α-wwwiki

α-wwwiki :: history/dipert/20130624-111242.txt

editor : alpha [83.158.129.196] 2013/06/24 11:12:42
_h2 15 lines (see also [[evaluator]])

_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};
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( eval( evaluate( func_args[i] ) ) );
  if (operators.hasOwnProperty(func_name))
    return compiled_args.join(func_name);
  else // it's a JS math function
    return func_name + '(' + compiled_args.join(',') + ')';
}
function do_evaluate(input) { return eval(evaluate(eval(input))); }
// LISP CODE END

// user JS 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]] -> 25
['*', ['+',3,3], ['+',4,4]] -> 48
['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( eval( 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))); }

// user JS 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 ); }
// ...
°°}

_h3 next step
_p lambdas, defines, ...