Filter for variable replacement in code blocks

- Replacing of `${namespace:var}` in code blocks with values
  from the environment or from the meta data block.

codeblock-var-replace/Makefile

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+DIFF ?= diff --strip-trailing-cr -u
+
+.EXPORT_ALL_VARIABLES:
+
+BANANA = eat a banana
+
+
+test: sample.md
+	@pandoc --lua-filter=codeblock-var-replace.lua --to=native $< \
+	    | $(DIFF) expected.native -
+
+expected.native: sample.md
+	pandoc --lua-filter=codeblock-var-replace.lua --output $@ $<
+
+expected.json: sample.md
+	pandoc --lua-filter=codeblock-var-replace.lua -t json --output $@ $<
+
+
+.PHONY: test

codeblock-var-replace/README.md

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+# codeblock-var-replace
+
+Filter to replace variables in code blocks with values from environment
+or meta data. All variables in the form `${namespace:name}` in a code
+block with a class `.var-replace` are replaced.
+
+## Variables
+
+A variables needs to be of the form `${namespace:name}` where
+`namespace` is currently one of `env`,`meta` with the following replacement behavior:
+
+- `env` : Substituting the environment variable `name`.
+- `meta` : Substituting the **stringified** variable `name` from the meta data block.
+
+## Example
+
+Note that meta data is parsed as markdown, therefore use a
+general code blocks `` `text` ``:
+
+    ---
+    author: "`The fearful bear`"
+    title: Thesis
+
+    monkey: "`Hello:  I am a monkey`"
+    "giraffe and zebra" : "`cool and humble  !`"
+    mypath: "`chapters:   1/A B.txt`"
+    food: "chocolate"
+    ---
+
+    ## Replace
+
+    ``` {.var-replace}
+    ${meta:monkey} and ${env:BANANA}
+
+    Zebras and giraffes are ${meta:giraffe and zebra}
+
+    ${meta:author} thanks for everything in '${meta:mypath}'
+    and of course eat some ${meta:food}
+    ```
+

codeblock-var-replace/codeblock-var-replace.lua

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+--- codeblock-var-replace.lua – replace variables in code blocks
+---
+--- Copyright: © 2019–2020 Gabriel Nützi
+--- License:   MIT – see LICENSE file for details
+
+-- pandoc's List type
+local List = require 'pandoc.List'
+local sys = require 'pandoc.system'
+local utils = require 'pandoc.utils'
+
+-- Save env. variables
+local env = sys.environment()
+
+-- Save meta table and metadata
+local meta
+local metadata
+local vars
+function save_meta (m)
+  meta = m
+  metadata = m['metadata']
+  vars = m['variables']
+end
+
+--- Replace variable with values from environment
+--- and meta data (stringifing).
+local function replace(what, var)
+  if what == "env" then
+    return env[var]
+  elseif what == "meta" then
+    local v = meta[var]
+    if v then
+      return utils.stringify(v)
+    end
+  end
+  return nil
+end
+
+--- Replace variables in code blocks
+function var_replace_codeblocks (cb)
+  -- ignore code blocks which are not of class "var-replace".
+  if not cb.classes:includes 'var-replace' then
+    return
+  end
+
+  cb.text = cb.text:gsub("%${(%l+):([^}]+)}", replace)
+  return cb
+end
+
+return {
+  { Meta = save_meta },
+  { CodeBlock = var_replace_codeblocks }
+}

codeblock-var-replace/expected.native

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+[Header 2 ("replace",[],[]) [Str "Replace"]
+,CodeBlock ("",["var-replace"],[]) "Hello:  I am a monkey and eat a banana\n\nZebras and giraffes are cool and humble  !\n\nThe fearful bear thanks for everything in 'chapters:   1/A B.txt'\nand of course eat some chocolate"]

codeblock-var-replace/sample.md

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+---
+author: "`The fearful bear`"
+title: Thesis
+
+monkey: "`Hello:  I am a monkey`"
+"giraffe and zebra" : "`cool and humble  !`"
+mypath: "`chapters:   1/A B.txt`"
+food: "chocolate"
+---
+
+## Replace
+
+``` {.var-replace}
+${meta:monkey} and ${env:BANANA}
+
+Zebras and giraffes are ${meta:giraffe and zebra}
+
+${meta:author} thanks for everything in '${meta:mypath}'
+and of course eat some ${meta:food}
+```