Lua: add `pandoc.template` module

The module provides a `compile` function to use strings as templates.

9 files changed, 187 insertions, 2 deletions

diff --git a/doc/lua-filters.md b/doc/lua-filters.md
index c50fe95bf..3ca5e8a94 100644
--- a/doc/lua-filters.md
+++ b/doc/lua-filters.md
@@ -2121,7 +2121,7 @@ Fields:
 :   Include table of contents (boolean)
 
 `template`
-:   Template to use (pandoc Template|nil)
+:   Template to use ([Template](#type-template)|nil)
 
 `toc_depth`
 :   Number of levels to include in TOC (integer)
@@ -2231,6 +2231,10 @@ Fields:
 :   table rows ([List] of rows, where a row is a list of simple
     cells, i.e., [List] of [Blocks][])
 
+## Template {#type-template}
+
+Opaque type holding a compiled template.
+
 ## Version {#type-version}
 
 A version object. This represents a software version like
@@ -4393,6 +4397,55 @@ Returns:
 
 -   The result(s) of the call to `callback`
 
+# Module pandoc.template
+
+Handle pandoc templates.
+
+### compile {#pandoc.template.compile}
+
+`compile (template[, templates_path])`
+
+Compiles a template string into a [Template](#type-template)
+object usable by pandoc.
+
+If the `templates_path` parameter is specified, should be the
+file path associated with the template. It is used when checking
+for partials. Partials will be taken only from the default data
+files if this parameter is omitted.
+
+An error is raised if compilation fails.
+
+Parameters:
+
+`template`:
+:   template string (string)
+
+`templates_path`:
+:   parameter to determine a default path and extension for
+    partials; uses the data files templates path by default.
+    (string)
+
+Returns:
+
+-   compiled template (Template)
+
+### default {#pandoc.template.default}
+
+`default ([writer])`
+
+Returns the default template for a given writer as a string. An
+error if no such template can be found.
+
+Parameters:
+
+`writer`:
+:   name of the writer for which the template should be
+    retrieved; defaults to the global `FORMAT`.
+
+Returns:
+
+-   raw template (string)
+
 # Module pandoc.types
 
 Constructors for types which are not part of the pandoc AST.
diff --git a/pandoc.cabal b/pandoc.cabal
index e1d8ebaa7..a4a13ce0d 100644
--- a/pandoc.cabal
+++ b/pandoc.cabal
@@ -711,6 +711,7 @@ library
                    Text.Pandoc.Lua.Module.MediaBag,
                    Text.Pandoc.Lua.Module.Pandoc,
                    Text.Pandoc.Lua.Module.System,
+                   Text.Pandoc.Lua.Module.Template,
                    Text.Pandoc.Lua.Module.Types,
                    Text.Pandoc.Lua.Module.Utils,
                    Text.Pandoc.Lua.Orphans,
diff --git a/src/Text/Pandoc/Lua/Init.hs b/src/Text/Pandoc/Lua/Init.hs
index 9e126f214..c3d792a37 100644
--- a/src/Text/Pandoc/Lua/Init.hs
+++ b/src/Text/Pandoc/Lua/Init.hs
@@ -48,6 +48,7 @@ loadedModules =
   , ("pandoc.mediabag", "mediabag")
   , ("pandoc.path", "path")
   , ("pandoc.system", "system")
+  , ("pandoc.template", "template")
   , ("pandoc.types", "types")
   , ("pandoc.utils", "utils")
   , ("text", "text")
diff --git a/src/Text/Pandoc/Lua/Module/Pandoc.hs b/src/Text/Pandoc/Lua/Module/Pandoc.hs
index c0127cdfe..9864da0db 100644
--- a/src/Text/Pandoc/Lua/Module/Pandoc.hs
+++ b/src/Text/Pandoc/Lua/Module/Pandoc.hs
@@ -29,6 +29,7 @@ import HsLua hiding (pushModule)
 import HsLua.Class.Peekable (PeekError)
 import System.Exit (ExitCode (..))
 import Text.Pandoc.Definition
+import Text.Pandoc.Error (PandocError (..))
 import Text.Pandoc.Lua.Orphans ()
 import Text.Pandoc.Lua.Marshal.AST
 import Text.Pandoc.Lua.Marshal.Filter (peekFilter)
@@ -50,7 +51,6 @@ import qualified Data.ByteString.Lazy.Char8 as BSL
 import qualified Data.Text as T
 import qualified Text.Pandoc.Lua.Util as LuaUtil
 import qualified Text.Pandoc.UTF8 as UTF8
-import Text.Pandoc.Error
 
