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Re: C++ question on wrapper API for setting Guile fluids

From: David Kastrup
Subject: Re: C++ question on wrapper API for setting Guile fluids
Date: Thu, 21 Apr 2022 12:58:46 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

Jean Abou Samra <> writes:

> Le 21/04/2022 à 10:38, Han-Wen Nienhuys a écrit :
>> On Thu, Apr 21, 2022 at 12:04 AM Jean Abou Samra <> wrote:
>>> I am working on code that pervasively utilizes Guile fluids. They should
>>> be set in dynamic scopes.
>> That sounds scary. Care to tell more?
> What is scary about it exactly?
> I am trying to reimplement purity in terms of fluids, so the set of
> parameters is not hardcoded to 'start, end' and all the code doesn't
> have to be littered with functions working both as pure and impure and
> forwarding start/end to the property callbacks they
> cause.


> Essentially, this should look like {   Dynwind_context dwc;  
> dwc->make_assumption (Lily::prebreak_estimate, SCM_BOOL_T);  
> ... get_property (grob, "property") ... } or in Scheme:
> (under-assumptions ((*prebreak-estimate* #t))   ...) and by virtue of
> the dynamic context, the callback that computes the grob property
> understands that it should do pure estimates.

My personal take on this would move the magic out of the normal reach of
users.  You get (*start-column*) and (*end-column*) which you can use
for accessing the respective fluids but the function *start-column* does
not merely access %start-column but it also registers the associated
impurity for the sake of caching.

This also helps by not registering impurities that don't arise for a
particular grob (things like breakpoint-depending accidentals
comparatively rarely rise to the level of actual relevance but get
everything dealt with as impure, with the associated costs, all the

> I haven't gotten as far as a working proof of concept, but I expect a
> speedup. This is essentially a remix of an idea from David:

I probably repeat myself...

David Kastrup

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