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

byte unsigned 8-bit natural number uy 86uy

int16 signed 16-bit integer s 86s
uint16 unsigned 16-bit natural number us 86us

signed 32-bit integer l or none 86


unsigned 32-bit natural number u or ul 86u

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"




"""<book title="Paradise Lost">"""


"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

let y = "a" + "b" // evaluated at compile-time

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

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.

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

let Literal1 = "a" + "b"

let FileLocation =   __SOURCE_DIRECTORY__ + "/" + __SOURCE_FILE__

let Literal2 = 1 ||| 64

let Literal3 = System.IO.FileAccess.Read ||| System.IO.FileAccess.Write


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
val it: float = infinity