[racket] Defining a typed language

On Fri, Oct 31, 2014 at 5:57 AM, Konrad Hinsen
<konrad.hinsen at fastmail.net> wrote:
> Sam Tobin-Hochstadt writes:
>
>  > The rule is that for anything like:
>  >
>  > (module m lang
>  >   (form ...))
>  >
>  > `form` is expanded until it either produces `#%plain-module-begin` or
>  > another core form. In the former case, it just continues, in the
>  > latter case, it wraps the whole thing with `#%module-begin` as
>  > exported from the specified language, and then expands that.
>  >
>  > In your particular example, it will expand (displayln (foo 42)) to
>  > (#%app displayln (foo 42)) and then to (#%module-begin (#%app
>  > displayln (foo 42))).
>  >
>  > > Are they part of the language definition? More specifically, I am
>  > > looking for a way to override them.
>  >
>  > I'm not sure exactly what you want to override here.
>
> Whatever it takes to prevent the partial expansion to end in
> #%plain-module-begin. I just want to get rid of the special treatment
> for single-form modules. For my mini-language, I expect single-form
> modules to be frequent special cases. Right now they fail
> systematically because of the special treatment.

You can't turn this feature off -- it's a fundamental part of the
macro system. There are a few strategies that you can employ here:

1. If your language is written with #lang, then you can insert, at
`read` time, an extra form, perhaps just `(begin)`. Then all your
modules will have at least two forms.
2. You can also, in the #lang case, `read` directly to
`#%module-begin` wrapped around everything, which will mean that your
#%module-begin gets control right away.
3. You could change the bindings of some of your exports, such as
#%app or anything else that might appear as the head form in a single
body expression, to check for (syntax-local-context) begin
'module-begin, and then behaving differently -- perhaps wrapping in
#%expression or in #%module-begin.

Hopefully one of those will work for you.

Sam