You have seen the basic structure of an F# program, so it will be easy to understand other basic building blocks of the F# programming language.
An F# program consists of various tokens. A token could be a keyword, an identifier, a constant, a string literal, or a symbol. We can categorize F# tokens into two types −
The following table shows the keywords and brief descriptions of the keywords. We will discuss the use of these keywords in subsequent chapters.
|abstract||Indicates a method that either has no implementation in the type in which it is declared or that is virtual and has a default implementation.|
|and||Used in mutually recursive bindings, in property declarations, and with multiple constraints on generic parameters.|
|as||Used to give the current class object an object name. Also used to give a name to a whole pattern within a pattern match.|
|assert||Used to verify code during debugging.|
|base||Used as the name of the base class object.|
|begin||In verbose syntax, indicates the start of a code block.|
|class||In verbose syntax, indicates the start of a class definition.|
|default||Indicates an implementation of an abstract method; used together with an abstract method declaration to create a virtual method.|
|delegate||Used to declare a delegate.|
|do||Used in looping constructs or to execute imperative code.|
|done||In verbose syntax, indicates the end of a block of code in a looping expression.|
|downcast||Used to convert to a type that is lower in the inheritance chain.|
|downto||In a for expression, used when counting in reverse.|
|elif||Used in conditional branching. A short form of else if.|
|else||Used in conditional branching.|
In type definitions and type extensions, indicates the end of a section of member definitions.
In verbose syntax, used to specify the end of a code block that starts with the begin keyword.
|exception||Used to declare an exception type.|
|extern||Indicates that a declared program element is defined in another binary or assembly.|
|false||Used as a Boolean literal.|
|finally||Used together with try to introduce a block of code that executes regardless of whether an exception occurs.|
|for||Used in looping constructs.|
|fun||Used in lambda expressions, also known as anonymous functions.|
|function||Used as a shorter alternative to the fun keyword and a match expression in a lambda expression that has pattern matching on a single argument.|
|global||Used to reference the top-level .NET namespace.|
|if||Used in conditional branching constructs.|
|in||Used for sequence expressions and, in verbose syntax, to separate expressions from bindings.|
|inherit||Used to specify a base class or base interface.|
|inline||Used to indicate a function that should be integrated directly into the caller's code.|
|interface||Used to declare and implement interfaces.|
|internal||Used to specify that a member is visible inside an assembly but not outside it.|
|lazy||Used to specify a computation that is to be performed only when a result is needed.|
|let||Used to associate, or bind, a name to a value or function.|
|let!||Used in asynchronous workflows to bind a name to the result of an asynchronous computation, or, in other computation expressions, used to bind a name to a result, which is of the computation type.|
|match||Used to branch by comparing a value to a pattern.|
|member||Used to declare a property or method in an object type.|
|module||Used to associate a name with a group of related types, values, and functions, to logically separate it from other code.|
|mutable||Used to declare a variable, that is, a value that can be changed.|
|namespace||Used to associate a name with a group of related types and modules, to logically separate it from other code.|
Used to declare, define, or invoke a constructor that creates or that can create an object.
Also used in generic parameter constraints to indicate that a type must have a certain constructor.
|not||Not actually a keyword. However, not struct in combination is used as a generic parameter constraint.|
Indicates the absence of an object.
Also used in generic parameter constraints.
|of||Used in discriminated unions to indicate the type of categories of values, and in delegate and exception declarations.|
|open||Used to make the contents of a namespace or module available without qualification.|
Used with Boolean conditions as a Boolean or operator. Equivalent to ||.
Also used in member constraints.
|override||Used to implement a version of an abstract or virtual method that differs from the base version.|
|private||Restricts access to a member to code in the same type or module.|
|public||Allows access to a member from outside the type.|
|rec||Used to indicate that a function is recursive.|
|return||Used to indicate a value to provide as the result of a computation expression.|
|return!||Used to indicate a computation expression that, when evaluated, provides the result of the containing computation expression.|
|select||Used in query expressions to specify what fields or columns to extract. Note that this is a contextual keyword, which means that it is not actually a reserved word and it only acts like a keyword in appropriate context.|
|static||Used to indicate a method or property that can be called without an instance of a type, or a value member that is shared among all instances of a type.|
Used to declare a structure type.
Also used in generic parameter constraints.
Used for OCaml compatibility in module definitions.
Used in conditional expressions.
Also used to perform side effects after object construction.
|to||Used in for loops to indicate a range.|
|true||Used as a Boolean literal.|
|try||Used to introduce a block of code that might generate an exception. Used together with with or finally.|
|type||Used to declare a class, record, structure, discriminated union, enumeration type, unit of measure, or type abbreviation.|
|upcast||Used to convert to a type that is higher in the inheritance chain.|
|use||Used instead of let for values that require Dispose to be called to free resources.|
|use!||Used instead of let! in asynchronous workflows and other computation expressions for values that require Dispose to be called to free resources.|
|val||Used in a signature to indicate a value, or in a type to declare a member, in limited situations.|
|void||Indicates the .NET void type. Used when interoperating with other .NET languages.|
|when||Used for Boolean conditions (when guards) on pattern matches and to introduce a constraint clause for a generic type parameter.|
|while||Introduces a looping construct.|
|with||Used together with the match keyword in pattern matching expressions. Also used in object expressions, record copying expressions, and type extensions to introduce member definitions, and to introduce exception handlers.|
|yield||Used in a sequence expression to produce a value for a sequence.|
|yield!||Used in a computation expression to append the result of a given computation expression to a collection of results for the containing computation expression.|
Some reserved keywords came from the OCaml language −
Some other reserved keywords are kept for future expansion of F#.
F# provides two types of comments −
Generally, you don’t have any explicit entry point for F# programs. When you compile an F# application, the last file provided to the compiler becomes the entry point and all top level statements in that file are executed from top to bottom.
A well-written program should have a single top-level statement that would call the main loop of the program.
A very minimalistic F# program that would display ‘Hello World’ on the screen −
(* This is a comment *) (* Sample Hello World program using F# *) printfn "Hello World!"
When you compile and execute the program, it yields the following output −