Lua: add function `pandoc.template.meta_to_context`.

The functions converts Meta values to template contexts; the intended
use is in combination with `pandoc.template.apply`.

3 files changed, 65 insertions, 5 deletions

diff --git a/doc/lua-filters.md b/doc/lua-filters.md
index 362756404..0c88433ae 100644
--- a/doc/lua-filters.md
+++ b/doc/lua-filters.md
@@ -5238,6 +5238,29 @@ Returns:
 
 -   raw template (string)
 
+### meta_to_context {#pandoc.template.meta_to_context}
+
+`meta_to_context (meta, blocks_writer, inlines_writer)`
+
+Creates template context from the document's [Meta]{#type-meta}
+data, using the given functions to convert [Blocks] and [Inlines]
+to [Doc] values.
+
+Parameters:
+
+`meta`
+:   document metadata ([Meta])
+
+`blocks_writer`
+:   converter from [Blocks] to [Doc] values (function)
+
+`inlines_writer`
+:   converter from [Inlines] to [Doc] values (function)
+
+Returns:
+
+-   template context (table)
+
 # Module pandoc.types
 
 Constructors for types which are not part of the pandoc AST.
diff --git a/pandoc-lua-engine/src/Text/Pandoc/Lua/Marshal/Context.hs b/pandoc-lua-engine/src/Text/Pandoc/Lua/Marshal/Context.hs
index dfaa1ff87..26dffec21 100644
--- a/pandoc-lua-engine/src/Text/Pandoc/Lua/Marshal/Context.hs
+++ b/pandoc-lua-engine/src/Text/Pandoc/Lua/Marshal/Context.hs
@@ -18,7 +18,7 @@ module Text.Pandoc.Lua.Marshal.Context
   , pushContext
   ) where
 
-import Control.Monad ((<$!>))
+import Control.Monad (when, (<$!>))
 import Data.Text (Text)
 import HsLua as Lua
 import HsLua.Module.DocLayout (peekDoc, pushDoc)
@@ -36,7 +36,18 @@ peekContext idx = Context <$!> peekMap peekText peekVal idx
 
 -- | Pushes a template context to the Lua stack.
 pushContext :: LuaError e => Pusher e (Context Text)
-pushContext = pushMap pushText pushVal . unContext
+pushContext ctx = do
+  pushMap pushText pushVal $ unContext ctx
+  created <- Lua.newmetatable "pandoc Context"
+  when created $ do
+    pushName "__concat"
+    pushHaskellFunction $ do
+      c1 <- forcePeek $ peekContext (nthBottom 1)
+      c2 <- forcePeek $ peekContext (nthBottom 2)
+      pushContext (c1 <> c2)
+      return 1
+    rawset (nth 3)
+  setmetatable (nth 2)
 
 pushVal :: LuaError e => Pusher e (Val Text)
 pushVal = \case
diff --git a/pandoc-lua-engine/src/Text/Pandoc/Lua/Module/Template.hs b/pandoc-lua-engine/src/Text/Pandoc/Lua/Module/Template.hs
index be769e988..d84f0c6d7 100644
--- a/pandoc-lua-engine/src/Text/Pandoc/Lua/Module/Template.hs
+++ b/pandoc-lua-engine/src/Text/Pandoc/Lua/Module/Template.hs
@@ -12,11 +12,13 @@ module Text.Pandoc.Lua.Module.Template
   ) where
 
 import HsLua
-import HsLua.Module.DocLayout (pushDoc)
+import HsLua.Module.DocLayout (peekDoc, pushDoc)
 import Text.Pandoc.Error (PandocError)
-import Text.Pandoc.Lua.Marshal.Context (peekContext)
+import Text.Pandoc.Lua.Marshal.AST (peekMeta, pushBlocks, pushInlines)
+import Text.Pandoc.Lua.Marshal.Context (peekContext, pushContext)
 import Text.Pandoc.Lua.Marshal.Template (peekTemplate, pushTemplate)
 import Text.Pandoc.Lua.PandocLua (PandocLua (unPandocLua), liftPandocLua)
+import Text.Pandoc.Writers.Shared (metaToContext')
 import Text.Pandoc.Templates
   ( compileTemplate, getDefaultTemplate, renderTemplate
   , runWithPartials, runWithDefaultPartials )
@@ -40,7 +42,7 @@ functions :: [DocumentedFunction PandocError]
 functions =
   [ defun "apply"
      ### liftPure2 renderTemplate
-     <#> parameter peekTemplate "pandoc Template" "template" "template to apply"
+     <#> parameter peekTemplate "Template" "template" "template to apply"
      <#> parameter peekContext "table" "context" "variable values"
      =#> functionResult pushDoc "Doc" "rendered template"
      #? T.unlines
@@ -74,4 +76,28 @@ functions =
      =#> functionResult pushText "string"
            "string representation of the writer's default template"
 
+  , defun "meta_to_context"
+     ### (\meta blockWriterIdx inlineWriterIdx -> unPandocLua $ do
+             let blockWriter blks = liftPandocLua $ do
+                   pushvalue blockWriterIdx
+                   pushBlocks blks
+                   callTrace 1 1
+                   forcePeek $ peekDoc top
+             let inlineWriter blks = liftPandocLua $ do
+                   pushvalue inlineWriterIdx
+                   pushInlines blks
+                   callTrace 1 1
+                   forcePeek $ peekDoc top
+             metaToContext' blockWriter inlineWriter meta)
+     <#> parameter peekMeta "Meta" "meta" "document metadata"
+     <#> parameter pure "function" "blocks_writer"
+           "converter from Blocks to Doc values"
+     <#> parameter pure "function" "inlines_writer"
+           "converter from Inlines to Doc values"
+     =#> functionResult pushContext "table" "template context"
+     #? T.unlines
+     [ "Creates template context from the document's [Meta]{#type-meta}"
+     , "data, using the given functions to convert [Blocks] and [Inlines]"
+     , "to [Doc] values."
+     ]
   ]