Language Elements
| Item |
Description |
| Comments |
Specified using ## or //and extend to the end of line, e.g.
## This is a comment
Also specified using //, e.g.
// This is a comment
Multiple lines comments are specified using /*...*/, e.g.
/* This is a
multi-line comment */
|
| Identifiers / variables |
Must start with a-z, A-Z, _ or $.
Can then be followed by 0-9, a-z, A-Z, _ or $.
e.g.
- Valid:
var1,_a99,$1
- Invalid:
9v,!a99,1$
Variable names are case-sensitive, e.g. var1 and Var1 are different variables.
NOTE: JEXL does not support variables with hyphens in them, e.g.
commons-logging // invalid variable name (hyphenated) is not a valid variable, but instead is treated as a
subtraction of the variable logging from the variable commons
JEXL also supports ant-style variables, the following is a valid variable name:
my.dotted.var
N.B. the following keywords are reserved, and cannot be used as a variable name or property when using the dot operator:
or and eq ne lt gt le ge div mod not null true false new var do while break continue function return
For example, the following is invalid:
my.new.dotted.var // invalid ('new' is keyword)
In such cases, quoted identifiers or the [ ] operator can be used, for example:
my.'new'.dotted.var
my['new'].dotted.var
|
| Scripts |
A script in JEXL is made up of zero or more statements, optionally enclosed in a function definition block.
Scripts can include one or more pragmas.
Scripts can be read from a String, File or URL.
Scripts can be created with named parameters which allow a later evaluation to be performed with arguments.
By default, in the absence of an explicit return statement, scripts return the value of the
last evaluated statement.
Using the return keyword, a script will return the expression that follows (or null).
|
| #pragma |
Declares a pragma, a method to communicate information from a script to its execution environment, e.g.
#pragma execution.option 42 will declare a pragma named execution.option with
a value of 42.
Pragma keys can be identifiers or antish names, pragma values can be literals (boolean, integer,
real, string, null, NaN) and antish names
Although pragmas are statements, they are not evaluated at runtime; they are constants
associated to the script after parsing. It is expected that user code accesses the pragma map from
scripts to alter some functions behavior.
There are 3 built-in pragmas:
jexl.options overrides the evaluation options using the option-flags syntax.
#pragma jexl.options "+strict -safe +lexical +lexicalShade" will let the script run with
the options strict being true, safe false, lexical true and lexical shade true.
jexl.namespace.ns_prefix declares a namespace valid for the script
execution lifetime, the value being the fully-qualified namespace class-name.
#pragma jexl.namespace.str java.lang.String declare 'str' as the namespace using
the String class.
jexl.import works in conjunction with the 'new' operator.
It declares the packages that a new ClassIdentifier(...) will visit - in
declaration order - to resolve the class name into a fully-qualified class-name.
#pragma jexl.import java.net will allow executing let url = new URL("https://commons.apache.org");,
the URL class name being solved as the java.net.URL fully-qualified class-name.
|
| Statements |
A statement can be the empty statement, the semicolon (;), block, conditional, variable declaration or an expression.
Statements are optionally terminated with a semicolon.
A single statement or a statement block can be annotated.
|
| Block |
A block is simply multiple statements inside curly braces ({, }).
|
| Local variables |
Can be defined using the let, const and var keywords;
their identifying rules are the same as contextual variables.
let declares a local variable (or a parameter) with a lexical block scope. The
variable can only be accessed within its definition block and any nested sub-block. This also
forbids variable redeclaration within that scope. Note: This emulates Java behavior which differs
from ECMAScript.const behaves as let but will prevent the variable from being
reassigned by any side effect operator.-
var declares a variable whose scope is the whole script and allows redefinition.
This behavior is altered by the JexlFeature#setLexical(true) that will enforce a
lexical scope for all variables akin to let declaration.
- Basic declaration:
let x;
- Declaration with assignment:
const theAnswer = 42;
- Invalid declaration:
var x.y;
Local variables they take precedence in resolution over contextual variables.
When scripts are created with named parameters, those behave as local variables.
