Runtime evaluation: Difference between revisions
added note re Lua 5.3+ loadstring -> load |
|||
Line 906:
<lang lua>f = load("return 42")
f() --> returns 42</lang>
In Lua 5.3+ (5.3, 5.4) <code>loadstring</code> is no longer available, use <code>load</code> instead.
<lang lua>> f = load("return 42")
> f()
42
> n = load("return 42")()
> n
42</lang>
=={{header|M2000 Interpreter}}==
|
Revision as of 17:28, 10 July 2020
You are encouraged to solve this task according to the task description, using any language you may know.
- Task
Demonstrate a language's ability for programs to execute code written in the language provided at runtime.
Show what kind of program fragments are permitted (e.g. expressions vs. statements), and how to get values in and out (e.g. environments, arguments, return values), if applicable what lexical/static environment the program is evaluated in, and what facilities for restricting (e.g. sandboxes, resource limits) or customizing (e.g. debugging facilities) the execution.
You may not invoke a separate evaluator program, or invoke a compiler and then its output, unless the interface of that program, and the syntax and means of executing it, are considered part of your language/library/platform.
For a more constrained task giving a specific program fragment to evaluate, see Eval in environment.
ALGOL 68
Variable names are generally not visible at run time with classic compilers. However ALGOL 68G is an interpretor and it retains this ability. <lang algol68>print(evaluate("4.0*arctan(1.0)"))</lang>
- Output:
+3.14159265358979e +0
This example demonstrates the use of variables and that the Algol 68G evaluate uses the normal Algol 68 scoping rules:
<lang algol68># procedure to call the Algol 68G evaluate procedure #
- the environment of the evaluation will be the caller's environment #
- with "code", "x" and "y" defined as the procedure parameters #
PROC ev = ( STRING code, INT x, INT y )STRING: evaluate( code );
BEGIN
INT i := 1; INT j := 2; REAL x := 4.2; REAL y := 0.7164;
# evaluates "i + j" in the current environment # print( ( evaluate( "i + j" ), newline ) );
# evaluates "x + y" in the environment of the procedure body of ev # print( ( ev( "x + y", i, j ), newline ) );
# evaluates "x + y" in the current environment, so shows a different # # result to the previous call # print( ( evaluate( "x + y" ), newline ) );
# prints "code" because code is defined in the environment of the # # call to evaluate (in ev) although it is not defined in this # # environment # print( ( ev( "code", 1, 2 ), newline ) );
# prints "code + codecode + code" - see above # print( ( ev( "code + code", 1, 2 ), newline ) )
END
- if this next call was executed, a runtime error would occur as x and y #
- do not exist anymore #
- ;print( ( evaluate( "x + y" ), newline ) ) #</lang>
- Output:
+3 +3 +4.91640000000000e +0 code code + codecode + code
AutoHotkey
function addScript can be used to dynamically add lines of code to a running script. <lang AutoHotkey>; requires AutoHotkey_H or AutoHotkey.dll msgbox % eval("3 + 4") msgbox % eval("4 + 4") return
eval(expression)
{
global script
script =
(
expression(){ return %expression% }
) renameFunction("expression", "") ; remove any previous expressions gosub load ; cannot use addScript inside a function yet exp := "expression" return %exp%() }
load: DllCall(A_AhkPath "\addScript","Str",script,"Uchar",0,"Cdecl UInt") return
renameFunction(funcName, newname){ static x%newname% := newname ; store newname in a static variable so its memory is not freed strput(newname, &x%newname%, strlen(newname) + 1) if fnp := FindFunc(funcName)
numput(&x%newname%, fnp+0, 0, "uint")
}</lang>
BASIC
Evaluating expressions
VAL() function converts string into numeric value. On many Basic implementations, VAL only accepts simple numeric values. However, Sinclair Basic and its derivates such as Beta Basic and SAM Basic accept any expression that evaluates to numeric value.
The following example shows a functon that plots graph of any function f(x). The function is passed in string parameter f$.
100 DEF PROC graph f$ 110 LOCAL x,y 120 PLOT 0,90 130 FOR x = -2 TO 2 STEP 0.02 140 LET y = VAL(f$) 150 DRAW TO x*50+100, y*50+90 160 NEXT x 170 END PROC
Usage example:
500 graph "SIN(x) + SIN(x*3)/3"
Executing code
The KEYIN statement available on Beta Basic and SAM Basic executes a string as if it had been entered from keyboard in command mode. It can execute commands directly, or add (or replace) lines in the program while the program is executing. This allows creating self-modifying programs.
The function do_with_x in the following example loops variable x from 1 to 10 and within the loop executes any code passed to function in parameter p$.
100 DEF PROC do_with_x p$ 110 LOCAL x 130 FOR x = 1 TO 10 140 KEYIN p$ 160 NEXT x 170 END PROC
The usage example below creates a multiplication table by executing inner loop for y:
500 LET y$ = "FOR y=1 TO 10: PRINT AT y, x*3; x*y: NEXT y" 510 do_with_x y$
VAL and KEYIN execute code in the environment they are called from. In the above examples, VAL and KEYIN both see the local variable x. There is no sandbox functionality in Bata BASIC or SAM BASIC.
In ZX Spectrum Basic, loading a new program will replace the existing program. The new program will sutomatically run, if it was saved to do so by using SAVE together with LINE:
<lang zxbasic> 10 REM load the next program 20 LOAD "PROG2" </lang>
You can also include code in a text string as follows: <lang zxbasic>10 LET f$=CHR$ 187+"(x)+"+CHR$ 178+"(x*3)/2": REM LET f$="SQR (x)+SIN (x*3)/2" 20 FOR x=0 TO 2 STEP 0.2 30 LET y=VAL f$ 40 PRINT y 50 NEXT x </lang>
CHR$ 178 is the token of function SQR, and CHR$ 178 is the token of function SIN.
In 48 k mode, you can also write this: <lang zxbasic>10 LET f= SQR (x)+SIN (x*3)/2</lang> Then the type of the variable is changed and the formula is enclosed in quotation marks: <lang zxbasic>10 LET f$=" SQR (x)+SIN (x*3)/2"</lang>
BBC BASIC
Expressions
Expressions can be evaluated using the EVAL function: <lang bbcbasic> expr$ = "PI^2 + 1"
PRINT EVAL(expr$)</lang>
- Output:
10.8696044
Statements
Statements can be executed by being tokenised and then written to a temporary file: <lang bbcbasic> exec$ = "PRINT ""Hello world!"""
bbc$ = FNtokenise(exec$) tmpfile$ = @tmp$+"temp.bbc" tmpfile% = OPENOUT(tmpfile$) BPUT#tmpfile%, bbc$+CHR$0 CLOSE #tmpfile% CALL tmpfile$ END DEF FNtokenise(A$) LOCAL A% A% = EVAL("0:"+A$) A$ = $(!332+2) = CHR$(LENA$+4) + CHR$0 + CHR$0 + A$ + CHR$13
</lang>
- Output:
Hello world!
