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

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zhak



Joined: 12 Apr 2005
Posts: 490
Location: Belarus
zhak
Tomasz, in your new instruction set macros, I saw
Code:
if ~ defined x
restore x
define x
    

but I don't seem to understand how it works.

`if ~ defined x` checks whether a symbol is NOT defined anywhere in code (either backwards or forward referenced)
then `restore x` is to undefine x. But why restore if it is not yet defined?
As I see it, it should be
Code:
if defined x
restore x
define x
    

What am I missing?
Post 13 Feb 2017, 19:02
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7756
Location: Kraków, Poland
Tomasz Grysztar
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.
Post 13 Feb 2017, 19:21
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