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.




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