Local variables can not use ant-style naming, only one identifier.
|
| Expression |
An expression can be the literal, variable, assignment, access operator, function definition, function call, method call or
an evaluation operator.
|
| Assignment |
Assigns the value of a variable (my.var = 'a value') using a
JexlContext as initial resolver. Both beans and ant-ish
variables assignment are supported.
|
| Function definition |
Defines a function within the script, usually associated with a local variable assignment.
var fun = function(x, y) { x + y }
The following syntaxes are also supported:
var fun = (x, y) -> { x + y }var fun = (let x, let y) -> { x + y }const fun = (const x, const y) -> { x + y }function fun(const x, const y) { x + y }
If the function has only one argument the parentheses may be omitted
var fun = x -> { x * x }.
Functions with an expression as body can forego the curly brackets as in:
var fun = x -> x * x .
Note that functions can use local variables and parameters from their declaring script.
Those variables values are bound to the function environment at definition time.
var t = 20; var s = function(x, y) {x + y + t}; t = 54; s(15, 7)
The function closure captures 't' when defined; the result of the evaluation will
lead to 15 + 7 + 20 = 42.
|
| Function call |
Calling a function follows the usual convention, e.g.
fun(17, 25) will call the fun function with arguments 17
and 25
|
| Method call |
Calls a method of an object, e.g.
"hello world".hashCode() will call the hashCode method
of the "hello world" String.
In case of multiple arguments and overloading, JEXL will make the best effort to find
the most appropriate non ambiguous method to call.
|
| Access Operator |
Allows to evaluate a property of an object, a value of the collection or an array
by using either square brackets or a dotted numeral, e.g.
foo.bar will access the bar property
of the foo Object.
arr1[0] will access the first element of the arr1 array.
The safe-access operator foo?.bar shortcuts any null or non-existent references
along the navigation path, allowing a safe-navigation free of errors. In the previous expression,
if 'foo' is null, the whole expression will evaluate as null. This is an efficient shortcut
to defensive expressions such as x?.y?.z that would otherwise be expressed as
x? (x.y? (x.y.z ?: null) :null) : null.
Properties can also be quoted as in foo.'b a r' and can be dynamic
back-quoted interpolation strings as in cal.`${dd.year}-${dd.month}-${dd.day}`.
These syntaxes are mixable with safe-access as in foo.'b a r'?.quux or
foo?.`${bar}`.quux.
The safe-access array operator (as in foo?[bar]) provides the same behavior as
the safe-access operator and shortcuts any null or non-existent references
along the navigation path, allowing a safe-navigation free of errors.
In the previous expression, if 'foo' is null, the whole expression will evaluate as null.
Note that this can also be used in a chain as in x?[y]?[z].
Access operators can be overloaded in JexlArithmetic, so that
the operator behavior will differ depending on the type of the operator arguments
|
| Evaluation Operator |
Performs computational, logical or comparative action between one, two or three arguments
whose values are expressions, e.g.
40 + 2 will call the add operator
between two integer literals.
All operators, except when stated otherwise, can be overloaded in JexlArithmetic, so that the action taken
will differ depending on the type of the operator arguments
|
| @annotation |
Annotations in JEXL are 'meta-statements'; they allow to wrap the execution of the JEXL statement in a user provided
caller; typical example would be: @synchronized(x) x.someMethod();
Annotations may be declared with zero or more parameters;
@lenient x.someMethod();
@synchronized(x) x.someMethod();
@parallel(pool, 8) x.someMethod();
They also can be chained as in:
@lenient @silent x.someMethod();
Annotation processing is implemented by providing a JexlContext.AnnotationProcessor; its processAnnotation
method will call the annotated statement encapsulated in a Callable. Annotation arguments are evaluated
and passed as arguments to processAnnotation.
|
Literals
| Item |
Description |
| Integer Literals |
1 or more digits from 0 to 9, eg 42.
|
| Float Literals |
1 or more digits from 0 to 9, followed
by a decimal point and then one or more digits from
0 to 9,
optionally followed by f or F,
eg 42.0 or 42.0f.
|
| Long Literals |
1 or more digits from 0 to 9 suffixed with l or L
, eg 42l.
|
| Double Literals |
1 or more digits from 0 to 9, followed
by a decimal point and then one or more digits from
0 to 9 suffixed with d or D
, eg 42.0d.
