Original copyright © 2015 Lua.org, PUC-Rio. Freely available under the terms of the Lua license. Modified for Luan.
Luan is a high level programming language based on Lua. A great strength of Lua is its simplicity and Luan takes this even further, being even simpler than Lua. The goal is to provide a simple programming language for the casual programmer with as few concepts as possible so that one can quickly learn the language and then easily understand any code written in Luan.
Luan is implemented in Java and is tightly coupled with Java. So it makes a great scripting language for Java programmers.
Unlike Lua which is meant to be embedded, Luan is meant to be a full scripting language. This done not by adding features to Luan, but rather by providing a complete set of libraries.
This section describes the basic concepts of the language.
Luan is a dynamically typed language. This means that variables do not have types; only values do. There are no type definitions in the language. All values carry their own type.
All values in Luan are first-class values. This means that all values can be stored in variables, passed as arguments to other functions, and returned as results.
There are eight basic types in Luan: nil, boolean, number, string, binary, function, java, and table. Nil is the type of the value nil, whose main property is to be different from any other value; it usually represents the absence of a useful value. Nil is implemented as the Java value null. Boolean is the type of the values false and true. Boolean is implemented as the Java class Boolean. Number represents both integer numbers and real (floating-point) numbers. Number is implemented as the Java class Number. Any Java subclass of Number is allowed and this is invisible to the Luan user. Operations on numbers follow the same rules of the underlying Java implementation. String is implemented as the Java class String. Binary is implemented as the Java type byte[].
Luan can call (and manipulate) functions written in Luan and functions written in Java (see Function Calls). Both are represented by the type function.
The type java is provided to allow arbitrary Java objects to be stored in Luan variables. A java value is a Java object that isn't one of the standard Luan types. Java values have no predefined operations in Luan, except assignment and identity test. Java values are useful when Java access is enabled in Luan.
The type table implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any Luan value except nil. Tables can be heterogeneous; that is, they can contain values of all types (except nil). Any key with value nil is not considered part of the table. Conversely, any key that is not part of a table has an associated value nil.
Tables are the sole data-structuring mechanism in Luan;
they can be used to represent ordinary arrays, sequences,
symbol tables, sets, records, graphs, trees, etc.
To represent records, Luan uses the field name as an index.
The language supports this representation by
providing a.name
as syntactic sugar for a["name"]
.
There are several convenient ways to create tables in Luan
(see Table Constructors).
We use the term sequence to denote a table where the set of all positive numeric keys is equal to {1..n} for some non-negative integer n, which is called the length of the sequence (see The Length Operator).
Like indices, the values of table fields can be of any type. In particular, because functions are first-class values, table fields can contain functions. Thus tables can also carry methods (see Function Definitions).
The indexing of tables follows
the definition of raw equality in the language.
The expressions a[i]
and a[j]
denote the same table element
if and only if i
and j
are raw equal
(that is, equal without metamethods).
In particular, floats with integral values
are equal to their respective integers
(e.g., 1.0 == 1
).
Luan values are objects: variables do not actually contain values, only references to them. Assignment, parameter passing, and function returns always manipulate references to values; these operations do not imply any kind of copy.
The library function Luan.type
returns a string describing the type
of a given value.
The environment of a chunk starts with only one local variable: require
. This function is used to load and access libraries and other modules. All other variables must be added to the environment using local declarations.
As will be discussed in Variables and Assignment,
any reference to a free name
(that is, a name not bound to any declaration) var
can be syntactically translated to _ENV.var
if _ENV
is defined.
Luan code can explicitly generate an error by calling the
error
function.
If you need to catch errors in Luan,
you can use the Try Statement.
Whenever there is an error,
an error table
is propagated with information about the error.
See Luan.new_error
.
Every table in Luan can have a metatable.
This metatable is an ordinary Luan table
that defines the behavior of the original value
under certain special operations.
You can change several aspects of the behavior
of operations over a value by setting specific fields in its metatable.
For instance, when a table is the operand of an addition,
Luan checks for a function in the field "__add
" of the table's metatable.
If it finds one,
Luan calls this function to perform the addition.
The keys in a metatable are derived from the event names;
the corresponding values are called "add"
and the metamethod is the function that performs the addition.
You can query the metatable of any table
using the get_metatable
function.
You can replace the metatable of tables
using the set_metatable
function.
A metatable controls how a table behaves in arithmetic operations, bitwise operations, order comparisons, concatenation, length operation, calls, and indexing.
A detailed list of events controlled by metatables is given next.
Each operation is identified by its corresponding event name.
The key for each event is a string with its name prefixed by
two underscores, '__
';
for instance, the key for operation "add" is the
string "__add
".
Note that queries for metamethods are always raw;
the access to a metamethod does not invoke other metamethods.
You can emulate how Luan queries a metamethod for an object obj
with the following code:
raw_get(get_metatable(obj) or {}, "__" .. event_name)
Here are the events:
"add":
the +
operation.
If any operand for an addition is a table,
Luan will try to call a metamethod.
First, Luan will check the first operand (even if it is valid).
If that operand does not define a metamethod for the "__add
" event,
then Luan will check the second operand.
If Luan can find a metamethod,
it calls the metamethod with the two operands as arguments,
and the result of the call
(adjusted to one value)
is the result of the operation.
Otherwise,
it raises an error.
"sub":
the -
operation.
Behavior similar to the "add" operation.
"mul":
the *
operation.
Behavior similar to the "add" operation.
"div":
the /
operation.
Behavior similar to the "add" operation.
"idiv":
the //
operation.
Behavior similar to the "add" operation.
"mod":
the %
operation.
Behavior similar to the "add" operation.
"pow":
the ^
(exponentiation) operation.
Behavior similar to the "add" operation.
"unm":
the -
(unary minus) operation.
Behavior similar to the "add" operation.
"concat":
the ..
(concatenation) operation.
Behavior similar to the "add" operation.
"len":
the #
(length) operation.
If there is a metamethod,
Luan calls it with the object as argument,
and the result of the call
(always adjusted to one value)
is the result of the operation.
If there is no metamethod but the object is a table,
then Luan uses the table length operation (see The Length Operator).
