= zexps

zexps, similar to g-expressions in Guix, are a way to generate
S-expressions that are taggged with store paths. But that's where
the similarity ends.


To create a zexp, you can use either the full syntax, or the reader macro:

[,scheme]
----
#~(foo bar #$baz)
; is identical to:
(zexp (foo bar (zexp-unquote baz)))
----

`(zexp-unquote VAL)` returns the value that the zexp (or any compatible record)
contains, while gathering the `zexp-unquote`d values used.

Like quasiquotation, zexps can use `unquote`, including ``zexp-unquote``d values
inside the ``unquote``d code. ``unquote``d code is evaluated when the `zexp` is evaluated.

[,scheme]
----
(define world #~,(begin (write "hello") "world"))
(define hello #~("hello" ,(string-append "very " "cute") #$world))
; When hello is used as zexp, it will also write "hello" to the output port.
----