KEYBOARD Initiate Interactive Debug Session

Section: Flow Control

Usage

The keyboard statement is used to initiate an interactive session at a specific point in a function. The general syntax for the keyboard statement is 
   keyboard

A keyboard statement can be issued in a script, in a function, or from within another keyboard session. The result of a keyboard statement is that execution of the program is halted, and you are given a prompt of the form: 

 [scope,n] -->

where scope is the current scope of execution (either the name of the function we are executing, or base otherwise). And n is the depth of the keyboard session. If, for example, we are in a keyboard session, and we call a function that issues another keyboard session, the depth of that second session will be one higher. Put another way, n is the number of return statements you have to issue to get back to the base workspace. Incidentally, a return is how you exit the keyboard session and resume execution of the program from where it left off. A retall can be used to shortcut execution and return to the base workspace. The keyboard statement is an excellent tool for debugging FreeMat code, and along with eval provide a unique set of capabilities not usually found in compiled environments. Indeed, the keyboard statement is equivalent to a debugger breakpoint in more traditional environments, but with significantly more inspection power.

Example

Here we demonstrate a two-level keyboard situation. We have a simple function that calls keyboard internally: 

     key_one.m
function c = key_one(a,b)
c = a + b;
keyboard

Now, we execute the function from the base workspace, and at the keyboard prompt, we call it again. This action puts us at depth 2. We can confirm that we are in the second invocation of the function by examining the arguments. We then issue two return statements to return to the base workspace. 

--> key_one(1,2)
[key_one,3]--> key_one(5,7)
[key_one,3]--> a

ans = 
 5 

[key_one,3]--> b

ans = 
 7 

[key_one,3]--> c

ans = 
 12 

[key_one,3]--> return

ans = 
 12 

[key_one,3]--> a

ans = 
 1 

[key_one,3]--> b

ans = 
 2 

[key_one,3]--> c

ans = 
 3 

[key_one,3]--> return

ans = 
 3