Otherwise, Luan raises an error.
"eq":
the ==
(equal) operation.
Behavior similar to the "add" operation,
except that Luan will try a metamethod only when the values
being compared are both tables
and they are not primitively equal.
The result of the call is always converted to a boolean.
"lt":
the <
(less than) operation.
Behavior similar to the "add" operation.
The result of the call is always converted to a boolean.
"le":
the <=
(less equal) operation.
Unlike other operations,
The less-equal operation can use two different events.
First, Luan looks for the "__le
" metamethod in both operands,
like in the "lt" operation.
If it cannot find such a metamethod,
then it will try the "__lt
" event,
assuming that a <= b
is equivalent to not (b < a)
.
As with the other comparison operators,
the result is always a boolean.
"index":
The indexing access table[key]
.
This event happens
when key
is not present in table
.
The metamethod is looked up in table
.
Despite the name,
the metamethod for this event can be any type.
If it is a function,
it is called with table
and key
as arguments.
Otherwise
the final result is the result of indexing this metamethod object with key
.
(This indexing is regular, not raw,
and therefore can trigger another metamethod if the metamethod object is a table.)
"new_index":
The indexing assignment table[key] = value
.
Like the index event,
this event happens when
when key
is not present in table
.
The metamethod is looked up in table
.
Like with indexing,
the metamethod for this event can be either a function or a table.
If it is a function,
it is called with table
, key
, and value
as arguments.
If it is a table,
Luan does an indexing assignment to this table with the same key and value.
(This assignment is regular, not raw,
and therefore can trigger another metamethod.)
Whenever there is a "new_index" metamethod,
Luan does not perform the primitive assignment.
(If necessary,
the metamethod itself can call raw_set
to do the assignment.)
"gc":
This is when a table is garbage collected. When the table's finalize method is called by the Java garbage collector, if there is a "__gc
" metamethod then it is called with the table as a parameter.
Luan uses Java's garbage collection.
This section describes the lexis, the syntax, and the semantics of Luan. In other words, this section describes which tokens are valid, how they can be combined, and what their combinations mean.
Language constructs will be explained using the usual extended BNF notation, in which {a} means 0 or more a's, and [a] means an optional a. Non-terminals are shown like non-terminal, keywords are shown like kword, and other terminal symbols are shown like ‘=’. The complete syntax of Luan can be found in §9 at the end of this manual.
Luan ignores spaces and comments between lexical elements (tokens), except as delimiters between names and keywords. Luan considers the end of a line to be the end of a statement. This catches errors and encourages readability. If you want to continue a statement on another line, you can use a backslash followed by a newline which will be treated as white space.
Names (also called identifiers) in Luan can be any string of letters, digits, and underscores, not beginning with a digit. Identifiers are used to name variables, table fields, and labels.
The following keywords are reserved and cannot be used as names:
and break catch continue do else elseif end_do end_for end_function end_if end_try end_while false finally for function if in local nil not or repeat return then true try until while
Luan is a case-sensitive language:
and
is a reserved word, but And
and AND
are two different, valid names.
The following strings denote other tokens:
+ - * / // % ^ # & ~ | == ~= <= >= < > = ( ) { } [ ] ; , . .. ... %> <% <%=
Literal strings
can be delimited by matching single or double quotes,
and can contain the following C-like escape sequences:
'\a
' (bell),
'\b
' (backspace),
'\f
' (form feed),
'\n
' (newline),
'\r
' (carriage return),
'\t
' (horizontal tab),
'\v
' (vertical tab),
'\\
' (backslash),
'\"
' (quotation mark [double quote]),
and '\'
' (apostrophe [single quote]).
A backslash followed by a real newline
results in a newline in the string.
The escape sequence '\z
' skips the following span
of white-space characters,
including line breaks;
it is particularly useful to break and indent a long literal string
into multiple lines without adding the newlines and spaces
into the string contents.
Luan can specify any character in a literal string by its numerical value.
This can be done
with the escape sequence \xXX
,
where XX is a sequence of exactly two hexadecimal digits,
or with the escape sequence \uXXXX
,
where XXXX is a sequence of exactly four hexadecimal digits,
or with the escape sequence \ddd
,
where ddd is a sequence of up to three decimal digits.
(Note that if a decimal escape sequence is to be followed by a digit,
it must be expressed using exactly three digits.)
Literal strings can also be defined using a long format
enclosed by long brackets.
We define an opening long bracket of level n as an opening
square bracket followed by n equal signs followed by another
opening square bracket.
So, an opening long bracket of level 0 is written as [[
,
an opening long bracket of level 1 is written as [=[
,
and so on.
A closing long bracket is defined similarly;
for instance,
a closing long bracket of level 4 is written as ]====]
.
A long literal starts with an opening long bracket of any level and
ends at the first closing long bracket of the same level.
It can contain any text except a closing bracket of the same level.
Literals in this bracketed form can run for several lines,
do not interpret any escape sequences,
and ignore long brackets of any other level.
Any kind of end-of-line sequence
(carriage return, newline, carriage return followed by newline,
or newline followed by carriage return)
is converted to a simple newline.
Any character in a literal string not explicitly affected by the previous rules represents itself. However, Luan opens files for parsing in text mode, and the system file functions may have problems with some control characters. So, it is safer to represent non-text data as a quoted literal with explicit escape sequences for non-text characters.
For convenience, when the opening long bracket is immediately followed by a newline, the newline is not included in the string. As an example the five literal strings below denote the same string:
a = 'alo\n123"'
a = "alo\n123\""
a = '\97lo\10\04923"'
a = [[alo
123"]]
a = [==[
alo
123"]==]
A numerical constant (or numeral)
can be written with an optional fractional part
and an optional decimal exponent,
marked by a letter 'e
' or 'E
'.
Luan also accepts hexadecimal constants,
which start with 0x
or 0X
.
Hexadecimal constants also accept an optional fractional part
plus an optional binary exponent,
marked by a letter 'p
' or 'P
'.
A numeric constant with a fractional dot or an exponent
denotes a float;
otherwise it denotes an integer.