Burlesque
In Burlesque "Code" is actually just a list of identifiers. It is therefore possible to create and manipulate code at runtime. Evaluating a block:
<lang burlesque> blsq ) {5 5 .+}e! 10 </lang>
Creating code at runtime (changing map (+5) to map (+6) at runtime):
<lang burlesque> blsq ) 1 10r@{5.+}m[ {6 7 8 9 10 11 12 13 14 15} blsq ) 1 10r@{5.+}6 0sam[ {7 8 9 10 11 12 13 14 15 16} </lang>
Code from string at runtime:
<lang burlesque> blsq ) 1 10r@"5.+"psm[ {6 7 8 9 10 11 12 13 14 15} </lang>
Evaluating strings at runtime (reverse is just for demonstration):
<lang burlesque> blsq ) "[m}+.5{@r01 1"<-pe {6 7 8 9 10 11 12 13 14 15} </lang>
Injecting other functions into code:
<lang burlesque> blsq ) {3 2}(?*)[+e! 6 </lang>
Identifiers not contained in a block require to be quoted to push them to the stack. Note the difference:
<lang burlesque> blsq ) ?+ ERROR: Burlesque: (.+) Invalid arguments! blsq ) ?+to "Error" blsq ) (?+) ?+ blsq ) (?+)to "Ident" </lang>
(also note the fallback to .+ from ?+).
Caché ObjectScript
The 'XECUTE' command performs substantially the same operation as the '$XECUTE' function, except the latter must specify a return value.
- Examples:
USER>Set cmd="Write ""Hello, World!""" USER>Xecute cmd Hello, World! USER>Set fnc="(num1, num2) Set res=num1+num2 Quit res" USER>Write $Xecute(fnc, 2, 3) 5
Common Lisp
Brief eval
tutorial
The eval function evaluates Lisp code at runtime.
<lang lisp>(eval '(+ 4 5)) ; returns 9</lang>
In Common Lisp, programs are represented as trees (s-expressions). Therefore, it is easily possible to construct a program which includes externally specified values, particularly using backquote template syntax:
<lang lisp>(defun add-four-complicated (a-number)
(eval `(+ 4 ',a-number)))</lang>
Or you can construct a function and then call it. (If the function is used more than once, it would be good to use compile
instead of eval
, which compiles the code before returning the function. eval
is permitted to compile as well, but compile
requires it.)
<lang lisp>(defun add-four-by-function (a-number)
(funcall (eval '(lambda (n) (+ 4 n)))) a-number)</lang>
If your program came from a file or user input, then you have it as a string, and read
or read-from-string
will convert it to s-expression form:
<lang lisp>(eval (read-from-string "(+ 4 5)"))</lang>
Common Lisp has lexical scope, but eval
always evaluates “in the null lexical environment”. In particular, eval
does not inherit the lexical variables from the enclosing code. (Note that eval
is an ordinary function and as such does not have access to that environment anyway.)
<lang lisp>(let ((x 11) (y 22))
;; This is an error! Inside the eval, x and y are unbound! (format t "~%x + y = ~a" (eval '(+ x y))))</lang>
One way to fix the error is to (declare (special x y)) for dynamic variables; but the easier and shorter way is to insert the values of x and y with the backquote template syntax.
<lang lisp>(let ((x 11) (y 22))
(format t "~%x + y = ~a" (eval `(+ ,x ,y))))</lang>
Sandboxing Discussion
Sandboxing in Common Lisp can be approached in a variety of ways, none of which are standardized.
One approach to define a sublanguage and validate expressions before passing them to compile
or eval
. Of course, a whole different language entirely can be defined, and translated to Lisp. This is essentially the classic "trusted compiler generating safe code in an untrusted target language" approach.
One way to simplify the validator is to use the package system to create a sandbox. This is done by defining a package arbitrarily called sandbox
. The validator then simply has to make sure that all symbols used in the expression are restricted to those which are visible inside the sandbox
package. Inside sandbox
, we include only those functions, operators, variables and other symbols from system packages that are safe: materials which don't allow sandboxed code to do anything harmful from within the sandbox, or to escape from the sandbox. For instance, suppose that some package system
has a function called run-shell-command
. We do not import run-shell-command
into the sandbox
package, and our validator will reject code which has references such as <lang lisp>(system:run-shell-command ...)</lang>. Therefore, the sandboxed code has no direct way to run that function. To gain access to it, it must exploit some flaw in the sandbox. One flaw in the sandbox would be the inclusion of certain package-related functions like find-symbol
. The expression <lang lisp>(find-symbol "FOO" "BAR")</lang> will retrieve symbol foo::bar
if it exists. The validator will not find this code because it has no embedded symbolic references to package foo
; they are disguised as character string. A cautious approach to the sandbox should be taken: include less rather than more, and consider each expansion of the sandbox with meticulous care.
Debugging Notes
There are no standardized debugging facilities specific to the eval
operation itself, but code evaluted may be affected by the current global declarations, particularly the optimize
declaration's debug
and safety
qualities.
Déjà Vu
The compiler, module system and interactive interpreter are all implemented in Déjà Vu itself, and the first two are part of the standard library.
Each compiled fragment is considered to be a single "file", and cannot access any local variables from outside of itself.
<lang dejavu>!run-blob !compile-string "(fake filename)" "!print \qHello world\q"</lang>
- Output:
Hello world
E
In E, eval is a method of expression ASTs (EExprs). (Other types of program fragment ASTs such as methods and patterns may not be directly evaluated, and must be inserted into an expression.)
The lexical environment is provided as a parameter and cannot be omitted. The evaluated program has no access to anything but the provided environment.
<lang e>? e`1 + 1`.eval(safeScope)
- value: 2</lang>
eval
returns the value of the expression. evalToPair
also returns the modified environment for use with further evaluation, e.g. for implementing a REPL.
<lang e>? def [value, env] := e`def x := 1 + 1`.evalToPair(safeScope)
- value: [2, ...]
? e`x`.eval(env)
- value: 2</lang>
Eval from a string may be done by invoking the parser.
<lang e>? def prog := <elang:syntax.makeEParser>.run("1 + 1")
- value: e`1.add(1)`
? prog.eval(safeScope)
- value: 2</lang>
EchoLisp
eval : The evaluation of the eval argument must give a symbolic expression, which is in turn evaluated. Alternatively, read-from-string produces a s-expression - any kind of program - from a string. <lang scheme> (eval (list * 6 7))
→ 42
(eval '(* 6 7)) ;; quoted argument
→ 42
(eval (read-from-string "(* 6 7)"))
→ 42
</lang>
Elena
ELENA 5.0: Using ELENA Script engine: <lang elena>import extensions'scripting;
public program() {
lscript.interpret("system'console.writeLine(""Hello World"")");
}</lang>
- Output:
Hello World
Elixir
<lang elixir>iex(1)> Code.eval_string("x + 4 * Enum.sum([1,2,3,4])", [x: 17]) {57, [x: 17]} iex(2)> Code.eval_string("c = a + b", [a: 1, b: 2]) {3, [a: 1, b: 2, c: 3]} iex(3)> Code.eval_string("a = a + b", [a: 1, b: 2]) {3, [a: 3, b: 2]}</lang>
Erlang
Erlang eval is a bit complex/verbose and requires the interaction of 3 modules: erl_scan (tokenizes), erl_parse (returns an abstract form) and erl_eval (variable binding, evaluate abstract form, etc). <lang erlang>1> {ok, Tokens, _} = erl_scan:string("X + 4 * lists:sum([1,2,3,4])."). ... 2> {ok, [Form]} = erl_parse:parse_exprs(Tokens). ... 3> Bindings = erl_eval:add_binding('X', 17, erl_eval:new_bindings()). [{'X',17}] 4> {value, Value, _} = erl_eval:expr(Form, Bindings). {value,57,[{'X',17}]} 5> Value. 57 </lang>
Factor
Arbitrary strings can be eval'd, but you must provide their stack effect.
