reveal.js: Explicitly setting boolean options to false fails

[salim-b]

Problem

Since Pandoc 2.14.0.3 (more specifically 14b2eb2), all reveal.js config options are explicitly set to their default values.

While leaving the default of a boolean variable untouched (1) or flipping it to true (2) works fine, setting it to false (3) fails.

1

echo "---
title: 'Test'
---

# My presentation

blabla
" | pandoc --to=revealjs --from=markdown --self-contained | grep mouseWheel

        mouseWheel: false,

2

echo "---
title: 'Test'
mouseWheel: true
---

# My presentation

blabla
" | pandoc --to=revealjs --from=markdown --self-contained | grep mouseWheel

        mouseWheel: true,

3

echo "---
title: 'Test'
mouseWheel: false
---

# My presentation

blabla
" | pandoc --to=revealjs --from=markdown --self-contained | grep mouseWheel

        mouseWheel: ,

Workaround

Explicitly setting the boolean false as a string works:

echo "---
title: 'Test'
mouseWheel: 'false'
---

# My presentation

blabla
" | pandoc --to=revealjs --from=markdown --self-contained | grep mouseWheel

        mouseWheel: false,

Pandoc version

2.14.0.3

[jgm]

The root problem is that the template engine (doctemplates) wasn't designed with boolean values in mind.
True gets rendered as "true", but False gets rendered as "". Why? Because the way conditionals work is that the condition is evaluated and tested for emptiness (empty = false, nonempty = true).

See jgm/doctemplates#16 -- ultimately we need to sort this out there.
In the mean time, it might be possible to change the writer so that it rewrites booleans as strings for these values (ugly).

[jgm]

One solution would be to

• change the HTML writer so that the default values are real booleans (instead of strings 'true', 'false' as now)
• change the templates like

mouseWheel: $if(mouseWheel)$true$else$false$endif$,

This is kind of ugly, but more straightforward than the present approach.

[jgm]

Of course, that approach would break with the workaround above.
It would be nice if people could write either false or 'false' and have it work.

[jgm]

Another possibility would be to add a "pipe" to doctemplates that converts a boolean value to the string true or false, leaves strings the same, and (does what?) with other types?
Or a more general pipe that turns a value into its canonical JSON representation?

[salim-b]

One solution would be to

* change the HTML writer so that the default values are real booleans (instead of strings 'true', 'false' as now)

* change the templates like

mouseWheel: $if(mouseWheel)$true$else$false$endif$,

This is kind of ugly, but more straightforward than the present approach.

I guess this makes sense until underlying issue jgm/doctemplates#16 is resolved, thus see #7403.

[jgm]

I've pushed commit 7dcf727
which uses a tweaked version of doctemplates. This seems to fix the problem.
(Of course, there's always the danger that it will cause other problems or break something for someone who was relying on false values rendering as empty strings...)