Macros¶

As with other lisps, Disp supports macros. Macros are evaluated compile time and resolve to an ast that is then evaluated by the compiler.

For example, one could create something equivalent to an ‘unless’ keyword by writing:

macro! unless [conditional] body
        ' while (not conditional) body

This will expand:

unless (eq i x)
        print i
        mut i (+ i x)

into:

while (not (eq i x))
        print i
        mut i (+ i x)

Design Decisions¶

Why are macro invocations not prefixed with a “!”?¶

A compiled macro can be argued as a bang expression, since it invokes behavior on compile time (transforming the arguments into a different syntax tree).

There rationale for making it look like a regular function is:

macros should be seen as equivalent to builtins, which are not

prefixed with a bang.
bang symbols imply that the full statement will be executed on compile

time. In the case of macros, the macro expansion will be invoked, but the body of the code will not.
this also enables execution of a macro, and it’s contents, during compile time. so both of these situations are possible:

# runtime evaluation
let x (unless true (print bar))
# compile time evaluation
let y (unless! true (print bar))