For example, take a look at this code (from tspl4):
(define proc1
(lambda (x y)
(proc2 y x)))
If I run this as my program in scheme...
#!r6rs
(import (rnrs))
(define proc1
(lambda (x y)
(proc2 y x)))
I get this error:
expand: unbound identifier in module in: proc2
...This code works fine though:
#!r6rs
(import (rnrs))
(define proc2
+)
(define proc1
(lambda (x y)
(proc2 y x)))
(display (proc1 2 3)) ;output: 5
They all have to be defined in the same module (= "library" in r6rs lingo). But you can define them in any order you want -- for example, in your last snip you can swap the two definitions and it will work fine. But note that you cannot put the definitions after the display
line -- this is an expression that uses their value, so if you move the function definitions after it, you'll get a runtime error. (Note the fact that it's a runtime error rather than a compile-time one.)