Examples of valid integer constants are
3 345 0xff 0xBEBADA
Examples of valid float constants are
3.0 3.1416 314.16e-2 0.31416E1 34e1 0x0.1E 0xA23p-4 0X1.921FB54442D18P+1
A comment starts with a double hyphen (--
)
anywhere outside a string.
If the text immediately after --
is not an opening long bracket,
the comment is a short comment,
which runs until the end of the line.
Otherwise, it is a long comment,
which runs until the corresponding closing long bracket.
Long comments are frequently used to disable code temporarily.
Variables are places that store values. There are three kinds of variables in Luan: global variables, local variables, and table fields.
A single name can denote a global variable or a local variable (or a function's formal parameter, which is a particular kind of local variable):
var ::= Name
Name denotes identifiers, as defined in Lexical Conventions.
Local variables are lexically scoped: local variables can be freely accessed by functions defined inside their scope (see Visibility Rules).
Before the first assignment to a variable, its value is nil.
Square brackets are used to index a table:
var ::= prefixexp ‘[’ exp ‘]’
The meaning of accesses to table fields can be changed via metatables.
An access to an indexed variable t[i]
is equivalent to
a call gettable_event(t,i)
.
(See Metatables and Metamethods for a complete description of the
gettable_event
function.
This function is not defined or callable in Luan.
We use it here only for explanatory purposes.)
The syntax var.Name
is just syntactic sugar for
var["Name"]
:
var ::= prefixexp ‘.’ Name
Global variables are not available by default. To enable global variable, you must define _ENV
as a local variable whose value is a table. If _ENV
is not defined, then an unrecognized variable name will produce a compile error. If _ENV
is defined then an access to an unrecognized variable name will be consider a global variable. So then an acces to global variable x
is equivalent to _ENV.x
.
Due to the way that chunks are compiled,
_ENV
is never a global name (see Environments).
Luan supports an almost conventional set of statements, similar to those in Pascal or C. This set includes assignments, control structures, function calls, and variable declarations.
A block is a list of statements, which are executed sequentially:
block ::= {stat}
Luan has empty statements that allow you to separate statements with semicolons, start a block with a semicolon or write two semicolons in sequence:
stat ::= ‘;’
A block can be explicitly delimited to produce a single statement:
stat ::= do block end_do end_do ::= end_do | end
Explicit blocks are useful to control the scope of variable declarations. Explicit blocks are also sometimes used to add a return statement in the middle of another block (see Control Structures).
The unit of compilation of Luan is called a chunk. Syntactically, a chunk is simply a block:
chunk ::= block
Luan handles a chunk as the body of an anonymous function with a variable number of arguments (see Function Definitions). As such, chunks can define local variables, receive arguments, and return values.
A chunk can be stored in a file or in a string inside the host program. To execute a chunk, Luan first loads it, compiling the chunk's code, and then Luan executes the compiled code.
Luan allows multiple assignments. Therefore, the syntax for assignment defines a list of variables on the left side and a list of expressions on the right side. The elements in both lists are separated by commas:
stat ::= varlist ‘=’ explist varlist ::= var {‘,’ var} explist ::= exp {‘,’ exp}
Expressions are discussed in Expressions.
Before the assignment, the list of values is adjusted to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are fewer values than needed, the list is extended with as many nil's as needed. If the list of expressions ends with a function call, then all values returned by that call enter the list of values, before the adjustment (except when the call is enclosed in parentheses; see Expressions).
The assignment statement first evaluates all its expressions and only then the assignments are performed. Thus the code
i = 3
i, a[i] = i+1, 20
sets a[3]
to 20, without affecting a[4]
because the i
in a[i]
is evaluated (to 3)
before it is assigned 4.
Similarly, the line
x, y = y, x
exchanges the values of x
and y
,
and
x, y, z = y, z, x
cyclically permutes the values of x
, y
, and z
.
The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable t[i] = val
is equivalent to
settable_event(t,i,val)
.
(See Metatables and Metamethods for a complete description of the
settable_event
function.
This function is not defined or callable in Luan.
We use it here only for explanatory purposes.)
An assignment to a global name x = val
is equivalent to the assignment
_ENV.x = val
(see Environments).
Global names are only available when _ENV
is defined.
The control structures if, while, and repeat have the usual meaning and familiar syntax:
stat ::= while exp do block end_while stat ::= repeat block until exp stat ::= if exp then block {elseif exp then block} [else block] end_if end_while ::= end_while | end end_if ::= end_if | end
Luan also has a for statement (see For Statement).
The condition expression of a control structure must be a boolean. Any other value type will produce an error. This helps catch errors and makes code more readable.
In the repeat–until loop, the inner block does not end at the until keyword, but only after the condition. So, the condition can refer to local variables declared inside the loop block.
The break statement terminates the execution of a while, repeat, or for loop, skipping to the next statement after the loop:
stat ::= break
A break ends the innermost enclosing loop.
The continue statement jumps to the beginning of a while, repeat, or for loop for next iteration, skipping the execution of statements inside the body of loop for the current iteration:
stat ::= continue
The return statement is used to return values from a function or a chunk (which is an anonymous function). Functions can return more than one value, so the syntax for the return statement is
stat ::= return [explist] [‘;’]
The for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is nil. The for loop has the following syntax:
stat ::= for namelist in exp do block end_for namelist ::= Name {‘,’ Name} end_for ::= end_for | end
A for statement like
for var_1, ···, var_n in exp do block end
is equivalent to the code:
do
local f = exp
while true do
local var_1, ···, var_n = f()
if var_1 == nil then break end
block
end
end
Note the following:
exp
is evaluated only once.
Its result is an iterator function.
f
is an invisible variable.
The name is here for explanatory purposes only.
var_i
are local to the loop;
you cannot use their values after the for ends.
If you need these values,
then assign them to other variables before breaking or exiting the loop.
The try statement has the same semantics as in Java.
stat ::= try block [catch Name block] [finally block] end_try end_try ::= end_try | end
To allow possible side-effects, function calls can be executed as statements:
stat ::= functioncall
In this case, all returned values are thrown away. Function calls are explained in Function Calls.
