Re: [racket-users] syntax-local-lift-expression vs. syntax-local-lift-require

[Matthew Flatt]

At Mon, 27 Feb 2017 18:43:28 -0800, Matthew Butterick wrote:
> > On Feb 27, 2017, at 5:34 PM, gfb  wrote:
> > 
> > If you want that to be equivalent to having `(require
> > math/number-theory)` at the top-level then I'm not sure the precise
> > semantics: should it affect the meaning of already-transformed code
> > from before that `lifted-require` was encountered?
> 
> Currently, I'm doing the job by hand: that is, in the expansion of the 
> `#%module-begin` of my #lang, I search the inbound parse tree and move the 
> `require` forms to the top level. This works fine. 
> 
> So I suppose my question boils down to "is there something in the 
> syntax-function zoo that does this more neatly?" 
> 
> As it stands, I think the answer is "no" because my manual technique is 
> essentially pre-empting the macro expander. 

Right.

The `syntax-local-lift-require` function works the way it does to avoid
the potential ambiguity Gary suggests, where adding a `require` from a
nested expression could create a binding that would affect expansion of
surrounding forms (but it's too late for that).

By adding the `require` in a fresh scope and then indirectly giving you
the scope, the effect of the `require` is consistently limited to
expressions that are afterwards given the new scope.


At Mon, 27 Feb 2017 17:34:53 -0800 (PST), gfb wrote:
> Recall that `requires` are noted before expansion of other forms, 
> so, e.g., this works:
>
> #lang racket
> divides? ; Even though the require for this appears after
> (require math/number-theory)

Well, ok... there is some ambiguity here. It's not that `require`s are
noted before other expansion, but that module-level expansions are
partial until all the `defines` and `requires` are found. So `divides?`
is determined not to be a definition by partial expansion, and further
expansion is delayed under the assumption that `divides?` is an
expression form. If `divides?` turns out to be a definition form, it
will trigger an error. In contrast, if `divides?` is put after
`(require math/number-theory)`, then it's ok for `divides?` to expand
as a definition.

This partial-expansion idea has worked ok, and so we've put up with
this much order-sensitive behavior, but we try to minimize it.

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Re: [racket-users] syntax-local-lift-expression vs. syntax-local-lift-require

[Matthew Butterick]

> On Feb 27, 2017, at 4:51 PM, Jay McCarthy  wrote:
> 
> You don't want local-require?



I don't think so — the `require` forms can be nested anywhere in the program. I 
want them to jump to the top level, and have their usual "global" effect.


> On Feb 27, 2017, at 5:34 PM, gfb  wrote:
> 
> If you want that to be equivalent to having `(require math/number-theory)` at 
> the top-level then I'm not sure the precise semantics: should it affect the 
> meaning of already-transformed code from before that `lifted-require` was 
> encountered? 

Currently, I'm doing the job by hand: that is, in the expansion of the 
`#%module-begin` of my #lang, I search the inbound parse tree and move the 
`require` forms to the top level. This works fine. 

So I suppose my question boils down to "is there something in the 
syntax-function zoo that does this more neatly?" 

As it stands, I think the answer is "no" because my manual technique is 
essentially pre-empting the macro expander. 

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[racket-users] syntax-local-lift-expression vs. syntax-local-lift-require

[Matthew Butterick]

Is there a simple way to lift a `require` form to the top level of a module? 
i.e., in the example below, is there something that would replace 
`WHAT-MAGIC-GOES-HERE?` to make it work?

I understand that `syntax-local-lift-expression` won't work. 

But `syntax-local-lift-require`, as best I can divine, needs you to furnish a 
particular identifier as the second argument, and incurs other housekeeping 
with scopes. So, though I'm not really sure what it's for, I'm pretty sure it's 
not for this.

(FWIW `syntax-local-lift-module-end-declaration` didn't work either.)




#lang racket

(define-syntax (lifted-require stx)
  (syntax-case stx ()
[(_ spec) (WHAT-MAGIC-GOES-HERE? #'(require spec))]))

(let ()
  (lifted-require math/number-theory))

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