When is it possible to override top-level bindings in (R7RS) scheme?
- by Marc
I have read the current draft of the forthcoming R7RS scheme standard (small language), but I don't understand under which conditions it is not an error to redefine top-level bindings.
I guess that it is possible to define or set! a binding that has been introduced at the top-level of a program a second time. But what about imported bindings from an external library? Is it possible to override these bindings by the standard?
On page 26/27 of the report, it says:
The top level of a program may also include import declarations.
In a library declaration, it is an error to import
the same identifier more than once with different bindings,
or to redefine or mutate an imported binding with define,
define-syntax or set!. However, a REPL should permit
these actions.
Does it mean that redefining is only an error when it does happen in libraries for imported bindings?
I understand that it prohibits optimisations by compilers if the compiler does not know whether, say + still means the built-in addition or is any other user-specified error. But from this perspective, it does not make sense to restrict forbidding to rebind on the library level, when it would also make sense (at least) for imported bindings in programs.
P.S.: As this is all about the environment of a scheme program: am I right in saying that environments are not first class citizens because one cannot get hold of the current environment? (Which, in turn, allows a compiled program to forget about the chosen names of the bindings.)
