F# - Basic Syntax

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.

Tokens in F#

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.

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 −

asr land lor lsl lsr lxor 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 tailcall 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!