Keyword Reference (F#)

This topic contains links to information about all F# language keywords.

F# Keyword Table

The following table shows all F# keywords in alphabetical order, together with brief descriptions and links to relevant topics that contain more information.

Keyword	Link	Description
abstract	Members (F#) Abstract Classes (F#)	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	let Bindings (F#) Members (F#) Constraints (F#)	Used in mutually recursive bindings, in property declarations, and with multiple constraints on generic parameters.
as	Classes (F#) Pattern Matching (F#)	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	Assertions (F#)	Used to verify code during debugging.
base	Classes (F#) Inheritance (F#)	Used as the name of the base class object.
begin	Verbose Syntax (F#)	In verbose syntax, indicates the start of a code block.
class	Classes (F#)	In verbose syntax, indicates the start of a class definition.
default	Members (F#)	Indicates an implementation of an abstract method; used together with an abstract method declaration to create a virtual method.
delegate	Delegates (F#)	Used to declare a delegate.
do	do Bindings (F#) Loops: for...to Expression (F#) Loops: for...in Expression (F#) Loops: while...do Expression (F#)	Used in looping constructs or to execute imperative code.
done	Verbose Syntax (F#)	In verbose syntax, indicates the end of a block of code in a looping expression.
downcast	Casting and Conversions (F#)	Used to convert to a type that is lower in the inheritance chain.
downto	Loops: for...to Expression (F#)	In a for expression, used when counting in reverse.
elif	Conditional Expressions: if... then...else (F#)	Used in conditional branching. A short form of else if.
else	Conditional Expressions: if... then...else (F#)	Used in conditional branching.
end	Structures (F#) Discriminated Unions (F#) Records (F#) Type Extensions (F#) Verbose Syntax (F#)	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	Exception Handling (F#) Exception Types (F#)	Used to declare an exception type.
extern	External Functions (F#)	Indicates that a declared program element is defined in another binary or assembly.
false	Primitive Types (F#)	Used as a Boolean literal.
finally	Exceptions: The try...finally Expression (F#)	Used together with try to introduce a block of code that executes regardless of whether an exception occurs.
for	Loops: for...to Expression (F#) Loops: for...in Expression (F#)	Used in looping constructs.
fun	Lambda Expressions: The fun Keyword (F#)	Used in lambda expressions, also known as anonymous functions.
function	Match Expressions (F#) Lambda Expressions: The fun Keyword (F#)	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	Namespaces (F#)	Used to reference the top-level .NET namespace.
if	Conditional Expressions: if... then...else (F#)	Used in conditional branching constructs.
in	Loops: for...in Expression (F#) Verbose Syntax (F#)	Used for sequence expressions and, in verbose syntax, to separate expressions from bindings.
inherit	Inheritance (F#)	Used to specify a base class or base interface.
inline	Functions (F#) Inline Functions (F#)	Used to indicate a function that should be integrated directly into the caller's code.
interface	Interfaces (F#)	Used to declare and implement interfaces.
internal	Access Control (F#)	Used to specify that a member is visible inside an assembly but not outside it.
lazy	Lazy Computations (F#)	Used to specify a computation that is to be performed only when a result is needed.
let	let Bindings (F#)	Used to associate, or bind, a name to a value or function.
let!	Asynchronous Workflows Computation Expressions	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	Match Expressions (F#)	Used to branch by comparing a value to a pattern.
member	Members (F#)	Used to declare a property or method in an object type.
module	Modules (F#)	Used to associate a name with a group of related types, values, and functions, to logically separate it from other code.
mutable	let Bindings (F#)	Used to declare a variable, that is, a value that can be changed.
namespace	Namespaces (F#)	Used to associate a name with a group of related types and modules, to logically separate it from other code.
new	Constructors (F#) Constraints (F#)	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	Symbol and Operator Reference (F#) Constraints (F#)	Not actually a keyword. However, not struct in combination is used as a generic parameter constraint.
null	Null Values (F#) Constraints (F#)	Indicates the absence of an object. Also used in generic parameter constraints.
of	Discriminated Unions (F#) Delegates (F#) Exception Types (F#)	Used in discriminated unions to indicate the type of categories of values, and in delegate and exception declarations.
open	Import Declarations: The open Keyword (F#)	Used to make the contents of a namespace or module available without qualification.
or	Symbol and Operator Reference (F#) Constraints (F#)	Used with Boolean conditions as a Boolean or operator. Equivalent to ||. Also used in member constraints.
override	Members (F#)	Used to implement a version of an abstract or virtual method that differs from the base version.
private	Access Control (F#)	Restricts access to a member to code in the same type or module.
public	Access Control (F#)	Allows access to a member from outside the type.
rec	Functions (F#)	Used to indicate that a function is recursive.
return	Asynchronous Workflows (F#) Computation Expressions (F#)	Used to indicate a value to provide as the result of a computation expression.
return!	Computation Expressions Asynchronous Workflows	Used to indicate a computation expression that, when evaluated, provides the result of the containing computation expression.
select	Query Expressions (F#)	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	Members (F#)	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.
struct	Structures (F#) Constraints (F#)	Used to declare a structure type. Also used in generic parameter constraints. Used for OCaml compatibility in module definitions.
then	Conditional Expressions: if... then...else (F#) Constructors (F#)	Used in conditional expressions. Also used to perform side effects after object construction.
to	Loops: for...to Expression (F#)	Used in for loops to indicate a range.
true	Primitive Types (F#)	Used as a Boolean literal.
try	Exceptions: The try...with Expression (F#) Exceptions: The try...finally Expression (F#)	Used to introduce a block of code that might generate an exception. Used together with with or finally.
type	F# Types Classes (F#) Records (F#) Structures (F#) Enumerations (F#) Discriminated Unions (F#) Type Abbreviations (F#) Units of Measure (F#)	Used to declare a class, record, structure, discriminated union, enumeration type, unit of measure, or type abbreviation.
upcast	Casting and Conversions (F#)	Used to convert to a type that is higher in the inheritance chain.
use	Resource Management: The use Keyword (F#)	Used instead of let for values that require Dispose to be called to free resources.
use!	Computation Expressions Asynchronous Workflows	Used instead of let! in asynchronous workflows and other computation expressions for values that require Dispose to be called to free resources.
val	Explicit Fields: The val Keyword (F#) Signatures (F#) Members (F#)	Used in a signature to indicate a value, or in a type to declare a member, in limited situations.
void	Primitive Types (F#)	Indicates the .NET void type. Used when interoperating with other .NET languages.
when	Constraints (F#)	Used for Boolean conditions (when guards) on pattern matches and to introduce a constraint clause for a generic type parameter.
while	Loops: while...do Expression (F#)	Introduces a looping construct.
with	Match Expressions (F#) Object Expressions (F#) Type Extensions (F#) Exceptions: The try...with Expression (F#)	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	Sequences (F#)	Used in a sequence expression to produce a value for a sequence.
yield!	Computation Expressions Asynchronous Workflows	Used in a computation expression to append the result of a given computation expression to a collection of results for the containing computation expression.

In addition, the following tokens are reserved in F# because they are keywords in the OCaml language:

asr

land

lor

lsl

lsr

lxor

mod

sig

If you use the --mlcompatibility compiler option, these keywords are available for use as identifiers.

The following tokens are reserved as keywords for future expansion of the F# language:

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

See Also

Reference

Symbol and Operator Reference (F#)

Other Resources

F# Language Reference

Compiler Options (F#)