IN: scratchpad "\"Hello, World!\" print" ( -- ) eval Hello, World! IN: scratchpad 4 5 "+" ( a b -- c ) eval 9
You can use the infer word to infer a quotation's stack effect. You can combine infer with parse-string to eval an arbitrary string without writing the stack effect yourself.
( scratchpad ) "USE: math 8 9 +" dup parse-string "USE: math 8 9 +" [ 8 9 + ] ( scratchpad ) infer "USE: math 8 9 +" ( x x -- x ) ( scratchpad ) eval 17
Forth
EVALUATE invokes the interpreter on a string of Forth code, using and modifying the current dictionary and stack state. <lang forth>s" variable foo 1e fatan 4e f*" evaluate
f. \ 3.14159... 1 foo !</lang>
Sandboxing can be achieved in general by using MARKER, which defines a checkpoint for the dictionary state which can later be restored.
<lang forth>unused . \ show how much dictionary space is available marker restore
create foo 30 allot
- my-def 30 0 do cr i . ." test" loop ;
unused . \ lower than before
restore unused . \ same as first unused; restore, foo, and my-def no longer defined</lang>
Frink
The eval[]
function can be used to evaluate arbitrary Frink code in the current environment, or in a new context.
<lang frink>
eval["length = 1234 feet + 2 inches"]
</lang>
There is also a two-argument version, eval[expression, rethrows]
where the rethrows
argument is a boolean flag indicating if we want evaluation errors to be thrown or just suppressed and undef
returned. If it is true, errors will be rethrown as Java exceptions, otherwise an error returns undef.
There is also a three-argument version, eval[expression, rethrows, hidesLocals]
where the hidesLocal
argument is a boolean flag indicating if we want to hide local variables (that is, create a new context) before evaluation.
Frink has an extensive security manager which allows the eval statement to prevent unsecure operations such as reading or writing a file or URL, creating new functions or classes, altering systemwide flags, evaluate arbitrary Java code, and so on. If code needs to evaluate unsecure statments, you can use the intentionally frighteningly-named unsafeEval[str]
(which may itself be disallowed in secure contexts.)
Go
As a compiled, strongly typed language, eval()
is not the strong suit of Go. Nevertheless, an eval
package exists that does that. Just don't expect it to be as easy or efficient as in interpreted languages. The eval package was originally part of the Go standard library but is now hosted and maintained externally.
<lang go>package main
import (
"fmt"
"bitbucket.org/binet/go-eval/pkg/eval"
"go/token"
)
func main() { w := eval.NewWorld(); fset := token.NewFileSet();
code, err := w.Compile(fset, "1 + 2") if err != nil { fmt.Println("Compile error"); return }
val, err := code.Run(); if err != nil { fmt.Println("Run time error"); return; } fmt.Println("Return value:", val) //prints, well, 3
}</lang>
Groovy
Each of these solutions evaluates a Groovy script based on some variation of the solution to the "Yuletide Holiday" task.
Each variation has been verified to give the same output:
[2011, 2016, 2022, 2033, 2039, 2044, 2050, 2061, 2067, 2072, 2078, 2089, 2095, 2101, 2107, 2112, 2118]
Simple evaluation
The GroovyShell class allows the evaluation of a string or of the text contents of a File or InputStream as a Groovy script. A script is a either a set of statements to be executed in order, or a Groovy class with a main() method, or a Groovy Thread subclass or Runnable implementation. The return value is the value of the last statement executed, or the value of an explicit return statement (if any). <lang groovy>def years1 = new GroovyShell().evaluate( (2008..2121).findAll {
Date.parse("yyyy-MM-dd", "${it}-12-25").format("EEE") == "Sun"
} )
println years1</lang>
The last expression evaluated in the script, a list of years found, is the return value of the evaluate() method.
Evaluation with variables
There are several approaches to evaluating a script with variables:
- GString embedded values
- Binding variables
- Eval shortcut
GString embedded values
Setting up the script as a GString with embedded value parsing is a "natural" ad hoc solution for Groovy programmers, but there are possible pitfalls if the script itself contains GStrings.
<lang groovy>def startYear = 2008
def endYear = 2121
def years2 = new GroovyShell().evaluate("""
(${startYear}..${endYear}).findAll {
Date.parse("yyyy-MM-dd", "\${it}-12-25").format("EEE") == "Sun"
} """)
println years2</lang> The variables "startYear" and "endYear" are dynamically pulled into the script GString as embedded values before the script itself ever executes.
Notice that in the script the embedded value "${it}" must be quoted with backslash (\) to prevent parsing as a part of the script GString. However, it is still correctly parsed within the internal GString when the script is run.
Binding variables
GroovyShell uses a Binding object to pass variable values to a script. This is the only way to pass variables if the script comes from a File or InputStream, but even if the script is a string Binding avoids the nested quoting issue caused by the ad hoc use of GString.
<lang groovy>def context = new Binding()
context.startYear = 2008
context.endYear = 2121
def years3 = new GroovyShell(context).evaluate(
(startYear..endYear).findAll {
Date.parse("yyyy-MM-dd", "${it}-12-25").format("EEE") == "Sun"
} )</lang>
We may instantiate Binding with the variables as named parameters, allowing a more terse syntax: <lang groovy>def years4 = new GroovyShell( new Binding(startYear: 2008, endYear: 2121) ).evaluate( (startYear..endYear).findAll {
Date.parse("yyyy-MM-dd", "${it}-12-25").format("EEE") == "Sun"
} )
println years4</lang>
We may also access the Binding object after script evaluation to extract values of any global variables set during the evaluation: <lang groovy>def binding = new Binding(startYear: 2008, endYear: 2121) new GroovyShell( binding ).evaluate( yearList = (startYear..endYear).findAll {
Date.parse("yyyy-MM-dd", "${it}-12-25").format("EEE") == "Sun"
} )
println binding.yearList</lang>
Eval shortcut
For simple evaluation of string-based scripts with only a few variables (like this one), the Eval class has static shortcut methods that do the Binding setup and GroovyShell evaluation under the surface. Eval.me(script) evaluates a script with no variables. Eval.x(x,script), Eval.xy(x,y,script), or Eval.xyz(x,y,z,script) each evaluates a script with 1, 2, or 3 variables, respectively. Here is an example with start and end years as script variables x and y.