Logical expressions can be statements. This is useful in cases like this:
x==5 or error "x should be 5"
Local variables can be declared anywhere inside a block. The declaration can include an initial assignment:
stat ::= local namelist [‘=’ explist]
If present, an initial assignment has the same semantics of a multiple assignment (see Assignment). Otherwise, all variables are initialized with nil.
A chunk is also a block (see Chunks), and so local variables can be declared in a chunk outside any explicit block.
The visibility rules for local variables are explained in Visibility Rules.
Template statements provide the full equivalent of JSP but in a general way. Template statements write to standard output. For example:
local name = "Bob"
%>
Hello <%= name %>!
Bye <%= name %>.
<%
is equivalent to the code:
local name = "Bob"
require("luan:Io.luan").stdout.write( "Hello ", name , "!\nBye ", name , ".\n" )
The basic expressions in Luan are the following:
exp ::= prefixexp exp ::= nil | false | true exp ::= Numeral exp ::= LiteralString exp ::= functiondef exp ::= tableconstructor exp ::= ‘...’ exp ::= exp binop exp exp ::= unop exp prefixexp ::= var | functioncall | ‘(’ exp ‘)’
Numerals and literal strings are explained in Lexical Conventions;
variables are explained in Variables;
function definitions are explained in Function Definitions;
function calls are explained in Function Calls;
table constructors are explained in Table Constructors.
Vararg expressions,
denoted by three dots ('...
'), can only be used when
directly inside a vararg function;
they are explained in Function Definitions.
Binary operators comprise arithmetic operators (see Arithmetic Operators), relational operators (see Relational Operators), logical operators (see Logical Operators), and the concatenation operator (see Concatenation). Unary operators comprise the unary minus (see Arithmetic Operators), the unary logical not (see Logical Operators), and the unary length operator (see The Length Operator).
Both function calls and vararg expressions can result in multiple values. If a function call is used as a statement (see Function Calls as Statements), then its return list is adjusted to zero elements, thus discarding all returned values. If an expression is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the expression is enclosed in parentheses). In all other contexts, Luan adjusts the result list to one element, either discarding all values except the first one or adding a single nil if there are no values.
Here are some examples:
f() -- adjusted to 0 results g(f(), x) -- f() is adjusted to 1 result g(x, f()) -- g gets x plus all results from f() a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil) a,b = ... -- a gets the first vararg parameter, b gets -- the second (both a and b can get nil if there -- is no corresponding vararg parameter) a,b,c = x, f() -- f() is adjusted to 2 results a,b,c = f() -- f() is adjusted to 3 results return f() -- returns all results from f() return ... -- returns all received vararg parameters return x,y,f() -- returns x, y, and all results from f() {f()} -- creates a list with all results from f() {...} -- creates a list with all vararg parameters {f(), nil} -- f() is adjusted to 1 result
Any expression enclosed in parentheses always results in only one value.
Thus,
(f(x,y,z))
is always a single value,
even if f
returns several values.
(The value of (f(x,y,z))
is the first value returned by f
or nil if f
does not return any values.)
Luan supports the following arithmetic operators:
+
: addition-
: subtraction*
: multiplication/
: float division//
: floor division%
: modulo^
: exponentiation-
: unary minusAddition, subtraction, multiplication, division, and unary minus are the same as these operators in Java. Exponentiation uses Java's Math.pow function.
Floor division (//) is a division that rounds the quotient towards minus infinity, that is, the floor of the division of its operands.
Modulo is defined as the remainder of a division that rounds the quotient towards minus infinite (floor division). (The Java modulo operator is not used.)
Luan generally avoids automatic conversions. String concatenation automatically converts all of its arguments to strings.
Luan provides library functions for explicit type conversions.
Luan supports the following relational operators:
==
: equality~=
: inequality<
: less than>
: greater than<=
: less or equal>=
: greater or equalThese operators always result in false or true.
Equality (==
) first compares the type of its operands.
If the types are different, then the result is false.
Otherwise, the values of the operands are compared.
Strings, numbers, and binary values are compared in the obvious way (by value).
Tables are compared by reference: two objects are considered equal only if they are the same object. Every time you create a new table, it is different from any previously existing table. Closures are also compared by reference.
You can change the way that Luan compares tables by using the "eq" metamethod (see Metatables and Metamethods).
Java values are compared for equality with the Java equals
method.
Equality comparisons do not convert strings to numbers
or vice versa.
Thus, "0"==0
evaluates to false,
and t[0]
and t["0"]
denote different
entries in a table.
The operator ~=
is exactly the negation of equality (==
).
The order operators work as follows.
If both arguments are numbers,
then they are compared following
the usual rule for binary operations.
Otherwise, if both arguments are strings,
then their values are compared according to the current locale.
Otherwise, Luan tries to call the "lt" or the "le"
metamethod (see Metatables and Metamethods).
A comparison a > b
is translated to b < a
and a >= b
is translated to b <= a
.
The logical operators in Luan are and, or, and not. The and and or operators consider both false and nil as false and anything else as true. Like the control structures (see Control Structures), the not operator requires a boolean value.
The negation operator not always returns false or true. The conjunction operator and returns its first argument if this value is false or nil; otherwise, and returns its second argument. The disjunction operator or returns its first argument if this value is different from nil and false; otherwise, or returns its second argument. Both and and or use short-circuit evaluation; that is, the second operand is evaluated only if necessary. Here are some examples:
10 or 20 --> 10 10 or error() --> 10 nil or "a" --> "a" nil and 10 --> nil false and error() --> false false and nil --> false false or nil --> nil 10 and 20 --> 20
(In this manual,
-->
indicates the result of the preceding expression.)
The string concatenation operator in Luan is
denoted by two dots ('..
').
All operands are converted to strings.
The length operator is denoted by the unary prefix operator #
.
The length of a string is its number of characters.
The length of a binary is its number of bytes.
A program can modify the behavior of the length operator for
any table through the __len
metamethod (see Metatables and Metamethods).
Unless a __len
metamethod is given,
the length of a table t
is defined
as the number of elements in sequence,
that is,
the size of the set of its positive numeric keys is equal to {1..n}
for some non-negative integer n.
In that case, n is its length.
Note that a table like
{10, 20, nil, 40}
has a length of 2
, because that is the last key in sequence.
