To Do: If Only ...

One of the simplest and most pervasive macros in arc is "do". "do" is useful when you need to evaluate a sequence of expressions, but you only have room for one. A common case is when evaluating conditionals:

(if a b
    c d
    e)

This corresponds to the following in java:

if (a) { 
  return b; 
} else if (c) {
  return d;
} else {
  return e;
}

In java, we can execute as many statements as we desire within a { } block; arc however allows only a single expression as return value. So if we wanted

if (a) { 
  b();
  c();
  return d; 
} else if (e) {
  return f;
} else {
  return g;
}

we can't just write

(if a b c d
    e f
    g)

Hence, do.

(if a (do b c d)
    e f
    g)

do expands into a call to a no-arg function containing b c d -

((fn () b c d))

- it defines an anonymous zero-arg function and immediately calls it. Hence, b c d are grouped in a single expression, and will work as desired inside a conditional:

(if a ((fn () b c d))
    e f
    g)