The idea behind Guile is to provide a fully-functional scripting
language that can be embedded within make. My belief is that the world
already has quite enough scripting languages, thanks, and it certainly
does not need another one in the form of a very rich suite of GNU make
builtin functions.
Not only would Guile provide the ability to perform all sorts of
operations (read/write files, math, etc. etc.--all the things that
people are requesting new builtin make functions to do) but I intend it
to be able to "look back" into GNU make; that is I intend to provide
Guile functions that can perform operations on GNU make's internals.
Some simple examples are things like querying and setting make variables
from Guile scripts, calling GNU make functions (eval being the most
obviously useful one, but maybe not the only useful one), querying the
various make databases for rule information, etc.
As with eval, I'm sure all of you will invent cool things to do with
this capability that I can't think of right now :-).