A special literal NaN can be used to denote Double.NaN constant
|
| BigInteger Literals |
1 or more digits from 0 to 9 suffixed with h or H
(for Huge ala OGNL, "does not interfere with hexa-decimal digits"), eg 42h.
|
| BigDecimal Literals |
1 or more digits from 0 to 9, followed
by a decimal point and then one or more digits from
0 to 9 suffixed with b or B)
, eg 42.0b.
|
| Natural literals - octal and hex support |
Natural numbers (i.e. Integer, Long, BigInteger) can also be expressed as octal or hexadecimal using the same format as Java.
i.e. prefix the number with 0 for octal, and prefix with 0x or 0X for hexadecimal.
For example 010 or 0x10.
|
| Real literals - exponent support |
Real numbers (i.e. Float, Double, BigDecimal) can also be expressed using standard Java exponent notation.
i.e. suffix the number with e or E followed by the sign + or -
followed by one or more decimal digits.
For example 42.0E-1D or 42.0E+3B.
|
| String literals |
Can start and end with either ' or " delimiters, e.g.
"Hello world" and
'Hello world' are equivalent.
The escape character is \ (backslash).
Unicode characters can be used in string literals;
Unicode escape sequences consist of:
- a backslash '\'
- a 'u'
- 4 hexadecimal digits ([0-9],[A-H],[a-h]).
Such sequences represent the UTF-16 encoding of a Unicode character,
for example, 'a' is equivalent to '\u0061'.
|
| Multiline format literals |
Start and end with ` delimiter - back-quote -, e.g. `Hello world`
The escape character is \ (backslash); Unicode escape sequences can also be used.
These format literals can span multiple lines and allow Unified JEXL expressions (JSTL like expressions)
to be interpolated. If a variable user valued JEXLis present in the environment - whether
as a local or global variable -, the format `Hello ${user}` will evaluate as Hello JEXL.
|
| Regular expression (regex) literals |
Start with ~/ and ends with / delimiters, e.g.
~/ABC.*/
The escape character is \ (backslash); it only escapes the string delimiter \ (slash)
|
| Boolean literals |
The literals true and false can be used, e.g.
val1 == true
|
| Null literal |
The null value is represented as in java using the literal null, e.g.
val1 == null
|
| Array literal |
A [ followed by zero or more expressions separated by , and ending
with ], e.g.
[ 1, 2, "three" ]
This syntax creates an Object[].
Empty array literal can be specified as [] with result of creating Object[]
JEXL will attempt to strongly type the array; if all entries are of the same class or if all
entries are Number instance, the array literal will be an MyClass[] in the former
case, a Number[] in the latter case.
Furthermore, if all entries in the array literal are of the same class
and that class has an equivalent primitive type, the array returned will be a primitive array. e.g.
[1, 2, 3] will be interpreted as int[].
|
| List literal |
A [ followed by zero or more expressions separated by , and ending
with ,...], e.g.
[ 1, 2, "three",...]
This syntax creates an ArrayList<Object>.
Empty list literal can be specified as [...]
|
| Set literal |
A { followed by zero or more expressions separated by , and ending
with }, e.g.
{ "one" , 2, "more"}
This syntax creates a HashSet<Object>.
Empty set literal can be specified as {}
|
| Map literal |
A { followed by zero or more sets of key : value pairs separated by , and ending
with }, e.g.
{ "one" : 1, "two" : 2, "three" : 3, "more": "many more" }
This syntax creates a HashMap<Object,Object>.
Empty map literal can be specified as {:}
|
| Range literal |
A value followed by .. and ending with other value, e.g.
1 .. 42
This syntax creates a 'range' object in the form of a java iterable which can be used in for statement, e.g.
for (i : 1..42) a = a + b[i]
|
Functions
| Function |
Description |
| empty |
Evaluates whether an expression if 'empty'.
This is true when the argument is:
-
null
- An instance of class C and the derived JexlArithmetic overloads a method 'public boolean empty(C arg)'
that returns true when the argument is considered empty
- An empty string
- An array of length zero
- A collection of size zero
- An empty map
- Defining a method 'public boolean isEmpty()'
that returns true when the instance is considered empty
This is false in other cases (besides errors).
empty(arg)
|
| size |
Evaluates the 'size' of an expression.
This returns:
- 0 if the argument is null
- The result of calling a method from a derived JexlArithmetic overload 'public int size(C arg)',
C being the class of the argument
- Length of an array
- Length of a string
- Size of a Collection
- Size of a Map
- The result of calling a method 'public int size()' defined by the argument class
This returns 0 in other cases (besides errors).
size("Hello") returns 5.
|
| new |
Creates a new instance using a fully-qualified class name or Class:
new("java.lang.Double", 10) returns 10.0.
