -> findSequence tail | [] -> () let testSequence = [1; 2; 3; 4; 5; 6; 7; 8; 9; 8; 7; 6; 5; 4; 3; 2; 1] findSequence testSequence The results of this example, when compiled and executed, are as follows: Found sequence 1, 2, 3 within the list Last 3 numbers in the list were 3 2 1 Because pattern matching is such a common task in F#, the language provides an alternative shorthand syntax. If the sole purpose of a function is to pattern match over something, then it may be worth using this syntax. In this version of the pattern matching syntax, you use the keyword function, place the pattern where the function s parameters would usually go, and then separate all the alternative rules with |. The next example shows this syntax in action in a simple function that recursively processes a list of strings and concatenates them into a single string. #light let rec conactStringList = function head :: tail -> head + conactStringList tail | [] -> "" let jabber = ["'Twas "; "brillig, "; "and "; "the "; "slithy "; "toves "; "..."] let completJabber = conactStringList jabber print_endline completJabber The results of this example, when compiled and executed, are as follows: 'Twas brillig, and the slithy toves ...
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Pattern matching has a couple of other uses within F#, but I have not yet covered in detail the types on which these other kinds of pattern matching are based. You can find further details on pattern matching with record types and union types in the next section, Defining Types. You can find details of pattern matching and exception handling in the section Exceptions and Exception Handling, and you can find details of how to pattern match over types from non-F# libraries in 3.
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Defining Types
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The type system in F# provides a number of features for defining custom types. All of F# s type definitions fall into two categories. The first category is types that are tuples or records. These are a set of types composed to form a composite type (similar to structs in C or classes in C#). The second category is sum types, sometimes referred to as union types. Tuples are a way of quickly and conveniently composing values into a group of values. Values are separated by commas and can then be referred to by one identifier, as shown in the first line of the next example. You can then retrieve the values by doing the reverse, as shown in the second and third lines, where identifiers separated by commas appear on the left side of the equals sign, with each identifier receiving a single value from the tuple. If you want to ignore a value in the tuple, you can use _ to tell the compiler you are not interested in the value, as in the second and third lines. #light let pair = true, false let b1, _ = pair let _, b2 = pair Tuples are different from most user-defined types in F# because you do not have to explicitly declare them using the type keyword. To define a type, you use the type keyword followed by the type name, an equals sign, and then the type you are defining. In its simplest form, you can use this to give an alias to any existing type, including tuples. Giving aliases to single types is not often useful, but giving aliases to tuples can be very useful, especially when you want to use a tuple as a type constraint. The next example shows how to give an alias to a single type and a tuple and also how to use an alias as a type constraint: #light type Name = string type Fullname = string * string let fullNameToSting (x : Fullname) = let first, second = x in first + " " + second Record types are similar to tuples in that they compose multiple types into a single type. The difference is that in record types each field is named. The next example illustrates the syntax for defining record types. You place field definitions between braces and separate them with semicolons. A field definition is composed of the field name followed by a colon and the field s type. The type definition Organization1 is a record type where the field names are unique. This means you can use a simple syntax to create an instance of this type where there
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