[racket] Defining a typed language

On Tue, Oct 28, 2014 at 10:30 AM, Konrad Hinsen
<konrad.hinsen at fastmail.net> wrote:
> Konrad Hinsen writes:
>
>  > I have added lots of comments to my minimalistic typed language
>  > implementation, in the hope that future language definers will
>  > be able to pick up from there:
>  >
>  >    http://github.com/khinsen/racket-typed-lang
>
> There is still one problem, although I am not convinced that it can be
> fixed in the language definition.
>
> The following code fails because 'foo' is considered an unbound identifier:
>
> --- test1.rkt ------------------------------------
> #lang racket/base
>
> (module example a-typed-lang
>   (displayln (foo 42)))
>
> (require 'example)
> --------------------------------------------------
>
> Strangely enough, as soon as my submodule has two forms it works, e.g.
>
> --- test2.rkt ------------------------------------
> #lang racket/base
>
> (module example a-typed-lang
>   (displayln (foo 42))
>   (displayln (foo 42)))
>
> (require 'example)
> --------------------------------------------------
>
> The documentation for "module" at
> http://docs.racket-lang.org/reference/module.html?q=module#%28form._%28%28quote._~23~25kernel%29._module%29%29
>
> suggests that single-form modules are handled differently than multi-form
> modules, which might well explain this phenomenon. However, it doesn't provide
> any clue to a solution. Apparently, my single form is
>
>   "partially expanded in a module-begin context. If the expansion
>   leads to #%plain-module-begin, then the body of the
>   #%plain-module-begin is the body of the module."
>
> Furthermore:
>
>    "Finally, if multiple forms are provided, they are wrapped with
>    #%module-begin, as in the case where a single form does not expand
>    to #%plain-module-begin."
>
> This suggests that if I want my single-form module to be handled exactly
> like a multi-form module, my single form must expand to something else
> than #%plain-module-begin. Fine. But who or what decides what my single
> form becomes when it is "partially expanded in a module-begin context" ?

This always happens in a single-form module. The macro system
partially-expands the macro to see if it turns into a version of
#%module-begin.

Sam