Literals

This article provides a table that shows how to specify the type of a literal in F#.

Literal types

The following table shows the literal types in F#. Characters that represent digits in hexadecimal notation are not case-sensitive; characters that identify the type are case-sensitive.

Type	Description	Suffix or prefix	Examples
sbyte	signed 8-bit integer	y	86y 0b00000101y
byte	unsigned 8-bit natural number	uy	86uy 0b00000101uy
int16	signed 16-bit integer	s	86s
uint16	unsigned 16-bit natural number	us	86us
int int32	signed 32-bit integer	l or none	86 86l
uint uint32	unsigned 32-bit natural number	u or ul	86u 86ul
nativeint	native pointer to a signed natural number	n	123n
unativeint	native pointer as an unsigned natural number	un	0x00002D3Fun
int64	signed 64-bit integer	L	86L
uint64	unsigned 64-bit natural number	UL	86UL
single, float32	32-bit floating point number	F or f	4.14F or 4.14f or infinityf or -infinityf
		lf	0x00000000lf
float; double	64-bit floating point number	none	4.14 or 2.3E+32 or 2.3e+32 or infinity or -infinity
		LF	0x0000000000000000LF
bigint	integer not limited to 64-bit representation	I	9999999999999999999999999999I
decimal	fractional number represented as a fixed point or rational number	M or m	0.7833M or 0.7833m
Char	Unicode character	none	'a' or '\u0061'
String	Unicode string	none	"text\n" or @"c:\filename" or """<book title="Paradise Lost">""" or "string1" + "string2" See also Strings.
byte	ASCII character	B	'a'B
byte[]	ASCII string	B	"text"B
String or byte[]	verbatim string	@ prefix	@"\\server\share" (Unicode) @"\\server\share"B (ASCII)

Named literals

Values that are intended to be constants can be marked with the Literal attribute.

This attribute has the effect of causing a value to be compiled as a constant. In the following example, both x and y below are immutable values, but x is evaluated at run-time, whereas y is a compile-time constant.

let x = "a" + "b" // evaluated at run-time

[<Literal>]
let y = "a" + "b" // evaluated at compile-time

Note

Functions cannot be used to compute [<Literal>] values because literals must be determined at compile-time and cannot depend on runtime evaluation.

Why functions cannot compute literals

The [<Literal>] attribute requires values to be known at compile-time. Functions, even if they seem to produce constant outputs, are evaluated at runtime, making them unsuitable for [<Literal>]. This restriction ensures that literals can be safely used in scenarios like pattern matching, attribute arguments, and interop with external functions.

For instance, attempting to assign the result of a function to a literal will fail:

[<Literal>]
let yFunc() = "a" + "b" // error FS0267: this is not a valid constant expression

This distinction also matters when calling an external function. For example, DllImport is an attribute that needs to know the value of myDLL during compilation. Without the [<Literal>] declaration, this code would fail to compile:

[<Literal>]
let myDLL = "foo.dll"

[<DllImport(myDLL, CallingConvention = CallingConvention.Cdecl)>]
extern void HelloWorld()

In pattern matching expressions, identifiers that begin with lowercase characters are always treated as variables to be bound, rather than as literals, so you should generally use initial capitals when you define literals.

[<Literal>]
let SomeJson = """{"numbers":[1,2,3,4,5]}"""

[<Literal>]
let Literal1 = "a" + "b"

[<Literal>]
let FileLocation =   __SOURCE_DIRECTORY__ + "/" + __SOURCE_FILE__

[<Literal>]
let Literal2 = 1 ||| 64

[<Literal>]
let Literal3 = System.IO.FileAccess.Read ||| System.IO.FileAccess.Write

Example of concise pattern matching using Named literals

Named literals can make pattern matching more concise by avoiding the need for when clauses or additional logic. For example:

[<Literal>]
let ErrorCode = 404

let handleResponse code =
    match code with
    | ErrorCode -> "Not Found"
    | _ -> "Other Response"

Remarks

Named literals are useful for:

• Pattern matching without a when clause.
• Attribute arguments.
• Static type provider arguments.

Unicode strings can contain explicit encodings that you can specify by using \u followed by a 16-bit hexadecimal code (0000 - FFFF), or UTF-32 encodings that you can specify by using \U followed by a 32-bit hexadecimal code that represents any Unicode code point (00000000 - 0010FFFF).

The use of bitwise operators other than ||| isn't allowed.

Integers in other bases

Signed 32-bit integers can also be specified in hexadecimal, octal, or binary using a 0x, 0o or 0b prefix, respectively.

let numbers = (0x9F, 0o77, 0b1010)
// Result: numbers : int * int * int = (159, 63, 10)

Underscores in numeric literals

You can separate digits with the underscore character (_).

let value = 0xDEAD_BEEF

let valueAsBits = 0b1101_1110_1010_1101_1011_1110_1110_1111

let exampleSSN = 123_45_6789

Special floating-point infinity values

Both the float and single floating-point numeric types have associated special values representing positive and negative infinity.

F# value	F# type	Corresponding .NET value
infinity or +infinity	float	PositiveInfinity
-infinity	float	NegativeInfinity
infinityf or +infinityf	single	PositiveInfinity
-infinityf	single	NegativeInfinity

These values can be used directly or are returned when dividing by a floating-point zero or a number too small to be represented by the given type. For example:

> 1.0/0.0;;
val it: float = infinity

> 1.0/(-0.0);;
val it: float = -infinity

> 1.0/0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
;;
val it: float = infinity