Note that the first argument of new can be a variable or any
expression evaluating as a String or Class; the rest of the arguments are used
as arguments to the constructor for the class considered.
In case of multiple constructors, JEXL will make the best effort to find
the most appropriate non ambiguous constructor to call.
Alternatively, using #pragma jexl.import java.langcode>, one can use the
following syntax: new Double(10).
|
| Top level function |
Top level function is a function which can be invoked without specifying a namespace.
Top level function can be defined by the function definition method inside the script
A JexlContext can define methods which can be invoked as top level functions.
This can allow expressions like:
string(23.0)
Another way to define top level function is to register to JexlEngine objects or classes
with null namespace.
|
| ns:function |
A JexlEngine can register objects or classes used as function namespaces.
This can allow expressions like:
math:cosinus(23.0)
|
Operators
| Operator |
Description |
Boolean and |
The usual && operator can be used as well as the word and, e.g.
cond1 and cond2 and
cond1 && cond2 are equivalent.
Note that this operator can not be overloaded
|
Boolean or |
The usual || operator can be used as well as the word or, e.g.
cond1 or cond2 and
cond1 || cond2 are equivalent.
Note that this operator can not be overloaded
|
Boolean not |
The usual ! operator can be used as well as the word not, e.g.
!cond1 and
not cond1 are equivalent.
Note that this operator can not be overloaded
|
Bitwise and |
The usual & operator is used, e.g.
33 & 4, 0010 0001 & 0000 0100 = 0.
|
Bitwise or |
The usual | operator is used, e.g.
33 | 4, 0010 0001 | 0000 0100 = 0010 0101 = 37.
|
Bitwise xor |
The usual ^ operator is used, e.g.
33 ^ 4, 0010 0001 ^ 0000 0100 = 0010 0100 = 37.
|
Bitwise complement |
The usual ~ operator is used, e.g.
~33, ~0010 0001 = 1101 1110 = -34.
|
Left-shift << |
The left shift operator (<<) shifts the first operand the specified number of bits to the left.
1 << 2 = 4
|
Right-shift >> |
The right shift operator (>>) shifts the first operand the specified number of bits to the right.
4 >> 2 = 1
|
Unsigned Right-shift >>> |
(zero-fill right shift) shifts the first operand the specified number of bits to the right.
The sign bit becomes 0, so the result is always non-negative.
|
Ternary conditional ?: |
The usual ternary conditional operator condition ? if_true : if_false operator can be
used as well as the abbreviation value ?: if_false which returns the value if
its evaluation is defined, non-null and non-false, e.g.
val1 ? val1 : val2 and
val1 ?: val2 are equivalent.
NOTE: The condition will evaluate to false when it
refers to an undefined variable or null for all JexlEngine
flag combinations. This allows explicit syntactic leniency and treats the condition
'if undefined or null or false' the same way in all cases.
Note that this operator can not be overloaded
|
Null coalescing operator ?? |
The null coalescing operator returns the result of its first operand if it is defined and is not null.
When xandyare null or undefined,
x ?? 'unknown or null x' evaluates as 'unknown or null x'
y ?? "default" evaluates as "default".
When var x = 42 and var y = "forty-two",x??"other"
evaluates as 42 and y??"other" evaluates as "forty-two".
NOTE: this operator does not behave like the ternary conditional since it
does not coerce the first argument to a boolean to evaluate the condition.
When var x = false and var y = 0,x??true
evaluates as false and y??1 evaluates as 0.
Note that this operator can not be overloaded
|
| Equality |
The equality == operator checks whether its two operands are equal, returning a
Boolean result. Unlike the strict equality operator, it attempts to convert and compare
operands that are of different types.
-
null is only ever equal to null, that is if you compare null
to any non-null value, the result is false.
- Equality uses the java
equals method
|
| Inequality |
The inequality != operator checks whether its two operands are not equal,
returning a Boolean result. Unlike the strict inequality operator, it attempts to convert and
compare operands that are of different types.
|
| Strict-Equality |
The strict equality === operator checks whether its two operands are equal,
returning a Boolean result. Unlike the equality operator, the strict equality operator always
considers operands of different types to be different.
|
| Strict Inequality |
The strict inequality !== operator checks whether its two operands are not equal,
returning a Boolean result. Unlike the inequality operator, the strict inequality operator
always considers operands of different types to be different
|
| Less Than |
The usual < operator can be used as well as the abbreviation lt.
