FSharp Functional Programming—Keywords

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F# Functional Programming Identifiers Keywords Literals Values and Functions Scope Recursion Anonymous Functions Operators Lists List Comprehensions Control Flow Types and Type Inference Pattern Matching Defining Types Exception Handling Lazy Evaluation Summary

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© 2007 Robert Pickering

Keywords

Most, if not all, programming languages have the concept of keywords. A keyword is a language token that the compiler reserves for special use. In F# you cannot use a keyword as an identifier name or a type name (I discuss types later in this chapter in "Defining Types"). The following are the F# keywords:

abstract	lsl
and	lsr
as	lxor
assert	match member
asr	mod
begin	module
class	mutable namespace
default	new
delegate	null
do	of
done	open
downcast	or
downto	override
else	rec
end	sig
exception	static
false	struct
finally	then
for	to
fun	true
function	try
if	type
in	val
inherit	when
inline	upcast
interface	while
land	with
lor

The words listed next are not currently F# keywords but have been reserved for possible future use. It is possible to use them as identifiers or type names now, but the compiler will issue a warning if you do. If you care that your code is compatible with future versions of the compiler, then it is best to avoid using them. I will not use them in the examples in this book.

async	method
atomic	mixin
break	namespace
checked	object
component	process
const	property
constraint	protected
constructor	public
continue	pure
decimal	readonly
eager	return
enum	sealed
event	switch
external	virtual
fixed	void
functor	volatile
include	where

If you really need to use a keyword as an identifier or type name, you can use the double back quote syntax:

let ``class`` = "style"

The usual reason for doing this is when you need to use a member from a library that was not written in F# and that uses one of F#’s keywords as a name (you’ll learn more about using non-F# libraries in Chapter 4). The best practice is to avoid using keywords as identifiers if possible.