Combining multiple metadata blocks
gfelbing opened this issue · 4 comments
Hello,
I am using a shared metadata yaml for all my documents, which contains definitions, which I need in all of my documents, for example:
---
documentclass: article
header-includes:
- \usepackage{tikz}
---
Now, I have some document, in which I want to overwrite/extend some values:
---
documentclass: IEEEtran
header-includes:
- \IEEEspecialpapernotice{Some notice}
---
I would have expected, that the documentclass (since it is a single-value type) gets overridden and the header-includes (since it is an array) gets extended.
Instead, the values specified in my document got completely ignored.
I compiled the document with the following options:
pandoc --standalone --filter pandoc-citeproc -f markdown+yaml_metadata_block+example_lists+implicit_figures+citations -o ./subject/document.pdf ./pandoc/meta.yaml ./subject/document.md
Is there a way to achieve my expected behaviour?
When a value is set multiple times in the same document, the
earlier one takes priority. See the MANUAL:
A document may contain multiple metadata blocks. The
metadata fields will be combined through a left-biased
union: if two metadata blocks attempt to set the same
field, the value from the first block will be taken.
Technical explanation for pandoc developers: This is because
the monoid instance for Meta (in Text.Pandoc.Definition)
uses Data.Map.union, which is left-biased. (Monoid append
is used for each new metadata block in the document; see the
end of yamlMetaBlock in the Markdown reader.)
So the solution for you would be to include the common
metadata after your document rather than before.
This would work for now.
For List-Typed metadata, you could apply Data.Map.unionWith curried with a (++), so arrays would get extendend.
Here is an hacked-together prove of concept of how this could be done using lua filters (requires #3584). Would this be sufficient to close this issue?
--- Define ways in which metadata can be merged.
local merge_methods = {
replace = function(v1, v2) return v2 or v1 end,
keep = function(v1, v2) return v1 or v2 end,
extendlist = function(v1, v2)
local res
if type(v1) == "table" and v1.tag == "MetaList" then
res = v1
else
res = pandoc.MetaList{v1}
end
if type(v2) == "table" and v2.tag == "MetaList" then
for i = 1, #v2 do
res[#res + 1] = v2[i]
end
else
res[#res + 1] = v2
end
return res
end
}
--- Merge second metadata table into the first.
function merge_metadata(md1, md2, field_methods)
for k, v in pairs(md2) do
-- the default method is to overwrite the current value.
local method = field_methods[k] or "replace"
md1[k] = merge_methods[method](md1[k], md2[k])
end
return md1
end
return {
{
Meta = function(meta)
local metafile = io.open('/home/albert/tmp/metadata-file.yaml', 'r')
local content = metafile:read('*a')
local doc = pandoc.read(content, "markdown")
metafile:close()
local field_methods = {
author = "extendlist",
title = "replace",
date = "replace",
classoptions = "keep",
}
return merge_metadata(meta, doc.meta, field_methods)
end,
}
}