For example
val1 < val2 and
val1 lt val2 are equivalent.
|
| Less Than Or Equal To |
The usual <= operator can be used as well as the abbreviation le.
For example
val1 <= val2 and
val1 le val2 are equivalent.
|
| Greater Than |
The usual > operator can be used as well as the abbreviation gt.
For example
val1 > val2 and
val1 gt val2 are equivalent.
|
| Greater Than Or Equal To |
The usual >= operator can be used as well as the abbreviation ge.
For example
val1 >= val2 and
val1 ge val2 are equivalent.
|
In or Match=~ |
The syntactically Perl inspired =~ operator can be used to check that a string matches
a regular expression (expressed either a Java String or a java.util.regex.Pattern).
For example
"abcdef" =~ "abc.* returns true.
It also checks whether any collection, set or map (on keys) contains a value or not; in that case, it behaves
as an "in" operator.
Note that arrays and user classes exposing a public 'contains' method will allow their instances
to behave as right-hand side operands of this operator.
"a" =~ ["a","b","c","d","e",f"] returns true.
|
Not-In or Not-Match!~ |
The syntactically Perl inspired !~ operator can be used to check that a string does not
match a regular expression (expressed either a Java String or a java.util.regex.Pattern).
For example
"abcdef" !~ "abc.* returns false.
It also checks whether any collection, set or map (on keys) does not contain a value; in that case, it behaves
as "not in" operator.
Note that arrays and user classes exposing a public 'contains' method will allow their instances
to behave as right-hand side operands of this operator.
"a" !~ ["a","b","c","d","e",f"] returns false.
|
Starts With=^ |
The =^ operator is a short-hand for the 'startsWith' method.
For example, "abcdef" =^ "abc" returns true.
Note that through duck-typing, user classes exposing a public 'startsWith' method will allow their instances
to behave as left-hand side operands of this operator.
|
Not Starts With!^ |
This is the negation of the 'starts with' operator.
a !^ "abc" is equivalent to !(a =^ "abc")
|
Ends With=$ |
The =$ operator is a short-hand for the 'endsWith' method.
For example, "abcdef" =$ "def" returns true.
Note that through duck-typing, user classes exposing an 'endsWith' method will allow their instances
to behave as left-hand side operands of this operator.
|
Not Ends With!$ |
This is the negation of the 'ends with' operator.
a !$ "abc" is equivalent to !(a =$ "abc")
|
| Addition |
The usual + operator is used.
For example
val1 + val2
|
| Subtraction |
The usual - operator is used.
For example
val1 - val2
|
| Multiplication |
The usual * operator is used.
For example
val1 * val2
|
| Division |
The usual / operator is used, or one can use the div operator.
For example
val1 / val2
or
val1 div val2
|
| Modulus (or remainder) |
The % operator is used. An alternative is the mod
operator.
For example
5 mod 2 gives 1 and is equivalent to 5 % 2
|
| Side-effect operators |
Some operators exist in side-effect forms.
Their default behavior is to execute the operator and assign the left-hand side with the result.
For instance a += 2 is equivalent to a = a + 2
The list of operators is:
- +=
- -=
- *=
- /=
- %=
- &=
- |=
- ^=
- <<=
- >>=
- >>>=
|
| Negate |
The unary - operator is used. It changes the sign of its numeric argument.
For example -12 or -(a * b)
|
| Positivize |
The unary + operator is used. It performs an integral promotion meaning
that byte, short, char arguments will be promoted to integer as a result.
For example +12 or +(a * b)
|
| Empty |
The unary empty operator behaves exactly as the corresponding function empty().
For example empty arg and empty(arg) are equivalent
|
| Size |
The unary size operator behaves exactly as the corresponding function size().
For example size [1,2,3] and size([1,2,3]) are equivalent
|
| instanceof/!instanceof |
The instanceof operator allows to check whether an object belongs to a certain class.