<lang groovy>def years5 = Eval.xy(2008, 2121,
(x..y).findAll {
Date.parse("yyyy-MM-dd", "${it}-12-25").format("EEE") == "Sun"
} )
println years5</lang>
GW-BASIC
<lang qbasic>10 LINE INPUT "Type an expression: ",A$ 20 OPEN "CHAIN.TMP" FOR OUTPUT AS #1 30 PRINT #1, "70 LET Y=("+A$+")" 40 CLOSE #1 50 CHAIN MERGE "CHAIN.TMP",60,ALL 60 FOR X=0 TO 5 70 REM 80 PRINT X,Y 90 NEXT X 100 GOTO 10</lang>
Harbour
<lang harbour> Procedure Main()
local bAdd := {|Label,n1,n2| Qout( Label ), QQout( n1 + n2 )} Eval( bAdd, "5+5 = ", 5, 5 ) Eval( bAdd, "5-5 = ", 5, -5 ) return
Upon execution you see: 5+5 = 10 5-5 = 0 </lang>
HicEst
XEQ invokes the interpreter on a string of HicEst code, but keeps the current dictionary and stack state. Blocks of expressions are not possible. <lang HicEst>value = XEQ( " temp = 1 + 2 + 3 ") ! value is assigned 6 ! temp is undefined outside XEQ, if it was not defined before.
XEQ(" WRITE(Messagebox) 'Hello World !' ")
OPEN(FIle="my_file.txt") READ(FIle="my_file.txt", Row=6) string XEQ( string ) ! executes row 6 of my_file.txt</lang>
J
Use monadic ".
(Do) to execute a string.
<lang j>". 'a =: +/ 1 2 3' NB. execute a string to sum 1, 2 and 3 and assign to noun a</lang>
Only J expressions are allowed in strings used as as arguments for ".
(control words and blocks of expressions are not allowed).
Alterntively, you can use the conjunction :
(Explicit Definition) to create various kinds of functions and evaluate them. Arguments have names, such as "y", which are specified by the language definition. For example:
<lang j>monad :'+/y' 1 2 3</lang>
Rules of scope for such functions match those described on the Scope modifiers page. Also, control words (like if. or for. or while.) and blocks of expressions are allowed in strings which are evaluated in this fashion.
The context for these evaluations will always be the current locale (which might typically be the current object [or class]). If only expressions are allowed, then local variables will be local to the current explicit definition. Otherwise a new local context will be created for the evaluation (and this will be discarded when evaluation has completed). Local contexts are lexical while locales may also be manipulated programatically.
Debugging facilities [currently] require that the operation be given a name.
J relies on the OS for sandboxing and does not offer any additional resource constraints.
Java
You can kind-of do this in Java. The compiler has the relevant APIs, so it is "considered part of your language/library/platform". You have to get a compiler (which may fail), make a pseudo-file-system, compile your class, and make a class loader that will load it. Then you can use regular Java reflection to make an instance and call methods on it.
Longest "Hello world" program ever?<lang Java>import java.io.ByteArrayOutputStream; import java.io.IOException; import java.io.OutputStream; import java.lang.reflect.InvocationTargetException; import java.net.URI; import java.util.HashMap; import java.util.LinkedList; import java.util.List; import java.util.Map; import javax.tools.FileObject; import javax.tools.ForwardingJavaFileManager; import javax.tools.JavaCompiler; import javax.tools.JavaFileObject; import javax.tools.SimpleJavaFileObject; import javax.tools.StandardJavaFileManager; import javax.tools.StandardLocation; import javax.tools.ToolProvider;
public class Evaluator{
public static void main(String[] args){ new Evaluator().eval( "SayHello", "public class SayHello{public void speak(){System.out.println(\"Hello world\");}}", "speak" ); }
void eval(String className, String classCode, String methodName){ Map<String, ByteArrayOutputStream> classCache = new HashMap<>(); JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
if ( null == compiler ) throw new RuntimeException("Could not get a compiler.");
StandardJavaFileManager sfm = compiler.getStandardFileManager(null, null, null); ForwardingJavaFileManager<StandardJavaFileManager> fjfm = new ForwardingJavaFileManager<StandardJavaFileManager>(sfm){ @Override public JavaFileObject getJavaFileForOutput(Location location, String className, JavaFileObject.Kind kind, FileObject sibling) throws IOException{ if (StandardLocation.CLASS_OUTPUT == location && JavaFileObject.Kind.CLASS == kind) return new SimpleJavaFileObject(URI.create("mem:///" + className + ".class"), JavaFileObject.Kind.CLASS){ @Override public OutputStream openOutputStream() throws IOException{ ByteArrayOutputStream baos = new ByteArrayOutputStream(); classCache.put(className, baos); return baos; } }; else throw new IllegalArgumentException("Unexpected output file requested: " + location + ", " + className + ", " + kind); } }; List<JavaFileObject> files = new LinkedList<JavaFileObject>(){{ add( new SimpleJavaFileObject(URI.create("string:///" + className + ".java"), JavaFileObject.Kind.SOURCE){ @Override public CharSequence getCharContent(boolean ignoreEncodingErrors){ return classCode; } } ); }};
// Now we can compile! compiler.getTask(null, fjfm, null, null, null, files).call();
try{ Class<?> clarse = new ClassLoader(){ @Override public Class<?> findClass(String name){ if (! name.startsWith(className)) throw new IllegalArgumentException("This class loader is for " + className + " - could not handle \"" + name + '"'); byte[] bytes = classCache.get(name).toByteArray(); return defineClass(name, bytes, 0, bytes.length); } }.loadClass(className);
// Invoke a method on the thing we compiled clarse.getMethod(methodName).invoke(clarse.newInstance());
}catch(ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException x){ throw new RuntimeException("Run failed: " + x, x); } }
}</lang>
- Output:
Hello world
If you have a JRE and not a JDK, there is no compiler, so this doesn't work.
Exception in thread "main" java.lang.RuntimeException: Could not get a compiler. at Evaluator.eval(Evaluator.java:33) at Evaluator.main(Evaluator.java:21)
JavaScript
The eval method handles statements and expressions well: <lang javascript> var foo = eval('{value: 42}'); eval('var bar = "Hello, world!";');
typeof foo; // 'object' typeof bar; // 'string' </lang>
Jsish
From Javascript entry. <lang javascript>/* Runtime evaluation, in Jsish */ var foo = eval('{value: 42}'); eval('var bar = "Hello, world!";');
- typeof foo;
- foo.value;
- typeof bar;
- bar;
/*
!EXPECTSTART!
typeof foo ==> object foo.value ==> 42 typeof bar ==> string bar ==> Hello, world!
!EXPECTEND!
- /</lang>
- Output:
prompt$ jsish --U runtimeEvaluation.jsi typeof foo ==> object foo.value ==> 42 typeof bar ==> string bar ==> Hello, world!
Julia
To run an entire script in the current env:
<lang julia>include("myfile.jl")</lang>
To run a string in the current env:
<lang julia>include_string("""
x = sum([1, 2, 3]) @show x """)
@show typeof(x) # Int64</lang>
Kotlin
Kotlin has a REPL which is started from the command line by running the compiler without any parameters.
Any valid Kotlin code can then be entered and immediately evaluated.
Below is a sample session.
$ kotlinc Welcome to Kotlin version 1.2.31 (JRE 1.8.0_162-8u162-b12-0ubuntu0.16.04.2-b12) Type :help for help, :quit for quit >>> 20 + 22 42 >>> 5 * Math.sqrt(81.0) 45.0 >>> fun triple(x: Int) = x * 3 >>> triple(16) 48 >>> :quit
Lasso
"Sourcefile" when executed has access to all variables and other data that would be available in scope to an included file.
This means thread vars ($) and types/methods already defined will be accessible.