Operator precedence in Luan follows the table below, from lower to higher priority:
or and < > <= >= ~= == .. + - * / % unary operators (not # -) ^
As usual,
you can use parentheses to change the precedences of an expression.
The concatenation ('..
') and exponentiation ('^
')
operators are right associative.
All other binary operators are left associative.
Table constructors are expressions that create tables. Every time a constructor is evaluated, a new table is created. A constructor can be used to create an empty table or to create a table and initialize some of its fields. The general syntax for constructors is
tableconstructor ::= ‘{’ fieldlist ‘}’ fieldlist ::= [field] {fieldsep [field]} field ::= ‘[’ exp ‘]’ ‘=’ exp | Name ‘=’ exp | exp fieldsep ::= ‘,’ | ‘;’ | end_of_line
Each field of the form [exp1] = exp2
adds to the new table an entry
with key exp1
and value exp2
.
A field of the form name = exp
is equivalent to
["name"] = exp
.
Finally, fields of the form exp
are equivalent to
[i] = exp
, where i
are consecutive integers
starting with 1.
Fields in the other formats do not affect this counting.
For example,
a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
is equivalent to
do
local t = {}
t[f(1)] = g
t[1] = "x" -- 1st exp
t[2] = "y" -- 2nd exp
t.x = 1 -- t["x"] = 1
t[3] = f(x) -- 3rd exp
t[30] = 23
t[4] = 45 -- 4th exp
a = t
end
The order of the assignments in a constructor is undefined. (This order would be relevant only when there are repeated keys.)
If the last field in the list has the form exp
and the expression is a function call or a vararg expression,
then all values returned by this expression enter the list consecutively
(see Function Calls).
The field list can have an optional trailing separator, as a convenience for machine-generated code.
A function call in Luan has the following syntax:
functioncall ::= prefixexp args
In a function call, first prefixexp and args are evaluated. The value of prefixexp must have type function. This function is called with the given arguments.
Arguments have the following syntax:
args ::= ‘(’ [explist] ‘)’ args ::= tableconstructor args ::= LiteralString
All argument expressions are evaluated before the call.
A call of the form f{fields}
is
syntactic sugar for f({fields})
;
that is, the argument list is a single new table.
A call of the form f'string'
(or f"string"
or f[[string]]
)
is syntactic sugar for f('string')
;
that is, the argument list is a single literal string.
The syntax for function definition is
functiondef ::= function funcbody funcbody ::= ‘(’ [parlist] ‘)’ block end_function end_function ::= end_function | end
The following syntactic sugar simplifies function definitions:
stat ::= function funcname funcbody stat ::= local function Name funcbody funcname ::= Name {‘.’ Name} [‘:’ Name]
The statement
function f () body end
translates to
f = function () body end
The statement
function t.a.b.c.f () body end
translates to
t.a.b.c.f = function () body end
The statement
local function f () body end
translates to
local f; f = function () body end
not to
local f = function () body end
(This only makes a difference when the body of the function
contains references to f
.)
A function definition is an executable expression, whose value has type function. When Luan precompiles a chunk, all its function bodies are precompiled too. Then, whenever Luan executes the function definition, the function is instantiated (or closed). This function instance (or closure) is the final value of the expression.
Parameters act as local variables that are initialized with the argument values:
parlist ::= namelist [‘,’ ‘...’] | ‘...’
When a function is called,
the list of arguments is adjusted to
the length of the list of parameters if the list is too short,
unless the function is a vararg function,
which is indicated by three dots ('...
')
at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments and supplies them
to the function through a vararg expression,
which is also written as three dots.
The value of this expression is a list of all actual extra arguments,
similar to a function with multiple results.
If a vararg expression is used inside another expression
or in the middle of a list of expressions,
then its return list is adjusted to one element.
If the expression is used as the last element of a list of expressions,
then no adjustment is made
(unless that last expression is enclosed in parentheses).
As an example, consider the following definitions:
function f(a, b) end function g(a, b, ...) end function r() return 1,2,3 end
Then, we have the following mapping from arguments to parameters and to the vararg expression:
CALL PARAMETERS f(3) a=3, b=nil f(3, 4) a=3, b=4 f(3, 4, 5) runtime error f(r(), 10) runtime error f(r()) runtime error g(3) a=3, b=nil, ... --> (nothing) g(3, 4) a=3, b=4, ... --> (nothing) g(3, 4, 5, 8) a=3, b=4, ... --> 5 8 g(5, r()) a=5, b=1, ... --> 2 3
Results are returned using the return statement (see Control Structures). If control reaches the end of a function without encountering a return statement, then the function returns with no results.
A block between backticks is run and then whatever was sent to standard output is returned as a string. Examples:
local s = `%>1 + 1 = <%=1+1%><%`
local s = ` fn(whatever) `
local s = `%>
...
<%`
Backticks complement template statements.
Luan is a lexically scoped language. The scope of a local variable begins at the first statement after its declaration and lasts until the last non-void statement of the innermost block that includes the declaration. Consider the following example:
local x = 10 -- global to module
do -- new block
local x = x -- new 'x', with value 10
print(x) --> 10
x = x+1
do -- another block
local x = x+1 -- another 'x'
print(x) --> 12
end
print(x) --> 11
end
print(x) --> 10 (the global one)
Notice that, in a declaration like local x = x
,
the new x
being declared is not in scope yet,
and so the second x
refers to the outside variable.
Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. A local variable used by an inner function is called an upvalue, or external local variable, inside the inner function.
Notice that each execution of a local statement defines new local variables. Consider the following example:
local a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end
The loop creates ten closures
(that is, ten instances of the anonymous function).
Each of these closures uses a different y
variable,
while all of them share the same x
.
The standard Luan libraries provide useful functions
that are implemented both in Java and in Luan itself.
How each function is implemented shouldn't matter to the user.
Some of these functions provide essential services to the language
(e.g., type
and get_metatable
);
others provide access to "outside" services (e.g., I/O).
This is provided by default as a local variable for any Luan code as described in Environments.
