# Eval

# Overview

The eval function in Gerbil Scheme is a powerful tool for dynamic code execution. However, its behavior and usage differ between runtime and compile-time, and it has some key differences compared to Gambit Scheme. This document outlines how to use eval effectively in various scenarios.

# Runtime Behavior

# Interactive Environment

In the Gerbil interpreter (gxi), eval works similarly to other Scheme implementations:

> (def greeting "hello world")
> (eval '(displayln greeting))
hello world

# Compiled Binaries

When using eval in compiled binaries, additional steps are necessary. To demonstrate this let's slightly modify code from previous example, save it into the file:

eval-example.ss

(export main)
(def greeting "hello world")
(def (main . args)
  (eval '(displayln greeting)))

Then compile into executable and run:

$ gxc -exe eval-example.ss
...
$ ./eval-example
*** ERROR -- Unbound variable: greeting
--- continuation backtrace:
[0] ##primordial-exception-handler-hook

An error is reported because eval operates on top-level namespace while greeting is defined a module namespace which is added implicitly.

This may be fixed by adding a namespace to greeting:

(export main)
(def greeting "hello world")
(def (main . args)
  (eval '(displayln eval-example#greeting))

$ gxc -exe eval-example.ss
...
$ ./eval-example
hello world

Adding namespace to each symbol referenced under eval may be a bit tedious, there is a better way to do this with (extern ...) form. Eval expression may be modified to look like this:

(begin
  (extern namespace: eval-example
    greeting)
  (displayln greeting))

Using extern will make eval-example#greeting available as greeting at runtime. But there is one more thing to do before it will work without errors, because extern relies on :gerbil/expander which should be loaded. In interactive mode this happens implicitly, while inside compiled binaries expander need to be loaded manually. Here is the final version:

(import :gerbil/expander)
(export main)
(def greeting "hello world")
(def (main . args)
  (gerbil-load-expander!)
  (eval '(begin
           (extern namespace: eval-example
             greeting)
           (displayln greeting))))

Which works as expected:

$ gxc -exe eval-example.ss
...
$ ./eval-example
hello world