Recap

I originally wrote to my friend Tim and asked this question:

Have you ever thought of how this works?

<%
var a = {"qweqwe", "asdasd"}, i;
for (i in a)
{
%>
<%= i %> <%= a[i] %>
<%
}
%>

I can't see how it could be the case that the host of this code passes over the first part (up to the for's opening brace) to the scripting engine, then the two variable prints, then the closing brace. For one thing, how would the script host (ASP, XCODE, etc.) regain control at the print statements when the script language returns to the top of the loop?

I can see how this might be altered by the host to this: ...everything up to '{'
Response.Write (i);
Response.Write (" ");
Response.Write (a[i]);
Response.Write ("\n");
}
and then the translated block is sent to the scripting engine.

Put it this way: how could it work for the scripting language to stop and start (as the code is written)? I think that the host has control and translates the text, the scripting engine executes the translated text, and the host emits the text as written by the scripting engine.

This led to a later formulation of the problem:

There are many problems, BTW, in doing inclusion as an operation within the language — an include file does not have to be a complete translation unit, it can call methods that appear in later include files, etc. These observations about complete translation units underline why I don't think that you can change languages inside a block written in a different scripting language. The scripting engine needs to work on a complete block in its language. And that's why I think that this is very odd:

 <% for (i = 1; i <= 10; i++) 
    { %>
Tim has <%= i %> big warts.
 <% } %>

To parse this, a scripting engine needs a completely syntactic statement. The host can't pass the engine "for (i = 0; i < 10; i++) {" and then expect that the engine will execute it once, pass control back to the host, back and forth ten times and then the host knows that the scripting engine is done. I think that the host has to translate this immediately into an equivalent block that uses printf-like function calls for the plain text output, like this:

<% for (i = 1; i <= 10; i++) 
   { 
Response.Write ("Tim has ");
Response.Write (i);
Response.Write (" big warts.");
   } %>

But I have problems even with that because then the host has to know how to write the method call, and the main problem is that VBScript and JavaScript (and probably every other language) have different rules for terminating the statement and quoting text and other syntactic issues.

And this ended with remark:

I'd be interested to see how ASP really works.

© 2001-8 Hugh Brown. All rights reserved.