don't understand "if ~ defined x / restore x / define x"

zhak wrote:

`if ~ defined x` checks whether a symbol is NOT defined anywhere in code (either backwards or forward referenced

This is a crucial part. If a symbol is a variable symbol and cannot be forward referenced, it can only be detected through backward referencing, that is when it is defined earlier in the source.

RESTORE is a "nop" when a symbol has not yet been assigned any value, but "touching" a symbol with RESTORE forces it to become a variable symbol, ensuring that it cannot be forward-referenced. So you should interpret that "restore x" line as meaning "treat x as a variable (and disallow forward-referencing it)".

To make this snippet clearer you could make a macro with a name suggesting the purpose of this construction:

Code:

macro var? name
    define name 
    restore name
end macro

if ~ defined x
    var x
    define x
end if

The DEFINE is in the macro to ensure that it does not accidentally destroy any previous value of a symbol. This is similar to the macro I used in my text about namespace separation.