F Sharp Programming/Option Types

F Sharp Higher Order Functions Tuples and Records Option Types An option type can hold two possible values: Some(x) or None. Option types are frequently used to represent optional values in calculations, or to indicate whether a particular computation has succeeded or failed. Using Option Types Let's say we have a function that divides two integers. Normally, we'd write the function as follows: let div x y = x / y This function works just fine, but it's not safe: it's possible to pass an invalid value into this function which results in a runtime error. Here is a demonstration in fsi: > let div x y = x / y;; val div : int -> int -> int > div 10 5;; val it : int = 2 > div 10 0;; System.DivideByZeroException: Attempted to divide by zero. at <StartupCode$FSI_0035>.$FSI_0035._main() stopped due to error div 10 5 executes just fine, but div 10 0 throws a division by zero exception. Using option types, we can return Some(value) on a successful calculation, or None if the calculation fails: > let safediv x y = match y with | 0 -> None | _ -> Some(x/y);; val safediv : int -> int -> int option > safediv 10 5;; val it : int option = Some 2 > safediv 10 0;; val it : int option = None Notice an important difference between our div and safediv functions: val div : int -> int -> int val safediv : int -> int -> int option div returns an int, while safediv returns an int option. Since our safediv function returns a different data type, it informs clients of our function that the application has entered an invalid state. Option types are conceptually similar to nullable types in languages like C#, however F# option types do not use the CLR System.Nullable<T> representation in IL due to differences in semantics. Pattern Matching Option Types Pattern matching option types is as easy as creating them: the same syntax used to declare an option type is used to match option types: > let isFortyTwo = function | Some(42) -> true | Some(_) | None -> false;; val isFortyTwo : int option -> bool > isFortyTwo (Some(43));; val it : bool = false > isFortyTwo (Some(42));; val it : bool = true > isFortyTwo None;; val it : bool = false Other Functions in the Option Module val get : 'a option -> 'a Returns the value of a Some option. val isNone : 'a option -> bool Returns true for a None option, false otherwise. val isSome : 'a option -> bool Returns true for a Some option, false otherwise. val map : ('a -> 'b) -> 'a option -> 'b option Given None, returns None. Given Some(x), returns Some(f x), where f is the given mapping function. val iter : ('a -> unit) -> 'a option -> unit Applies the given function to the value of a Some option, does nothing otherwise. F Sharp Higher Order Functions Tuples and Records