lua-fmt - A fmtlib implement for Lua

Lua-fmt is a fmt like simple C module for Lua to format string.

The format syntax is same with fmtlib, which is same with Python's format routine.

Install

The simplest way to install it is using Luarocks:

luarocks install --server=https://luarocks.org/dev lua-fmt

Or, just compile the single C file:

# Linux
gcc -o lfmt.so -O3 -fPIC -shared lfmt.c
# macOS
gcc -o lfmt.so -O3 -fPIC -shared -undefined dynamic_lookup lfmt.c
# Windows
cl /Fefmt.dll /LD /MT /O2 /DLUA_BUILD_AS_DLL /Ipath/to/lua/include lfmt.c path/to/lua/lib

Example

local fmt = require "fmt"

-- automatic indexed argument
print(fmt("{} {} {}", 1,2, 3)) --> "1 2 3"

-- manual indexed argument
print(fmt("{2}, {1}", "World", "Hello")) --> "Hello, World"

-- named indexed argument
print(fmt("{name} is {type}", { name = "foo", type = "bar" })) --> "foo is bar"
print(fmt("{t.name} is {t.type}", {t = { name = "foo", type = "bar" }})) --> "foo is bar"

-- format specifier
print(fmt("{:b}", 42)) --> "101010"

Document

Mostly same as fmtlib.

Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.

The grammar for a replacement field is as follows:

replacement_field ::=  "{" [arg_id] [":" format_spec] "}"
arg_id            ::=  field_name accessor*
field_name        ::=  integer | identifier
accessor          ::=  "." field_name | "[" key_name "]"
key_name          ::=  field_name | <any chars except ']'>*
integer           ::=  digit+
digit             ::=  "0"..."9"
identifier        ::=  id_start id_continue*
id_start          ::=  "a"..."z" | "A"..."Z" | "_"
id_continue       ::=  id_start | digit

the mainly difference is the support of accessor, which is supported by Python but not by fmtlib.

---

“Format specifications” are used within replacement fields contained within a format string to define how individual values are presented. Each formattable type may define how the format specification is to be interpreted.

Most built-in types implement the following options for format specifications, although some of the formatting options are only supported by the numeric types.

The general form of a standard format specifier is:

format_spec ::=  [[fill]align][sign]["#"]["0"][width][grouping]["." precision][type]
fill        ::=  <a character other than '{' or '}'>
align       ::=  "<" | ">" | "^"
sign        ::=  "+" | "-" | " "
width       ::=  integer | "{" [arg_id] "}"
grouping    ::=  "_" | ","
precision   ::=  integer | "{" [arg_id] "}"
type        ::=  int_type | flt_type | str_type
int_type    ::=  "b" | "B" | "d" | "o" | "x" | "X" | "c"
flt_type    ::=  "e" | "E" | "f" | "F" | "g" | "G" | "%"
str_type    ::=  "p" | "s"

Differences (all exists in Python):

• add grouping support for int_type: "{:_}" e.g. "10_000"
• add "%" specifier for float type: "{:%}" e.g. "100.0%"

Lua type vs. type specifiers:

Lua Type	Type specifiers
"integer"	int_type
"float"	flt_type
Others	str_type