[racket-dev] Top level is hopeless or bug?

At Mon, 17 Sep 2012 23:48:06 -0400, Asumu Takikawa wrote:
> Is the following interaction the expected behavior here or is it a bug?
> 
> Welcome to Racket v5.3.0.24.
> -> (module foo racket (define x 0) (provide x))
> -> (require 'foo)
> -> x
> 0
> -> (module foo racket (module* bar #f (define x 1) (provide x)))
> -> x
> ; readline-input:4:0: link: module mismatch;
> ;  possibly, bytecode file needs re-compile because dependencies changed
> ;   exporting module: 'foo
> ;   exporting phase level: 0
> ;   internal explanation: variable not provided (directly or indirectly)
> ;   in: x
> ; [,bt for context]

That looks right to me. I don't think the submodule is relevant; it's
just that `x' has gone away:

 > (module foo racket (define x 1) (provide x))
 > (require 'foo)
 > (module foo racket)
 > x
 readline-input::80: link: module mismatch;
 ...


> This is also interesting:
> 
> Welcome to Racket v5.3.0.24.
> -> (module foo racket (define x 0) (provide x))
> -> (require 'foo)
> -> x
> 0
> -> (module foo racket (define x 1) (provide x))
> ; define-values: assignment disallowed;
> ;  cannot re-define a constant
> ;   constant: x
> ;   in module: 'foo
> ; [,bt for context]
> -> x
> 1

Neat. That's the inliner exposing a mismatch between the current module
code and the instantiated variable `x'.

Adding a use of `namespace-variable-value' (which forces a variable
look-up) at the end of your trace:

 > (namespace-variable-value 'x)
 0

As another example, using a non-literal right-hand side for `x' avoids
inlining:

 > (module foo racket (define x (random 1)) (provide x))
 > (require 'foo)
 > x
 0
 > (module foo racket (define x (add1 (random 1))) (provide x))
 define-values: assignment disallowed;
  cannot re-define a constant
   constant: x
   in module: 'foo
   context...:
    foo: [running body]
    /Applications/Racket v5.3/collects/racket/private/misc.rkt:87:7
 > x
 0

Maybe inlining of module bindings should be disabled in the top level,
but I'm inclined to treat this as just another limitation of the top
level.