 -- | Push the "pandoc" package to the Lua stack. Requires the `List`
 -- module to be loadable.
diff --git a/src/Text/Pandoc/Lua/Module/Template.hs b/src/Text/Pandoc/Lua/Module/Template.hs
new file mode 100644
index 000000000..cd66ce1c1
--- /dev/null
+++ b/src/Text/Pandoc/Lua/Module/Template.hs
@@ -0,0 +1,61 @@
+{-# LANGUAGE OverloadedStrings #-}
+{- |
+   Module      : Text.Pandoc.Lua.Module.Template
+   Copyright   : Copyright © 2022 Albert Krewinkel, John MacFarlane
+   License     : GNU GPL, version 2 or above
+   Maintainer  : Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
+
+Lua module to handle pandoc templates.
+-}
+module Text.Pandoc.Lua.Module.Template
+  ( documentedModule
+  ) where
+
+import HsLua
+import Text.Pandoc.Error (PandocError)
+import Text.Pandoc.Lua.Marshal.Template (pushTemplate)
+import Text.Pandoc.Lua.PandocLua (PandocLua (unPandocLua), liftPandocLua)
+import Text.Pandoc.Templates
+  (compileTemplate, getDefaultTemplate, runWithPartials, runWithDefaultPartials)
+
+import qualified Data.Text as T
+
+-- | The "pandoc.template" module.
+documentedModule :: Module PandocError
+documentedModule = Module
+  { moduleName = "pandoc.template"
+  , moduleDescription = T.unlines
+    [ "Lua functions for pandoc templates."
+    ]
+  , moduleFields = []
+  , moduleOperations = []
+  , moduleFunctions = functions
+  }
+
+-- | Template module functions.
+functions :: [DocumentedFunction PandocError]
+functions =
+  [ defun "compile"
+     ### (\template mfilepath -> unPandocLua $
+           case mfilepath of
+             Just fp -> runWithPartials (compileTemplate fp template)
+             Nothing -> runWithDefaultPartials
+                        (compileTemplate "templates/default" template))
+     <#> parameter peekText "string" "template" "template string"
+     <#> optionalParameter peekString "string" "templ_path" "template path"
+     =#> functionResult (either failLua pushTemplate) "pandoc Template"
+           "compiled template"
+
+  , defun "default"
+     ### (\mformat -> unPandocLua $ do
+           let getFORMAT = liftPandocLua $ do
+                 getglobal "FORMAT"
+                 forcePeek $ peekText top `lastly` pop 1
+           format <- maybe getFORMAT pure mformat
+           getDefaultTemplate format)
+     <#> optionalParameter peekText "string" "writer"
+           "writer for which the template should be returned."
+     =#> functionResult pushText "string"
+           "string representation of the writer's default template"
+
+  ]
diff --git a/src/Text/Pandoc/Lua/Packages.hs b/src/Text/Pandoc/Lua/Packages.hs
index 3e4a6d650..e2f0e59a5 100644
--- a/src/Text/Pandoc/Lua/Packages.hs
+++ b/src/Text/Pandoc/Lua/Packages.hs
@@ -26,6 +26,7 @@ import qualified HsLua.Module.Text as Text
 import qualified Text.Pandoc.Lua.Module.Pandoc as Pandoc
 import qualified Text.Pandoc.Lua.Module.MediaBag as MediaBag
 import qualified Text.Pandoc.Lua.Module.System as System
+import qualified Text.Pandoc.Lua.Module.Template as Template
 import qualified Text.Pandoc.Lua.Module.Types as Types
 import qualified Text.Pandoc.Lua.Module.Utils as Utils
 
