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Version: 1.3.0

#define

The booleans false and true are predefined. The way to define symbols (that is the traditional name of those identifiers) consists in using the #define directive, followed by the symbol, like so:

#define SYM
#if SYM
This IS copied to the output.
#else
This is NOT copied to the output, except the newline character.
#endif

This opens the possibility to use Boolean expressions made of

  • true and false already mentioned;
  • || for the disjunction ("or");
  • && for the conjunction ("and");
  • == for equality;
  • != for inequality;
  • ! for negation;
  • ( and ) around expressions to specify priorities.

Directives are processed in sequence in the input file. This preprocessor, like that of C#, allows us to undefine a symbol, that is, giving it the Boolean value false, like so:

#define SYM
#undef SYM
#if SYM
This is NOT copied to the output, except the newline character.
#else
This IS copied to the output.
#endif

The result is

# 1 "Tests/undef.txt"
This IS copied to the output.

Note: If you wish to redefine a symbol, you must undefine it first.

When we want to write a cascade of conditionals, the preprocessor enables a shortcut by means of the #elif directive, like so:

#if ...
...
#elif ...
...
#elif ...
...
#endif

Basically, a #elif directive is equivalent to #else followed by #if, but we only need to close with only one #endif.

The rationale for using conditional directives in LIGO is to enable in a single smart contract several versions of a standard.

#if STANDARD_1
...
#elif STANDARD_2
...
#else
#error Standard not implemented
#endif

A real life example could be Dexter. It provides another interesting use of a conditional directive, where a record type depends on the version of the standard.