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Any function that does not accept or return values is of type unit, which is similar to the type void in C# and System.Void in the CLR. To a functional programmer, a function that doesn t accept or return a value might not seem interesting, since if it doesn t accept or return a value, it does nothing. In the imperative paradigm, you know that side effects exist, so even if a function accepts or returns nothing, you know it can still have its uses. The unit type is represented as a literal value, a pair of parentheses (()). This means that whenever you want a function that doesn t take or return a value, you just put () in the code: #light let main() = () In this example, main is a function because you placed parentheses after the identifier, where its parameters would go. If you didn t this, it would mean main is not a function and instead just a value that is not a function. As you know, all functions are values, but here the difference between a function and a nonfunction value is important. If main were a nonfunction value, the expressions within it would be evaluated only once. Since it is a function, the expressions will be evaluated each time it is called.
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CHAPTER 4 I IMPERATIVE PROGRAMMING
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I Caution Just because a function is named main doesn t mean it is the entry point of the program and is
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executed automatically. If you wanted your main function to be executed, then you would need to add a call to main() at the end of the source file. 6 details exactly how the entry point is determined for an F# program.
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Similarly, by placing () after the equals sign, you tell the compiler you are going to return nothing. Ordinarily, you need to put something between the equals sign and the empty parentheses, or the function is pointless; however, for the sake of keeping things simple, I ll leave this function pointless. Now you ll see the type of main by using the fsc i switch; the results of this are as follows. (I explained the notation used by the compiler s i switch in 3 s Types and Type Inference. ) As you can see, the type of main is a function that accepts unit and transforms it into a value of type unit: val main : unit -> unit Because the compiler now knows the function doesn t return anything, you can now use it with some special imperative constructs. To call the function, you can use the let keyword followed by a pair of parentheses and the equals sign. This is a special use of the let keyword, which means call a function that does not return a value. Alternatively, you can simply call the function without any extra keywords at all: #light let () = main() // -- or -main() Similarly, you can chain functions that return unit together within a function simply make sure they all share the same indentation. The next example shows several print_endline functions chained together to print text to the console: #light let poem() = print_endline print_endline print_endline print_endline poem()
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"I wandered lonely as a cloud" "That floats on high o'er vales and hills," "When all at once I saw a crowd," "A host, of golden daffodils"
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It s not quite true that the only functions that return unit type can be used in this manner; however, using them with a type other than unit will generate a warning, which is something most programmers want to avoid. So, to avoid this, it s sometimes useful to turn a function that does return a value into a function of type unit, typically because it has a side effect. The need to do this is fairly rare when just using F# libraries written in F# (although situations where it is useful do exist), but it is more common when using .NET libraries that were not written in F#.
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