@@ -50,6 +51,7 @@ pandocPackageSearcher pkgName =
     "pandoc.mediabag" -> pushModuleLoader MediaBag.documentedModule
     "pandoc.path"     -> pushModuleLoader Path.documentedModule
     "pandoc.system"   -> pushModuleLoader System.documentedModule
+    "pandoc.template" -> pushModuleLoader Template.documentedModule
     "pandoc.types"    -> pushModuleLoader Types.documentedModule
     "pandoc.utils"    -> pushModuleLoader Utils.documentedModule
     "text"            -> pushModuleLoader Text.documentedModule
diff --git a/test/Tests/Lua/Module.hs b/test/Tests/Lua/Module.hs
index 2f1a54e5c..fd3fc8998 100644
--- a/test/Tests/Lua/Module.hs
+++ b/test/Tests/Lua/Module.hs
@@ -27,6 +27,8 @@ tests =
                   ("lua" </> "module" </> "pandoc-mediabag.lua")
   , testPandocLua "pandoc.path"
                   ("lua" </> "module" </> "pandoc-path.lua")
+  , testPandocLua "pandoc.template"
+                  ("lua" </> "module" </> "pandoc-template.lua")
   , testPandocLua "pandoc.types"
                   ("lua" </> "module" </> "pandoc-types.lua")
   , testPandocLua "pandoc.utils"
diff --git a/test/lua/module/pandoc-template.lua b/test/lua/module/pandoc-template.lua
new file mode 100644
index 000000000..c288b2016
--- /dev/null
+++ b/test/lua/module/pandoc-template.lua
@@ -0,0 +1,65 @@
+local tasty = require 'tasty'
+local template = require 'pandoc.template'
+
+local assert = tasty.assert
+local test = tasty.test_case
+local group = tasty.test_group
+
+return {
+  test('is table', function ()
+    assert.are_equal(type(template), 'table')
+  end),
+  group 'default' {
+    test('is function', function ()
+      assert.are_equal(type(template.default), 'function')
+    end),
+    test('returns a string for known format', function ()
+      assert.are_equal(
+        pandoc.utils.type(template.default 'json'),
+        'string'
+      )
+      assert.are_equal(
+        pandoc.utils.type(template.default 'markdown'),
+        'string'
+      )
+    end),
+    test('fails on unknown format', function ()
+      local success, msg = pcall(function ()
+          return pandoc.utils.type(template.default 'nosuchformat')
+      end)
+      assert.is_falsy(success)
+    end),
+  },
+  group 'compile' {
+    test('is function', function ()
+      assert.are_equal(type(template.compile), 'function')
+    end),
+    test('returns a Template', function ()
+      assert.are_equal(
+        pandoc.utils.type(template.compile('$title$')),
+        'pandoc Template'
+      )
+    end),
+    test('returns a Template', function ()
+      local templ_path = pandoc.path.join{'lua', 'module', 'default.test'}
+      assert.are_equal(
+        pandoc.utils.type(template.compile('${ partial() }', templ_path)),
+        'pandoc Template'
+      )
+    end),
+    test('fails if template has non-existing partial', function ()
+      assert.error_matches(
+        function () return template.compile('${ nosuchpartial() }') end,
+        'PandocCouldNotFindDataFileError'
+      )
+    end),
+    test('works with default template that uses partials', function ()
+      local jats_template = template.default 'jats'
+      assert.are_equal(type(jats_template), 'string')
+      assert.are_equal(
+        pandoc.utils.type(template.compile(jats_template)),
+        'pandoc Template'
+      )
+    end),
+  },
+}
diff --git a/test/lua/module/partial.test b/test/lua/module/partial.test
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/test/lua/module/partial.test