Types, methods, traits and thread vars created or modified will maintain state subsequently - i.e. if a type is defined in code executed in a sourcefile context then that type will be available after execution. if a thread var is modified in the sourcefile executed code then the var will maintain that value after execution.
Local variables (#) maintain scope behaviour as normal.
Output is governed by the "autocollect" boolean, the third parameter in the sourcefile invocation.
<lang Lasso>//code, fragment name, autocollect, inplaintext local(mycode = "'Hello world, it is '+date") sourcefile('['+#mycode+']','arbritraty_name', true, true)->invoke
'\r'
var(x = 100)
local(mycode = "Outside Lasso\r['Hello world, var x is '+var(x)]")
// autocollect (3rd param): return any output generated
// inplaintext (4th param): if true, assumes this is mixed Lasso and plain text,
// requires Lasso code to be in square brackets or other supported code block demarkation.
sourcefile(#mycode,'arbritraty_name', true, true)->invoke
'\r'
var(y = 2) local(mycode = "'Hello world, is there output?\r' var(x) *= var(y)") // autocollect (3rd param): as false, no output returned // inplaintext (4th param): as false, assumes this is Lasso code, no mixed-mode Lasso and text. sourcefile(#mycode,'arbritraty_name', false, false)->invoke 'var x is now: '+$x
'\r'
var(z = 3) local(mycode = "var(x) *= var(z)") sourcefile(#mycode,'arbritraty_name', false, false)->invoke 'var x is now: '+$x</lang>
- Output:
Hello world, it is 2013-11-10 15:54:19 Outside Lasso Hello world, var x is 100 var x is now: 200 var x is now: 600
Liberty BASIC
Liberty BASIC has the ability to evaluate arrays using a string for the array name and a variable for the element. <lang lb> 'Dimension a numerical and string array Dim myArray(5) Dim myStringArray$(5)
'Fill both arrays with the appropriate data For i = 0 To 5
myArray(i) = i myStringArray$(i) = "String - " + str$(i)
Next i
'Set two variables with the names of each array numArrayName$ = "myArray" strArrayName$ = "myStringArray"
'Retrieve the array data by evaluating a string 'that correlates to the array For i = 0 To 5
Print Eval$(numArrayName$ + "(" + str$(i) + ")") Print Eval$(strArrayName$ + "$(" + str$(i) + ")")
Next i </lang>
An example using a struct and a pointer. <lang lb> Struct myStruct, value As long, _
string As ptr
myStruct.value.struct = 10 myStruct.string.struct = "Hello World!"
structName$ = "myStruct" numElement$ = "value" strElement$ = "string"
Print Eval$(structName$ + "." + numElement$ + "." + "struct")
'Pay close attention that this is EVAL() because we are 'retrieving the PTR to the string which is essentially a ulong Print Winstring(Eval(structName$ + "." + strElement$ + "." + "struct")) </lang>
Lua
<lang lua>f = loadstring(s) -- load a string as a function. Returns a function.
one = loadstring"return 1" -- one() returns 1
two = loadstring"return ..." -- two() returns the arguments passed to it</lang>
In Lua 5.2 the loadstring
function is superseded by the more general load
function, which can be used in a compatible way. Nevertheless, loadstring
is still available.
<lang lua>f = load("return 42") f() --> returns 42</lang>
In Lua 5.3+ (5.3, 5.4) loadstring
is no longer available, use load
instead.
<lang lua>> f = load("return 42")
> f()
42
> n = load("return 42")()
> n
42</lang>
M2000 Interpreter
<lang M2000 Interpreter> Module checkit {
Module dummy { i++ Print Number } \\ using Stack New { } we open a new stack for values, and old one connected back at the end \\ using block For This {} we erase any new definition, so we erase i (which Local make a new one) a$={ Stack New { For this { Local i for i=1 to 10 : print i : next i } } If valid(k) then print k } i=500 k=600 Push 1000 inline a$ Print i=500 Print Number=1000 \\ eval an expression Print Eval("i+k") \\ eval a function Print Function("{read x : = x**2}", 2)=4 Dim k(10)=123 \\ eval array only Print array("k()", 2)=123 Push 10, 10 \\ call a module by make it inline first inline code dummy, dummy Print i=502
} CheckIt </lang>
Mathematica
Mathematica's ToExpression
evaluates an expression string as if it were placed directly in the code. Statements are just CompoundExpression
s, so they also work. Any evaluation can be limited with TimeConstrained
and MemoryConstrained
.
<lang Mathematica>Print[ToExpression["1 + 1"]];
Print[ToExpression["Print[\"Hello, world!\"]; 10!"]];
x = 5;
Print[ToExpression["x!"]];
Print[ToExpression["Module[{x = 8}, x!]"]];
Print[MemoryConstrained[ToExpression["Range[5]"], 10000, {}]];
Print[MemoryConstrained[ToExpression["Range[10^5]"], 10000, {}]];
Print[TimeConstrained[ToExpression["Pause[1]; True"], 2, False]];
Print[TimeConstrained[ToExpression["Pause[60]; True"], 2, False]];</lang>
- Output:
2 Hello, world! 3628800 120 40320 {1, 2, 3, 4, 5} {} True False
MATLAB
The eval and evalin functions handles any kind of code. It can handle multi-line code, although it needs the lines to be separated by the newline character. It can even allow you to program at runtime, as illustrated in the last example in the code and output below. Errors can occur when mixing eval statements with regular code, especially "compile-time" errors if the code appears to be missing key elements (ending brackets or end statements, etc). Some of these are also demonstrated. <lang MATLAB>function testEval
fprintf('Expressions:\n') x = eval('5+10^2') eval('y = (x-100).*[1 2 3]') eval('z = strcat(my, string)') try w eval(' = 45') catch fprintf('Runtime error: interpretation of w is a function\n\n') end % eval('v') = 5 % Invalid at compile-time as MATLAB interprets as using eval as a variable fprintf('Other Statements:\n') nl = sprintf('\n'); eval(['for k = 1:20' nl ... 'fprintf(%.3f\n, k)' nl ... 'if k == 3' nl ... 'break' nl ... 'end' nl ... 'end']) true == eval('1') try true eval(' == 1') catch fprintf('Runtime error: interpretation of == 1 is of input to true\n\n') end fprintf('Programming on the fly:\n') userIn = true; codeBlock = ; while userIn userIn = input('Enter next line of code: ', 's'); codeBlock = [codeBlock nl userIn]; end eval(codeBlock)
end</lang>
- Output:
Expressions: x = 105 y = 5 10 15 z = my string Runtime error: interpretation of w is a function Other Statements: 1.000 2.000 3.000 ans = 1 Runtime error: interpretation of == 1 is of input to true Programming on the fly: Enter next line of code: fprintf('Goodbye, World!\n') Enter next line of code: str = 'Ice and Fire'; Enter next line of code: words = textscan(str, '%s'); Enter next line of code: fprintf('%s ', words{1}{end:-1:1}) Enter next line of code: Goodbye, World! Fire and Ice
Maxima
<lang maxima>/* Here is how to create a function and return a value at runtime. In the first example, the function is made global, i.e. it still exists after the statement is run. In the second example, the function is declared local. The evaluated string may read or write any variable defined before eval_string is run. */
kill(f)$
eval_string("block(f(x) := x^2 + 1, f(2))"); 5
fundef(f); /* f(x) := x^2 + 1 */
eval_string("block([f], local(f), f(x) := x^3 + 1, f(2))"); 9
fundef(f); /* f(x) := x^2 + 1 */</lang>
Nanoquery
The exec() function runs code contained within a string as if it were being read from a file, so any valid code may be run. <lang nanoquery>exec("println \"hello, world\"") exec("a = 1\nif a = 1\nprintln \"a is 1\"\nend\nprintln \"test\"")</lang>
- Output:
hello, world a is 1 test
Oforth
Oforth can evaluate strings at runtime.