Example use:
local Table = require "luan:Table.luan"
Could be defined as:
local function require(mod_name)
return Package.load(mod_name) or Luan.error("module '"..mod_name.."' not found")
end
A special case is:
require "java"
This enables Java in the current chunk if that chunk has permission to use Java. If the chunk doesn't have permission to use Java, then an error is thrown.
Include this library by:
local Luan = require "luan:Luan.luan"
The basic library provides basic functions to Luan that don't depend on other libaries.
If Luan was run from the command line then this is a list of the command line arguments. For example if one runs Luan like this:
luan t.luan a b c
Then Luan.arg will contain:
{
[0] = "t.luan"
[1] = "a"
[2] = "b"
[3] = "c"
}
And of course #Luan.arg
will be 3
.
Could be defined as:
function Luan.do_file(uri)
local fn = Luan.load_file(uri) or Luan.error("file '"..uri.."' not found")
return fn()
end
Throws an error containing the message.
Could be defined as:
function Luan.error(message)
Luan.new_error(message).throw()
end
Evaluates text
as a Luan expression.
Could be defined as:
function Luan.eval(text,source_name, env)
return Luan.load( "return "..text, source_name or "eval", env )()
end
If table
does not have a metatable, returns nil.
Otherwise,
if the table's metatable has a "__metatable"
field,
returns the associated value.
Otherwise, returns the metatable of the given table.
Returns the hash code of v
.
Returns an iterator function so that the construction
for i,v in ipairs(t) do body end
will iterate over the key–value pairs
(1,t[1]
), (2,t[2]
), ...,
up to the first nil value.
Could be defined as:
function Luan.ipairs(t)
local i = 0
return function()
if i < #t then
i = i + 1
return i, t[i]
end
end
end
Loads a chunk.
The text
is compiled.
If there are no syntactic errors,
returns the compiled chunk as a function;
otherwise, throws an error.
The source_name
parameter is a string saying where the text came from. It is used to produce error messages. Defaults to "load".
If the env
parameter is supplied, it becomes the _ENV
of the chunk.
The persist
parameter is a boolean which determines if the compiled code is persistently cached to a temporary file. Defaults to false
.
Similar to load
,
but gets the chunk from file file_uri
.
file_uri
can be a string or a uri table.
Creates a new error table containing the message assigned to "message
". The error table also contains a throw
function which throws the error. The table also contains a list of stack trace elements where each stack trace element is a table containing "source
", "line
", and possible "call_to
". The table also has a metatable containing "__to_string
" to render the error.
To print the current stack trace, you could do:
Io.print( Luan.new_error "stack" )
If t
has a metamethod __pairs
,
calls it with t
as argument and returns the
result from the call.
Otherwise, returns a function so that the construction
for k,v in pairs(t) do body end
will iterate over all key–value pairs of table t
.
This Luan's equivalent to Javascript's JSON.parse(), but for a Luan value. In addition to the usual JSON values, Luan.parse allows long strings and allows specifying numeric types of double, float, integer, and long. For example:
local t = Luan.parse[=[
{
nothing = nil
t = true
f = false
s = "string"
ls = [[long string]]
n = 3
d = double(3)
f = float(3)
i = integer(3)
l = long(3)
list = { 1, 2, 3 }
table = {
one = 1
two = 2
three = 3
}
["ugly-key"] = "something"
}
]=]
Based on the Python range() function, this lets one iterate through a sequence of numbers.
Example use:
for i in range(1,10) do
Io.print("count up:",i)
end
for i in range(10,0,-1) do
Io.print("count down:",i)
end
Could be defined as:
function Luan.range(start, stop, step)
step = step or 1
step == 0 and Luan.error "bad argument #3 (step may not be zero)"
local i = start
return function()
if step > 0 and i <= stop or step < 0 and i >= stop then
local rtn = i
i = i + step
return rtn
end
end
end
Checks whether v1
is equal to v2
,
without invoking any metamethod.
Returns a boolean.
Gets the real value of table[index]
,
without invoking any metamethod.
table
must be a table;
index
may be any value.
Returns the length of the object v
,
which must be a table or a string,
without invoking any metamethod.
Returns an integer.
Sets the real value of table[index]
to value
,
without invoking any metamethod.
table
must be a table,
index
any value different from nil,
and value
any Luan value.
Sets the metatable for the given table.
If metatable
is nil,
removes the metatable of the given table.
If the original metatable has a "__metatable"
field,
raises an error.
This Luan's equivalent to Javascript's JSON.stringify(), but for a Luan value.
v
is a value of any type which is converted to a string that is a Luan expression. options
may be a table or a function. If options
is a table, it may contain the following flags whose true
value means:
["key"]
If options
is a function then this function should take an argument stack
and return an options
table. The stack
will be a list of keys indicating where stringify is currently processing. This allows different options to be applied at different places in a data structure.
Receives a value of any type and converts it to a string in a human-readable format.
If the metatable of v
has a "__to_string"
field,
then to_string
calls the corresponding value
with v
as argument,
and uses the result of the call as its result.
Returns the type of its only argument, coded as a string.
The possible results of this function are
"nil
" (a string, not the value nil),
"number
",
"string
",
"binary
",
"boolean
",
"table
",
"function
",
and "java
".
Returns a function so that the construction
for i, v in Luan.values(···) do body end
will iterate over all values of ···
.
A global variable (not a function) that holds a string containing the current Luan version.
Include this library by:
local Package = require "luan:Package.luan"
The package library provides basic facilities for loading modules in Luan.
Loads the given module.
The function starts by looking into the Package.loaded
table
to determine whether mod_uri
is already loaded.
If it is, then Package.load
returns the value stored
at Package.loaded[mod_uri]
.
Otherwise, it tries to load a new value for the module.
To load a new value, Package.load
first checks if mod_uri
starts with "java:". If yes, then this is a Java class which is loaded by special Java code.
Otherwise Package.load
tries to read the text of the file referred to by mod_uri
. If the file doesn't exist, then Package.load
returns false. If the file exists, then its content is compiled into a chunk by calling Luan.load
. This chunk is run passing in mod_uri
as an argument. The value returned by the chunk must not be nil and is loaded.
If a new value for the module successful loaded, then it is stored in Package.loaded[mod_uri]
. The value is returned.
A table used by Package.load
to control which
modules are already loaded.
