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editor : alpha [82.248.89.116] 2013/06/23 05:16:48
_h2 15 lines for a very small lisp

_p Following [[psil2]], this is a very very small lisp (s-expressionss) interpreter written in 15 lines. The code is adapted from [[Alan Dipert|http://alan.dipert.org/post/7193534410/discover-lisp-in-your-web-browser-with-javascript]] who gives a very clean and clear explanation about the lisp fundamental principle. In this tiny code, there are no defines, no lambdas, nothing but the 4 basic operators "+,-,*,/", the Javascript Math functions ... and any user function predefined in Javascript. From this point of view, it's endless, powerful and evil (because eval). You may :

_ul copy a line from the examples,
_ul paste it into the field below,
_ul and click on the button "evaluate".
 
{input {@ 
  id="input" 
  type="texte" 
  value="['Math.sqrt', ['+', ['*',3,3], ['*',4,4]]]" 
  style="width:100%; font:normal 1em courier; color:red;"
}}

{input {@ type="submit" value="evaluate" onclick="°°
// LISP CODE START :
var operators = {'+':0, '-':0, '*':0, '/':0}; // the basic dictionary
var evaluate = function(expression) { // ['*',1,2,3,4,5,6] , ['Math.sqrt', 2] or ['+', ['*',3,3], ['*',4,4]]
  if (!Array.isArray(expression)) 
    return expression.toString();								
  var func_name = expression[0]; // * , Math.sqrt
  var func_args = expression.slice(1); // [1, 2, 3, 4, 5, 6] , or [2]
  var compiled_args = [];
  for (var i=0; i< func_args.length; i++)
    compiled_args.push( evaluate( func_args[i] ) ); // ['1', '2', '3', '4', '5', '6'] or ['2'] , ['3*3', '4*4']
  if (operators.hasOwnProperty(func_name)) // it's in the dictionary
    return compiled_args.join(func_name);  // 1*2*3*4*5*6
  else // it's a JS math function
    return func_name + '(' + compiled_args.join(',') + ')'; // Math.sqrt(2)
}
function do_evaluate(input) { return eval(evaluate(eval(input))); }
// LISP CODE END

// user functions can be called 
function fac (n) { return (n< 2)? 1 : n*fac(n-1); }
function hypo (a,b) { return Math.sqrt( a*a + b*b ); }
// ...
getId('output').innerHTML = do_evaluate(getId('input').value);
°°"}}
{pre {@ id="output"}}
_h3 some examples to copy/paste
{pre 
['+',1,2,3,4,5,6]
['*',1,2,3,4,5,6]
['+', ['*',3,3], ['*',4,4]]
['Math.sqrt', 2]
['Math.pow',2,8]
['Math.sqrt', ['+', ['*',3,3], ['*',4,4]]]
['hypo',3,4]
['fac',6]
...
and so on.
}

_h3 lisp code
_p Here are the 15 lines of Javascript :
{pre °°
var operators = {'+':0, '-':0, '*':0, '/':0};
var evaluate = function(expression) { 
  if (!Array.isArray(expression)) 
    return expression.toString();								
  var func_name = expression.shift();
  var func_args = expression;
  var compiled_args = [];
  for (var i=0; i< func_args.length; i++)
    compiled_args.push( evaluate( func_args[i] ) );
  if (operators.hasOwnProperty(func_name))
    return compiled_args.join(func_name);
  else
    return func_name + '(' + compiled_args.join(',') + ')';
}
function do_evaluate(input) { return eval(evaluate(eval(input))); }
°°}

_h3 bug
{pre
1) ['+', ['*',3,3], ['*',4,4]] gives the good result : 25
2) ['*', ['+',3,3], ['+',4,4]] gives a bad result : 19 and not 48
because :
1) computes (3*3)+(4*4) = 9+16 = 25
2° computes 3+(3*4)+4 = 3+12+4 = 19 and not (3+3*(4+4) = 48

Some work to do ...
}