In order to restrict evaluation, perform is used on strings. With perform, only objects can be evaluated. If a function or a method is included into the string an exception is raised and the function is not evaluated.
<lang Oforth>"[ [ $a, 12], [$b, 1.2], [ $c, [ $aaa, $bbb, $ccc ] ], [ $torun, #first ] ]" perform .s [1] (List) [[a, 12], [b, 1.2], [c, [aaa, bbb, ccc]], [torun, #first]]
"12 13 +" perform [1:interpreter] ExCompiler : Can't evaluate <+></lang>
In order to evaluate any Oforth code, eval can be used. This method should not be used on unsafe strings.
<lang Oforth>"12 13 + println" eval 25 ": newFunction(a) a + ; 12 10 newFunction println" eval 22</lang>
ooRexx
The ooRexx INTERPRET instruction allows execution of dynamically constructed code. Almost any well-formed code can be executed dynamically, including multiple instructions at a time. The instructions are executed in the local context where the interpret instruction executes, so full access to the current variable context is available. For example: <lang ooRexx>
a = .array~of(1, 2, 3) ins = "loop num over a; say num; end" interpret ins
</lang> Executes the LOOP instruction, displaying the contents of the array pointed to by variable A.
OxygenBasic
Runtime (secondary) compiling is possible, with some restrictions. For instance, static variables may not be created by the compiled code, but parental variables are visible to it. This demo produces tables of Y values, given a formula, and a range of X values to step through.
<lang oxygenbasic>
function ExecSeries(string s,double b,e,i) as string '=================================================== ' sys a,p string v,u,tab,cr,er ' 'PREPARE OUTPUT BUFFER ' p=1 cr=chr(13) chr(10) tab=chr(9) v=nuls 4096 mid v,p,s+cr+cr p+=4+len s ' double x,y,z 'shared variables ' 'COMPILE ' a=compile s er=error if er then print "runtime error: " er : exit function end if ' 'EXECUTE ' for x=b to e step i if p+128>=len v then v+=nuls len(v) 'extend buffer end if call a u=str(x) tab str(y) cr mid v,p,u : p+=len u next ' freememory a 'release compiled code ' return left v,p-1 'results ' end function
'===== 'TESTS '=====
'Expression, StartVal, EndVal stepVal, Increment
print ExecSeries "y=x*x*x", 1, 10, 1 print ExecSeries "y=sqrt x",1, 9 , 1
</lang>
Oz
<lang oz>declare
%% simplest case: just evaluate expressions without bindings R1 = {Compiler.virtualStringToValue "{Abs ~42}"} {Show R1}
%% eval expressions with additional bindings and %% the possibility to kill the evaluation by calling KillProc KillProc R2 = {Compiler.evalExpression "{Abs A}" unit('A':~42) ?KillProc} {Show R2}
%% full control: add and remove bindings, eval expressions or %% statements, set compiler switches etc. Engine = {New Compiler.engine init} {Engine enqueue(setSwitch(expression false))} %% statements instead of expr. {Engine enqueue(mergeEnv(env('A':42 'System':System)))} {Engine enqueue(feedVirtualString("{System.show A}"))}</lang>
By restricting the environment it is possible to restrict what kind of programs can be run.
PARI/GP
Since GP is usually run from the REPL gp, it is trivial to evaluate programs at runtime (most are run this way). Slightly less trivial is passing code around as a first-class object:
<lang parigp>runme(f)={
f()
};
runme( ()->print("Hello world!") )</lang>
One facility designed for restricting such embedded programs is default(secure,1)
which denies scripts the ability to run system
and extern
. This cannot be turned off except interactively.
Perl
The eval
function accepts a block or a string as its argument. The difference is that a block is parsed at compile-time, whereas a string is parsed at runtime. The block or string may represent any valid Perl program, including a single expression. The subprogram executes in the same lexical and dynamic scope as the surrounding code. The return value of a call to eval
depends on how the subprogram terminates:
- If control reaches the end of the subprogram,
eval
returns the value of the last expression evaluated. - If the subprogram uses an explicit
return
,eval
returns the given value. - If the subprogram throws an exception,
eval
returnsundef
. The text of the exception is assigned to$@
. (When the subprogram terminates without an exception,$@
is set to the null string instead.)
<lang perl>my ($a, $b) = (-5, 7); $ans = eval 'abs($a * $b)'; # => 35</lang>
PHP
The eval construct allow string evaluation as PHP code. Opening and closing tags are not required. Return statements immediatly terminates evaluation . Eval returns NULL, unless return is called in evalued code. <lang php> <?php
$code = 'echo "hello world"'; eval($code); $code = 'return "hello world"'; print eval($code);
</lang>
PicoLisp
In PicoLisp there is a formal equivalence of code and data. Almost any piece of data is potentially executable. PicoLisp has three internal data types: Numbers, symbols and lists. Though in certain contexts (e.g. GUI objects) also atomic data (numbers and symbols) are evaluated as code entities, a typical executable item is a list.
The PicoLisp reference distinguishes between two terms: An 'exe' (expression) is an executable list, with a function as the first element, followed by arguments. A 'prg' (program) is a list of 'exe's, to be executed sequentially.
'exe's and 'prg's are implicit in the whole runtime system. For example, the body of a function is a 'prg', the "true" branch of an 'if' call is an 'exe', while the "false" branch again is a 'prg'.
For explicit execution, an 'exe' can be evaluated by passing it to the function 'eval', while a 'prg' can be handled by 'run'.
As PicoLisp uses exclusively dynamic binding, any 'exe' or 'prg' can be executed in arbitrary contexts. The environmet can be controlled in any conceivable way, through implicit function parameter bindings, or explicitly with the aid of functions like 'bind', 'let' or 'job'.
Pike
Pike provides compile_string()
and compile_file()
which can compile code into a class that can be instantiated:
<lang Pike>program demo = compile_string(#"
string name=\"demo\"; string hello() { return(\"hello, i am \"+name); }");
demo()->hello(); Result: "hello, i am demo"</lang>
an actual application of this is shown in Simple database.
PowerShell
Evaluate an expression: <lang PowerShell> $test2plus2 = '2 + 2 -eq 4' Invoke-Expression $test2plus2 </lang>
- Output:
True
Evaluate a [scriptblock]
(a statement or group of statements) with code surrounded by curly braces using the & (call) operator:
<lang PowerShell>
$say = {"Hello, world!"}
& $say
</lang>
- Output:
Hello, world!
Scriptblocks behave just as functions so they may have parameters: <lang PowerShell> $say = {param ([string]$Subject) "Hello, $Subject!"} & $say -Subject "my friend" </lang>
- Output:
Hello, my friend!
