make-image-dict

# To make "stub functions" in an image, solving c interop problems like this:

# I hoped that I would be able to unmarshal the runtime.jimage, load my C functions,
# and Janet would use my app's functions instead of the stubs. However, during
# compilation, Janet seems to precompile all the top-level function calls. This
# means hotswapping the functions does nothing, and the Janet code continues to 
# use the stubs instead.
#
# I was hoping that there would be some way of telling Janet "don't precompile 
# this please", either a (defstubn ...) or the ability to do 
# (defn internal/redraw-ui [opts] (dyn)), where (dyn) is some abstract type that
# Janet can't precompute directly.
# https://github.com/janet-lang/janet/issues/1642


# pretend this is the stub function we want to replace
(defn greet [] (print "hello world"))

# make-image-dict determines which values will be filled in at
# "deserialization" time
(put make-image-dict greet 'greet)
(def pretend-env @{
  'some-function (fn [] (greet) (greet))})

(def compiled (make-image pretend-env))
# notice: the string "hello world" does not appear in the compiled result
(pp compiled)

# if we just try to (load-image compiled) at this point, it will raise,
# because we haven't specified what to do about 'greet

# we have to make an entry in the load-image-dict. note that
# it does not have to be the same value!
(put load-image-dict 'greet (fn [] (print "something else!")))

# now we can call load-image
(def pretend-env-reparsed (load-image compiled))

# and when we run this, we will see the replacement that we gave
((pretend-env-reparsed 'some-function))