# Macros¶

Felix only provides a rudimentary macro system. Macros are scoped like other symbols. The primary purpose of macros is to support conditional compilation. Macros are usually gathered into a single file which is prepended using a compiler switch to all other files, after parsing.

For advanced synthetic code generation, Felix uses Scheme programming language in user action codes of the grammar.

## Syntax¶

stmt := "macro" "val" snames "=" sexpr ";"

stmt := "forall" sname "in" sexpr "do" stmt* "done"

x[sthename_pri] := "noexpand" squalified_name


## Semantics¶

The macro val statement is used to associate a macro name with an expression.

The forall statement is used to generate a sequence of statements repeatedly replacing occurences of the given name with each of the given expression in turn, and is useful for table generation.

The noexpand prefix is used to prevent expansion of a name which might have been a macro val. It is stripped out by the macro processor.

## Constant Folding¶

The Felix macro processor performs constant folding for certain types and operations.

Expressions involving only boolean constants true and false and the logical operations and, or and not are evaluated and simplified.

Adjacent string literals are concatenated.

## Conditional Compilation¶

Felix does not provide any specific directives for conditional compilation. Instead, conditional expressions and statements with compile time constant conditions are simplified by the macro processor, providing conditional compilation using the same syntax as run time conditionals.

For example:

macro val POSIX = true;
if POSIX do
typedef file = int;
else do
typedef file = HANDLE;
done


is reduced to:

typedef file = int;


by the macro processor, so that even if HANDLE is not defined, there will be no type error because the code using it is eliminated.

The macro processor also reduces pattern matches if possible, in fact, conditionals are represented by pattern matches!