F# Basic Syntax has seen the basic structure of an F# program, so it will be easy to understand other basic building blocks of the F# programming language.
Tokens in F# Basic Syntax
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 −
- Keywords
- Symbol and Operators
F# Keywords
The following table shows the keywords and brief descriptions of the keywords. We will discuss the use of these keywords in subsequent chapters.
| Keyword | Description |
|---|---|
| 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. |
| end | 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 | This 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 the 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. |
| new | 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. |
| null | 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. |
| or | 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 of 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 as a keyword inappropriate context. |
| static | Used to indicate a method or property that can be called without an instance of a type, or a valued member that is shared among all instances of a type. |
| struct | Used to declare a structure type. Also used in generic parameter constraints. Used for OCaml compatibility in module definitions. |
| then | 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 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 −
| asr | land | LOR | LSL | LSR | Luxor | mod | sig |
Some other reserved keywords are kept for future expansion of F#.
| atomic | break | checked | component | const | constraint | constructor |
| continue | eager | event | external | fixed | functor | include |
| method | mixin | object | parallel | process | protected | pure |
| sealed | tail-call | trait | virtual | volatile |
Comments in F#
F# provides two types of comments −
- One line comment starts with // symbol.
- Multi line comment starts with (* and ends with *).
A Basic Program and Application Entry Point in F#
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 −
Hello World!
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