A slightly more complex example: <lang PowerShell> $say = {param ([string]$Exclamation, [string]$Subject) "$Exclamation, $Subject!"} & $say -Exclamation "Goodbye" -Subject "cruel world" </lang>
- Output:
Goodbye, cruel world!
To reverse the normal behaviour of a [scriptblock]
use the GetNewClosure method. This makes the scriptblock self-contained or closed; ie, the variable will only be read when the scriptblock is initialised:
<lang PowerShell>
$title = "Dong Work For Yuda"
$scriptblock = {$title}
$closedScriptblock = $scriptblock.GetNewClosure()
& $scriptblock & $closedScriptblock </lang>
- Output:
Dong Work For Yuda Dong Work For Yuda
Change the variable and execute the scriptblock, the closed version will not reflect the change: <lang PowerShell> $title = "I'm Too Sexy" & $scriptblock & $closedScriptblock </lang>
- Output:
I'm Too Sexy Dong Work For Yuda
Since the [scriptblock] type is an anonymous function, the Begin {}, Process {} and End {} blocks may be added to a scriptblock, just like any function.
Python
The exec statement allows the optional passing in of global and local names via mappings (See the link for full syntax). The example below shows exec being used to parse and execute a string containing two statements:
<lang python>>>> exec x = sum([1,2,3,4]) print x 10</lang>
Note that in Python 3.x exec is a function:
<lang python>>>> exec( x = sum([1,2,3,4]) print(x) ) 10</lang>
R
In R, expressions may be manipulated directly as abstract syntax trees, and evaluated within environments.
quote() captures the abstract syntax tree of an expression. parse() does the same starting from a string. call() constructs an evaluable parse tree. Thus all these three are equivalent.
<lang r>expr1 <- quote(a+b*c) expr2 <- parse(text="a+b*c")1 expr3 <- call("+", quote(`a`), call("*", quote(`b`), quote(`c`)))</lang>
eval() evaluates a quoted expression. evalq() is a version of eval() which quotes its first argument.
<lang r>> a <- 1; b <- 2; c <- 3 > eval(expr1) [1] 7</lang>
eval() has an optional second environment which is the lexical environment to evaluate in.
<lang r>> env <- as.environment(list(a=1, b=3, c=2)) > evalq(a, env) [1] 1 > eval(expr1, env) #this fails; env has only emptyenv() as a parent so can't find "+" Error in eval(expr, envir, enclos) : could not find function "+" > parent.env(env) <- sys.frame() > eval(expr1, env) # eval in env, enclosed in the current context [1] 7 > assign("b", 5, env) # assign() can assign into environments > eval(expr1, env) [1] 11</lang>
Racket
Racket has the usual eval that is demonstrated here, and in addition, it has a sandbox environment that provides a safe evaluator that is restricted from accessing files, network, etc.
<lang Racket>
- lang racket
(require racket/sandbox) (define e (make-evaluator 'racket)) (e '(define + *)) (e '(+ 10 20)) (+ 10 20)
- (e '(delete-file "/etc/passwd"))
- --> delete-file
- `delete' access denied for /etc/passwd
</lang>
And, of course, both of these methods can use Racket's multilingual capabilities and evaluate the code in a language with different semantics.
Raku
(formerly Perl 6) Any syntactically valid sequence of statements may be run, and the snippet to be run can see its outer lexical scope at the point of the eval: <lang perl6>use MONKEY-SEE-NO-EVAL;
my ($a, $b) = (-5, 7); my $ans = EVAL 'abs($a * $b)'; # => 35</lang> Unlike in Perl 5, eval in Raku only compiles and executes the string, but does not trap exceptions. You must say try eval to get that behavior (or supply a CATCH block within the text to be evaluated).
REBOL
The do function evaluates a script file or a series of expressions and returns a result.
It performs the fundamental interpretive action of the Rebol language and is used internally within many other functions such as if, case, while, loop, repeat, foreach, and others. <lang rebol>a: -5 b: 7 answer: do [abs a * b] ; => 35</lang>
REXX
This REXX program does a:
- run-time evaluation of an internal expression, and
- run-time evaluation of a user-prompted expression.
<lang rexx>/*REXX program illustrates the ability to execute code entered at runtime (from C.L.)*/ numeric digits 10000000 /*ten million digits should do it. */ bee=51 stuff= 'bee=min(-2,44); say 13*2 "[from inside the box.]"; abc=abs(bee)' interpret stuff say 'bee=' bee say 'abc=' abc say
/* [↓] now, we hear from the user. */
say 'enter an expression:' pull expression say say 'expression entered is:' expression say
interpret '?='expression
say 'length of result='length(?)
say ' left 50 bytes of result='left(?,50)"···"
say 'right 50 bytes of result=···'right(?, 50) /*stick a fork in it, we're all done. */</lang>
output when using the input: 2**44497 - 1
which happens to be the 27th Mersenne prime.
26 [from inside the box.] bee= -2 abc= 2 enter an expression: 2**44497 - 1 expression entered is: 2**44497 - 1 length of result=13395 left 50 bytes of result=85450982430363380319330070531840303650990159130402··· right 50 bytes of result=···22977396345497637789562340536844867686961011228671
Ring
<lang ring> Eval("nOutput = 5+2*5 " ) See "5+2*5 = " + nOutput + nl Eval("for x = 1 to 10 see x + nl next") Eval("func test see 'message from test!' ") test() </lang> Output : <lang ring> 5+2*5 = 15 1 2 3 4 5 6 7 8 9 10 message from test! </lang>
We can create simple interactive programming environment using the next program <lang ring> while true
see nl + "code:> " give cCode try eval(cCode) catch see cCatchError done
end </lang>
Output <lang ring> code:> see "hello world" hello world code:> for x = 1 to 10 see x + nl next 1 2 3 4 5 6 7 8 9 10
code:> func test see "Hello from test" + nl
code:> test() Hello from test
code:> bye </lang>
Ruby
The eval method evaluates a string as code and returns the resulting object. With one argument, it evaluates in the current context: <lang ruby>a, b = 5, -7 ans = eval "(a * b).abs" # => 35</lang>
With two arguments, eval runs in the given Binding or Proc context: <lang ruby>def first(main_var, main_binding)
foo = 42 second [[main_var, main_binding], ["foo", binding]]
end
def second(args)
sqr = lambda {|x| x**2} deref(args << ["sqr", binding])
end
def deref(stuff)
stuff.each do |varname, context| puts "value of #{varname} is #{eval varname, context}" end
end
hello = "world" first "hello", binding</lang>
value of hello is world value of foo is 42 value of sqr is #<Proc:0x1002ef6c@eval.rb:7>
Scheme
In Scheme, the expression passed to eval is evaluated in the current interaction environment, unless otherwise specified. The result is read back as a Scheme value.
<lang scheme>> (define x 37) > (eval '(+ x 5)) 42 > (eval '(+ x 5) (interaction-environment)) 42 > (eval '(+ x 5) (scheme-report-environment 5)) ;; provides R5RS definitions
Error: identifier not visible x. Type (debug) to enter the debugger. > (display (eval (read))) (+ 4 5) ;; this is input from the user. 9</lang>
Sidef
The eval method evaluates a string as code and returns the resulting object.
