We are all familiar with the working of macros in languages like C. There are certain situations in which macro expansions can lead to undesirable results because of accidental capture of identifiers.
The code is actually equivalent to:
x = 4, y = 9
The variable a declared in the scope of the main function is overshadowed by the variable a in the macro definition so a = 4 never gets updated (known as accidental capture).
Hygienic macros are macros whose expansion is guaranteed not to cause the accidental capture of identifiers. A hygienic macro doesn’t use variable names that can risk interfering with the code under expansion.
The situation in the above code can be avoided simply by changing the name of the variable in the macro definition, which will produce a different output.
x = 5, y = 9
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