Re: eval inside let

[Mikael Djurfeldt <mdj at mdj dot nada dot kth dot se>]

thi <ttn@mingle.glug.org> writes:

> Ceri Storey writes:
> 
>  > I'd guess i'm missing something here. Any help would be much appreciated.
> 
> `local-eval' takes two args, the first an expression and the second a
> macro that references the environment to be considered "local".  at
> least, this is what i get from reading libguile/debug.c... anyway, try:
> 
>   (let ((foo 'bar)) (local-eval (display foo) the-environment))
> 
> note that you don't need to quote the first arg.  (hmm, when i use a
> quote, guile seems to hang -- does anyone else see this?)

Note that local-eval is a procedure, which means that

  (local-eval (display foo) <env>)

will evaluate #<unspecified> (the result of display) to
#<unspecified>.

The interesting case is the quoted one.

The hang is because you try to use a macro (the-environment) as
environment.  It should be

  (local-eval '(display foo) (the-environment))

The second arg to local-eval should be error checked.  But
environments are currently explicitly represented as a list structure,
and it is too costly to check that such a structure is well formed.

The way to solve this is to wrap environments in a smob whenever they
occur on the Scheme level.  This has been implemented in scm.