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45. is_defined


Indicate whether a variable or function defined.


Integer_Type is_defined (String_Type obj)


This function is used to determine whether or not a function or variable whose name is obj has been defined. If obj is not defined, the function returns 0. Otherwise, it returns a non-zero value that defpends on the type of object obj represents. Specifically, it returns one of the following values:

     +1 if an intrinsic function
     +2 if user defined function
     -1 if intrinsic variable
     -2 if user defined variable
      0 if undefined

For example, consider the function:

    define runhooks (hook)
       if (2 == is_defined(hook)) eval(hook);
This function could be called from another S-lang function to allow customization of that function, e.g., if the function represents a mode, the hook could be called to setup keybindings for the mode.
See Also

typeof, eval, autoload

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