Format Strings

The Hack typechecker checks that format strings are being used correctly.

Quick Start

// Correct.
Str\format("First: %d, second: %s", 1, "foo");

// Typechecker error: Too few arguments for format string.
Str\format("First: %d, second: %s", 1);

This requires that the format string argument is a string literal, not a variable.

$string = "Number is: %d";

// Typechecker error: Only string literals are allowed here.
Str\format($string, 1);

Defining Functions with Format Strings

You can define your own functions with format string arguments too.

function takes_format_string(
  \HH\FormatString<\PlainSprintf> $format,
  mixed ...$args
): void {}

function use_it(): void {
  takes_format_string("First: %d, second: %s", 1, "foo");
}

HH\FormatString<PlainSprintf> will check that you've used the right number of arguments. HH\FormatString<Str\SprintfFormat> will also check that arguments match the type in the format string.

Format Specifiers

See PlainSprintf for in-depth information on all format specifiers.

Specifier	Expected Input	Expected Output
b	int	Binary (63 => 111111)
c	int	ASCII character (63 => ?)
d	int	Signed int (-1 => -1)
u	int	Unsigned int (-1 => 18446744073709551615)
e	float	Scientific Notation, as lowercase (0.34 => 3.4e)
E	float	Scientific Notation, as uppercase (0.34 => 3.4E)
f	float	Locale Floating Point Number
F	float	Non-Locale Floating Point Number
g	float	Locale Floating Point Number or Scientific Notation (3.141592653 => 3.14159, 3141592653 => 3.1415e+9).
o	int	Octal (63 => 77)
s	string	string
x	int	Hexadecimal, as lowercase (63 => 3f)
X	int	Hexadecimal, as uppercase (63 => 3F)