When you load a module mod_uri
and
Package.loaded[mod_uri]
is not nil,
Package.load
simply returns the value stored there.
This variable is only a reference to the real table;
assignments to this variable do not change the
table used by Package.load
.
Include this library by:
local String = require "luan:String.luan"
This library provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. When indexing a string in Luan, the first character is at position 1 (not at 0, as in Java). Indices are allowed to be negative and are interpreted as indexing backwards, from the end of the string. Thus, the last character is at position -1, and so on.
Receives zero or more integers. Returns a string with length equal to the number of arguments, in which each character has the internal numerical code equal to its corresponding argument.
Returns a boolean indicating whether the s
contains s2
.
Returns a hex digest string of s
. Could be defined as:
function String.digest_message(algorithm,s)
return Binary.to_hex( Binary.digest_message( algorithm, String.to_binary(s) ) )
end
Encodes argument s
into a string that can be placed in quotes so as to return the original value of the string.
Returns a boolean indicating whether the s
ends with s2
.
Looks for the first substring
s2
in the string s
.
If it finds a match, then find
returns the indices of s
where this occurrence starts and ends;
otherwise, it returns nil.
A third, optional numerical argument init
specifies
where to start the search;
its default value is 1 and can be negative.
If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.
Returns a formatted version of its variable number of arguments
following the description given in its first argument (which must be a string).
The format string follows the same rules as the Java function String.format
because Luan calls this internally.
Note that Java's String.format
is too stupid to convert between ints and floats, so you must provide the right kind of number.
Receives a string and returns a copy of this string with all uppercase letters changed to lowercase. All other characters are left unchanged.
Returns a regex table for the pattern s
.
Returns a string which matches the literal string s
in a regular expression. This function is simply the Java method Pattern.quote
.
Returns a string that is the concatenation of n
copies of
the string s
separated by the string sep
.
The default value for sep
is the empty string
(that is, no separator).
Returns the empty string if n
is not positive.
Returns a string where each substring target
in s
is replaced by replacement
.
Returns a string that is the string s
reversed.
Splits s
using substring s2
and returns the results. If limit
is positive, then only returns at most that many results. If limit
is zero, then remove trailing empty results.
Returns a boolean indicating whether the s
starts with s2
.
Returns the substring of s
that
starts at i
and continues until j
;
i
and j
can be negative.
If j
is absent, then it is assumed to be equal to -1
(which is the same as the string length).
In particular,
the call string.sub(s,1,j)
returns a prefix of s
with length j
,
and string.sub(s, -i)
returns a suffix of s
with length i
.
If, after the translation of negative indices,
i
is less than 1,
it is corrected to 1.
If j
is greater than the string length,
it is corrected to that length.
If, after these corrections,
i
is greater than j
,
the function returns the empty string.
Converts a string to a binary by calling the Java method String.getBytes
.
When called with no base
,
to_number
tries to convert its argument to a number.
If the argument is
a string convertible to a number,
then to_number
returns this number;
otherwise, it returns nil.
The conversion of strings can result in integers or floats.
When called with base
,
then s
must be a string to be interpreted as
an integer numeral in that base.
In bases above 10, the letter 'A
' (in either upper or lower case)
represents 10, 'B
' represents 11, and so forth,
with 'Z
' representing 35.
If the string s
is not a valid numeral in the given base,
the function returns nil.
Removes the leading and trailing whitespace by calling the Java method String.trim
.
Returns the internal numerical codes of the characters s[i]
,
s[i+1]
, ..., s[j]
.
The default value for i
is 1;
the default value for j
is i
.
These indices are corrected
following the same rules of function String.sub
.
Receives a string and returns a copy of this string with all lowercase letters changed to uppercase. All other characters are left unchanged. The definition of what a lowercase letter is depends on the current locale.
Regular expressions are handled using a regex table generated by String.regex.
Pattern matching is based on the Java Pattern class.
Looks for the first match of
the regex in the string s
.
If it finds a match, then find
returns the indices of s
where this occurrence starts and ends;
otherwise, it returns nil.
A third, optional numerical argument init
specifies
where to start the search;
its default value is 1 and can be negative.
If the regex has captures, then in a successful match the captured values are also returned, after the two indices.
Returns an iterator function that,
each time it is called,
returns the next captures from the regex
over the string s
.
If the regex specifies no captures,
then the whole match is produced in each call.
As an example, the following loop
will iterate over all the words from string s
,
printing one per line:
local r = String.regex[[\w+]]
local s = "hello world from Lua"
for w in r.gmatch(s) do
print(w)
end
The next example collects all pairs key=value
from the
given string into a table:
local t = {}
local r = String.regex[[(\w+)=(\w+)]]
local s = "from=world, to=Lua"
for k, v in r.gmatch(s) do
t[k] = v
end
For this function, a caret '^
' at the start of a pattern does not
work as an anchor, as this would prevent the iteration.
Returns a copy of s
in which all (or the first n
, if given)
occurrences of the regex have been
replaced by a replacement string specified by repl
,
which can be a string, a table, or a function.
gsub
also returns, as its second value,
the total number of matches that occurred.
The name gsub
comes from Global SUBstitution.
If repl
is a string, then its value is used for replacement.
The character \
works as an escape character.
Any sequence in repl
of the form $d
,
with d between 1 and 9,
stands for the value of the d-th captured substring.
The sequence $0
stands for the whole match.
If repl
is a table, then the table is queried for every match,
using the first capture as the key.
If repl
is a function, then this function is called every time a
match occurs, with all captured substrings passed as arguments,
in order.
In any case, if the regex specifies no captures, then it behaves as if the whole regex was inside a capture.
If the value returned by the table query or by the function call is not nil, then it is used as the replacement string; otherwise, if it is nil, then there is no replacement (that is, the original match is kept in the string).