<lang ruby>var (a, b) = (-5, 7); say eval '(a * b).abs'; # => 35 say (a * b -> abs); # => 35</lang>
Slate
In Slate, programs are represented as Syntax Node trees, with methods defined on the various syntactic types. The backtick syntax provides a convenient quoting mechanism, and as objects, they have convenient methods defined for evaluation or evaluation within a specific environment: <lang slate>`(4 + 5) evaluate. `(4 + 5) evaluateIn: prototypes.</lang>
You can also explicitly invoke the Parser on a String, to convert it into syntactic objects: <lang slate>(Syntax Parser newOn: '4 + 5') upToEnd do: [| :each | print: each evaluate]</lang>
You can construct a program using externally-specified values using `unquote within a quoted expression: <lang slate>define: #x -> 4. `(x `unquote + 5) evaluate.</lang>
Or you can obviously construct a string: <lang slate>define: #x -> 4. (Syntax Parser newOn: x printString ; ' + 5')</lang>
The evaluate method also takes into consideration the current lexical scope, unless another environment is specified. The following returns 10, no matter what binding x has in the local namespace: <lang slate>define: #x -> 4. [| x | x: 5. `(x `unquote + 5) evaluate] do.</lang>
Slate can sandbox via constructing a fresh namespace and evaluating within it, but this mechanism is not strongly secure yet.
Smalltalk
<lang smalltalk>[ 4 + 5 ] value.</lang> Evaluating an expression without bindings: <lang smalltalk>e := ' 123 degreesToRadians sin '. Transcript show: (Compiler evaluate: e) .</lang> To get local bindings (x, y), evaluate an expression which yields a block given as a string, then call the resulting block: <lang smalltalk>e := '[ :x :y | (x*x + (y*y)) sqrt ]'. Transcript show: ((Compiler evaluate: e) value: 3 value: 4).</lang> this could be wrapped into a utility, which expects the names to bind as argument, if required.
SNOBOL4
The built in function eval() evaluates SNOBOL4 expressions and returns the value. The expression is evaluated in the current environment and has access to then-current variables.
<lang snobol4> expression = "' page ' (i + 1)"
i = 7 output = eval(expression)
end</lang>
- Output:
page 8
The built in function code() compiles complete SNOBOL4 source statements, or even complete programs. The compiled program is returned (as a value of type CODE), and when executed the program is executed in the then-current environment and has access to the then-current variables. Labels in the compiled program are added to the current program. Programs of type CODE are executed by a variant of the goto clause:
<lang SNOBOL4> compiled = code(' output = "Hello, world."') :s<compiled> end</lang>
When passing programs to code(), semicolons are used to separate lines.
The calling (already-compiled) program can call, for example, functions that are defined in the code compiled at runtime, and can include gotos to labels only defined in the code compiled at runtime. Likewise, the code compiled at runtime has access to not just variables, but also files, functions, etc., that are in the already-compiled program.
Sparkling
In Sparkling, the standard library provides functions to compile expressions and statements into functions. Each such function is considered a different top-level program, running in the execution context of it's "parent" program (i. e. the piece of code from within which it was created). Consequently, functions compiled at runtime share their environment (e. g. all globals) with their parent program.
Compiled expressions and statements can take arbitrary arguments and return values to the caller. As with any function, the expression or statement being compiled can refer to its arguments using the # prefix operator.
An expression always "returns" a value (i. e. evaluates to one) to the caller. Basically, compiling an expression is semantically (and syntactically) equivalent with creating a function with no declared arguments of which the body consists of a single return statement, returning the expression.
Evaluating expressions
Simple
<lang sparkling>let fn = exprtofn("13 + 37"); fn() // -> 50</lang>
With arguments
<lang sparkling>let fn = exprtofn("#0 * #1"); fn(3, 4) // -> 12</lang>
Evaluating statements
<lang sparkling>let fn = compile("for (var i = 0; i < 10; i++) { print(i); }"); fn(); // result: 0 1 2 3 4 5 6 7 8 9</lang>
Tcl
Simple Evaluation
Evaluation in the current interpreter: <lang tcl>set four 4 set result1 [eval "expr {$four + 5}"] ;# string input
set result2 [eval [list expr [list $four + 5]]] ;# list input</lang>
Evaluation in a restricted context
Tcl handles sandboxing by creating new interpreters. Each interpreter is strongly isolated from all other interpreters except in that the interpreter that creates a sub-interpreter retains management control over that “slave” interpreter. The exact capabilities exposed in the slave are controlled by what commands exist in it; commands in the slave may be aliases for other commands in the master interpreter, which allows for trapping into a more highly authorized context (which can be considered analogous to a system call to an OS kernel). <lang tcl># Create an interpreter with a default set of restrictions interp create -safe restrictedContext
- Our secret variable
set v "secret"
- Allow some guarded access to the secret from the restricted context.
interp alias restrictedContext doubleSecret {} example proc example {} {
global v lappend v $v return [llength $v]
}
- Evaluate a script in the restricted context
puts [restrictedContext eval {
append v " has been leaked" catch {file delete yourCriticalFile.txt} ;# Will be denied! return "there are [doubleSecret] words in the secret: the magic number is [expr {4 + 5}]"
}]; # --> there are 2 words in the secret: the magic number is 9 puts $v; # --> secret secret</lang> As can be seen, the result of the overall evaluation is the same as the result of the evaluation in the slave.
Note that with providing values to the restricted context, it is normal to do this by providing an alias/trap command in the restricted context to allow the script to pick up the value when it wants it. Although the value could also have been provided by setting a variable in the restricted context, this is fairly unusual in practice. The example above shows how this might be done with the result of the doubleSecret
command.
Evaluation within limits
Even stronger protection of the master interpreter is available from Tcl 8.5 onwards through the setting of resource limits on the slaves. These allow the master to prevent the evaluated script from going berserk: <lang tcl>set i [interp create] interp limit $i commands -value [expr [$i eval info cmdcount]+20] -granularity 1 interp eval $i {
set x 0 while {1} { # Infinite loop! Bwahahahaha! puts "Counting up... [incr x]" }
}</lang>
- Output:
(the last line is an error message)
Counting up... 1 Counting up... 2 Counting up... 3 Counting up... 4 Counting up... 5 Counting up... 6 Counting up... 7 Counting up... 8 Counting up... 9 Counting up... 10 command count limit exceeded
TI-89 BASIC
The function expr(string)
evaluates the string as an expression. It is evaluated in the environment of the calling program (it can see local variables).
The Exec string, args...
statement executes arbitrary 68k machine code (and is thus entirely unsafe).
TODO: Is there a way to execute statements as well as evaluate expressions?
UNIX Shell
eval is the command to use: <lang bash>$ a=42 $ b=a $ eval "echo \$$b" 42</lang>
Ursa
The eval statement in Ursa takes a string and evaluates it as a command, redirecting the console to the specified I/O device. <lang ursa># writes hello world to the console eval "out \"hello world\" endl console" console </lang>
zkl
In zkl, the compiler is part of the language and compiling a chunk of code returns an executable (which how the REPL works), so <lang zkl>Compiler.Compiler.compileText(
"fcn f(text){text.len()}").f("foobar")
//-->6</lang> All language constructs are allowed, the only sand boxing is the new code can only touch global resources or items explicitly passed in ("foobar" in the example).
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