It is using Class.isInstance to perform the check. As a convenience, {{ !instanceof }} is
supported as well.
|
Access
| Operator |
Description |
| Array access |
Array elements may be accessed using either square brackets or a dotted numeral, e.g.
arr1[0] and arr1.0 are equivalent
|
| List access |
List elements may be accessed using either square brackets or a dotted numeral, e.g.
list[0] and list.0 are equivalent
|
| Map access |
Map elements are accessed using square brackets, e.g.
map[0]; map['name']; map[var];
Note that map['7'] and map[7] refer to different elements.
Map elements with a numeric key may also be accessed using a dotted numeral, e.g.
map[0] and map.0 are equivalent.
Note that map.1 and map.01 refer to different elements,
while map.1 and map[01] are equivalent.
|
| JavaBean property access |
Properties of JavaBean objects that define appropriate getter methods can be accessed
using either square brackets or a dotted numeral, e.g.
foo['bar'] and foo.bar are equivalent.
The appropriate Foo.getBar() method will be called.
Note that both foo.Bar and foo.bar can be used
|
| Indexed JavaBean property access |
Indexed properties of JavaBean objects that define appropriate getter methods can be accessed
using either square brackets or a dotted numeral, e.g.
x.attribute['name'] and x.attribute.name are equivalent.
The appropriate Foo.getAttribute(String index) method will be called
|
| Public field access |
Public fields of java objects can be accessed using either square brackets or a dotted numeral, e.g.
foo['bar'] and foo.bar are equivalent.
|
| Duck-typed collection property access |
Properties of Java classes that define public Object get(String name) method can be accessed
using either square brackets or a dotted numeral, e.g.
foo['bar'] and foo.bar are equivalent.
The appropriate Foo.get(String index) method will be called with the argument of "bar" String
|
Conditional
| Statement |
Description |
| if |
Classic, if/else statement, e.g.
if ((x * 2) == 5) {
y = 1;
} else {
y = 2;
}
|
| for |
Loop through items of an Array, Collection, Map, Iterator or Enumeration, e.g.
for (let item : list) {
x = x + item;
}
Where list is a context variable pointing to any iterable structure.
The following syntax using a context variable is also supported:
for (item : list) {
x = x + item;
}
Note that the loop variable item is accessible after loop evaluation
Finally, the conventional syntax using a local variable, initial value, loop condition and
loop step is supported.
for (let i = 0; i < size(list); ++i) {
x = x + list[i];
}
Where list is a local variable pointing to an array-like structure.
The JEXL 1.1 syntax using foreach(item in list) is now unsupported.
|
| while |
Loop until a condition is satisfied, e.g.
while (x < 10) {
x = x + 2;
}
|
| do/while |
Loop until a condition is satisfied, e.g.
do {
x = x + 2;
} while (x < 10)
|
Control Flow
| Statement |
Description |
| if |
Classic, if/else statement, e.g.
if ((x * 2) == 5) {
y = 1;
} else {
y = 2;
}
|
| continue |
Within loops (do/while/for), terminates execution of the statements in the current iteration
and skips to the next loop iteration.
let text = '';
for (let i : (4..2)) {
if (i === 3) {
continue;
}
text += i;
}
text;
Will evaluate as "42".
|
| break |
Within loops (do/while/for), terminates execution of the statements in the current iteration
and exits the loop.
let i = 33;
while (i < 66) {
if (i == 42) {
break;
}
i += 1;
}
i;
Will evaluate as 42.
|
| return |
The return statement ends execution and specifies a value to be returned to the script
or lambda caller.
function f(x) {
if (x == 42) {
return "The answer to life, the universe, and everything";
}
return x
}
f(41);
Will evaluate as 41.
|
| throw |
The throw statement throws an object. Execution of the current script will stop
(the statements after throw won't be executed), and control will be passed to the first catch
block in the call stack. If no catch block exists among caller functions, script execution will
terminate.
|
| try/catch/finally |
The try...catch statement is comprised of a try block and either a catch block, a finally block,
or both. The code in the try block is executed first, and if it throws an exception, the code in
the catch block will be executed.
The code in the finally block will always be executed before control flow exits the entire construct.
try {
return 42/0;
} catch (let e) {
// handle the error
}
The following syntax is also supported:
try {
return 42/0;
} catch (var e) {
// handle the error
}
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| try-with-resources |
The try-with-resources statement is a try statement that declares one or more resources.
The try-with-resources statement ensures that each resource is closed at
the end of the statement; any object that exposes a close() method will see that
method invoked.
let g = open("g");
try(let f = open("f"), g) {
// more code
}
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