Here are some examples:
local r = String.regex[[(\w+)]]
local x = r.gsub("hello world", "$1 $1")
--> x="hello hello world world"
local r = String.regex[[(\w+)]]
local x = r.gsub("hello world", "$0 $0", 1)
--> x="hello hello world"
local r = String.regex[[(\w+)\s*(\w+)]]
local x = r.gsub("hello world from Luan", "$2 $1")
--> x="world hello Luan from"
local r = String.regex[[\$(.*?)\$]]
local x = r.gsub("4+5 = $return 4+5$", function(s)
return load(s)()
end)
--> x="4+5 = 9"
local r = String.regex[[\$(\w+)]]
local t = {name="lua", version="5.3"}
local x = r.gsub("$name-$version.tar.gz", t)
--> x="lua-5.3.tar.gz"
Looks for the first match of
the regex in the string s
.
If it finds one, then match
returns
the captures from the regex;
otherwise it returns nil.
If the regex specifies no captures,
then the whole match is returned.
A third, optional numerical argument init
specifies
where to start the search;
its default value is 1 and can be negative.
Returns a boolean indicating whether the regex can be found in string s
.
This function is equivalent to
return regex.match(s) ~= nil
Changes the regex pattern to pattern
.
Splits s
using the regex and returns the results. If limit
is positive, then only returns at most that many results. If limit
is zero, then remove trailing empty results.
Include this library by:
local Binary = require "luan:Binary.luan"
Same as Java's Base64.Decoder.decode.
Same as Java's Base64.Encoder.encodeToString.
Receives zero or more bytes (as integers). Returns a binary with length equal to the number of arguments, in which each byte has the internal numerical code equal to its corresponding argument.
Returns the internal numerical codes of the bytes b[i]
,
b[i+1]
, ..., b[j]
.
The default value for i
is 1;
the default value for j
is i
.
These indices are corrected
following the same rules of function String.sub
.
Implemented in Java as:
return MessageDigest.getInstance(algorithm).digest(b);
Converts a binary to a hex string.
If charset
is not nil then converts the binary b
to a string using the Java String constructor, else makes each byte a char.
Include this library by:
local Table = require "luan:Table.luan"
This library provides generic functions for table manipulation.
It provides all its functions inside the table Table
.
Returns a table with case-insensitive string keys. Copies tbl
or is empty.
Clears the table.
Given a list,
returns the string list[i]..sep..list[i+1] ··· sep..list[j]
.
The default value for sep
is the empty string,
the default for i
is 1,
and the default for j
is #list
.
If i
is greater than j
, returns the empty string.
If i
is nil
, returns a shallow copy of tbl
.
Otherwise returns a new table which is a list of the elements tbl[i] ··· tbl[j]
.
By default, j
is #tbl
.
Inserts element value
at position pos
in list
,
shifting up the elements
list[pos], list[pos+1], ···, list[#list]
.
Recursively applies java_to_table_shallow
to convert a Java object to nested tables. java_to_table_shallow
defaults to Table.java_to_table_shallow.
Converts a Java object to a table. Works for collection types List, Map, Set, and Java arrays.
Returns a new table with all parameters stored into keys 1, 2, etc.
and with a field "n
" with the total number of parameters.
Note that the resulting table may not be a sequence.
Removes from list
the element at position pos
,
returning the value of the removed element.
When pos
is an integer between 1 and #list
,
it shifts down the elements
list[pos+1], list[pos+2], ···, list[#list]
and erases element list[#list]
;
The index pos
can also be 0 when #list
is 0,
or #list + 1
;
in those cases, the function erases the element list[pos]
.
Sorts list elements in a given order, in-place,
from list[1]
to list[#list]
.
If comp
is given,
then it must be a function that receives two list elements
and returns true when the first element must come
before the second in the final order
(so that not comp(list[i+1],list[i])
will be true after the sort).
If comp
is not given,
then the standard Lua operator <
is used instead.
The sort algorithm is not stable; that is, elements considered equal by the given order may have their relative positions changed by the sort.
Returns the elements from the given list. This function is equivalent to
return list[i], list[i+1], ···, list[j]
By default, i
is 1 and j
is list.n or #list
.
Include this library by:
local Number = require "luan:Number.luan"
Returns x
as a double.
Returns x
as a float.
If the value x
is convertible to an integer,
returns that integer.
Otherwise throws an error.
If the value x
is convertible to an long,
returns that long.
Otherwise throws an error.
Converts long value i
to a string by calling Long.toString
.
Returns a string for the numeric type of x
. Possible return values include "integer
", "long
", "double
", and "float
".
Include this library by:
local Math = require "luan:Math.luan"
This library provides basic mathematical functions.
It provides all its functions and constants inside the table Math
.
Returns the absolute value of x
.
Returns the arc cosine of x
(in radians).
Returns the arc sine of x
(in radians).
Returns the arc tangent of a value; the returned angle is in the range -pi/2 through pi/2.
Returns the arc tangent of y/x
(in radians),
but uses the signs of both parameters to find the
quadrant of the result.
(It also handles correctly the case of x
being zero.)
Returns the smallest integral value larger than or equal to x
.
Returns the cosine of x
(assumed to be in radians).
Converts the angle x
from radians to degrees.
Returns the value e^{x}
(where e
is the base of natural logarithms).
Returns the largest integral value smaller than or equal to x
.
Returns the remainder of the division of x
by y
that rounds the quotient towards zero.
A value larger than any other numerical value.
Returns the logarithm of x
in the given base.
The default for base
is e
(so that the function returns the natural logarithm of x
).
Returns the argument with the maximum value,
according to the Lua operator <
.
An integer with the maximum value for an integer.
Returns the argument with the minimum value,
according to the Lua operator <
.
An integer with the minimum value for an integer.
Returns the integral part of x
and the fractional part of x
.
The value of π.
Converts the angle x
from degrees to radians.
When called without arguments,
returns a pseudo-random float with uniform distribution
in the range [0,1).
When called with two integers m
and n
,
Math.random
returns a pseudo-random integer
with uniform distribution in the range [m, n].
(The value m-n cannot be negative and must fit in a Luan integer.)
The call Math.random(n)
is equivalent to Math.random(1,n)
.
This function is an interface to the underling pseudo-random generator function provided by Java. No guarantees can be given for its statistical properties.
Returns the sine of x
(assumed to be in radians).
Returns the square root of x
.
(You can also use the expression x^0.5
to compute this value.)
Returns the tangent of x
(assumed to be in radians).