Lua: add Version type to simplify comparisons

Version specifiers like `PANDOC_VERSION` and `PANDOC_API_VERSION` are
turned into `Version` objects. The objects simplify version-appropriate
comparisons while maintaining backward-compatibility.

A function `pandoc.types.Version` is added as part of the newly
introduced module `pandoc.types`, allowing users to create version
objects in scripts.

12 files changed, 353 insertions, 23 deletions

diff --git a/doc/lua-filters.md b/doc/lua-filters.md
index 3b3bb2f17..a5a7f2922 100644
--- a/doc/lua-filters.md
+++ b/doc/lua-filters.md
@@ -151,21 +151,21 @@ variables.
 :   Table of the options which were provided to the parser.
 
 `PANDOC_VERSION`
-:   Contains the pandoc version as a numerically indexed table,
-    most significant number first. E.g., for pandoc 2.1.1, the
-    value of the variable is a table `{2, 1, 1}`. Use
-    `table.concat(PANDOC_VERSION, '.')` to produce a version
-    string. This variable is also set in custom writers.
+:   Contains the pandoc version as a [Version object] which
+    behaves like a numerically indexed table, most significant
+    number first. E.g., for pandoc 2.7.3, the value of the
+    variable is equivalent to a table `{2, 7, 3}`. Use
+    `tostring(PANDOC_VERSION)` to produce a version string. This
+    variable is also set in custom writers.
 
 `PANDOC_API_VERSION`
 :   Contains the version of the pandoc-types API against which
     pandoc was compiled. It is given as a numerically indexed
     table, most significant number first. E.g., if pandoc was
     compiled against pandoc-types 1.17.3, then the value of the
-    variable will be a table `{1, 17, 3}`. Use
-    `table.concat(PANDOC_API_VERSION, '.')` to produce a version
-    string from this table. This variable is also set in custom
-    writers.
+    variable will behave like the table `{1, 17, 3}`. Use
+    `tostring(PANDOC_API_VERSION)` to produce a version string.
+    This variable is also set in custom writers.
 
 `PANDOC_SCRIPT_FILE`
 :   The name used to involve the filter. This value can be used
@@ -178,6 +178,8 @@ variables.
     variable is of type [CommonState](#type-ref-CommonState) and
     is read-only.
 
+[Version object]: #type-ref-Version
+
 # Pandoc Module
 
 The `pandoc` lua module is loaded into the filter's lua
@@ -1353,6 +1355,32 @@ available to readers and writers.
 A pandoc log message. Object have no fields, but can be converted
 to a string via `tostring`.
 
+## Version {#type-ref-Version}
+
+A version object. This represents a software version like
+"2.7.3". The object behaves like a numerically indexed table,
+i.e., if `version` represents the version `2.7.3`, then
+
+    version[1] == 2
+    version[2] == 7
+    version[3] == 3
+    #version == 3   -- length
+
+Comparisons are performed element-wise, i.e.
+
+    Version '1.12' > Version '1.9'
+
+### `must_be_at_least`
+
+`must_be_at_least(actual, expected [, error_message])`
+
+Raise an error message if the actual version is older than the
+expected version.
+
+Usage:
+
+    PANDOC_VERSION:must_be_at_least('2.7.3')
+
 [Block]: #type-ref-Block
 [List]: #module-pandoc.list
 [MetaValue]: #type-ref-MetaValue
@@ -2726,3 +2754,8 @@ Parameters:
 Returns:
 
 -   The result(s) of the call to `callback`
+
+
+# Module pandoc.types
+
+Constructors for types which are not part of the pandoc AST.
diff --git a/pandoc.cabal b/pandoc.cabal
index 8bf09cdd1..25513d7db 100644
--- a/pandoc.cabal
+++ b/pandoc.cabal
@@ -339,8 +339,7 @@ extra-source-files:
                  test/odt/markdown/*.md
                  test/odt/native/*.native
                  test/lua/*.lua
-                 test/lua/module/pandoc.lua
-                 test/lua/module/pandoc.utils.lua
+                 test/lua/module/*.lua
 source-repository head
   type:          git
   location:      git://github.com/jgm/pandoc.git
@@ -593,9 +592,11 @@ library
                    Text.Pandoc.Lua.Marshaling.AnyValue,
                    Text.Pandoc.Lua.Marshaling.CommonState,
                    Text.Pandoc.Lua.Marshaling.ReaderOptions,
+                   Text.Pandoc.Lua.Marshaling.Version,
                    Text.Pandoc.Lua.Module.MediaBag,
                    Text.Pandoc.Lua.Module.Pandoc,
                    Text.Pandoc.Lua.Module.System,
+                   Text.Pandoc.Lua.Module.Types,
                    Text.Pandoc.Lua.Module.Utils,
                    Text.Pandoc.Lua.Packages,
                    Text.Pandoc.Lua.Util,
diff --git a/src/Text/Pandoc/Lua/Global.hs b/src/Text/Pandoc/Lua/Global.hs
index f303af6c5..b9b6c9cd9 100644
--- a/src/Text/Pandoc/Lua/Global.hs
+++ b/src/Text/Pandoc/Lua/Global.hs
@@ -17,7 +17,6 @@ module Text.Pandoc.Lua.Global
 
 import Prelude
 import Data.Data (Data)
-import Data.Version (Version (versionBranch))
 import Foreign.Lua (Lua, Peekable, Pushable)
 import Foreign.Lua.Userdata ( ensureUserdataMetatable, pushAnyWithMetatable
                             , metatableName)
@@ -52,7 +51,7 @@ setGlobal global = case global of
     Lua.push format
     Lua.setglobal "FORMAT"
   PANDOC_API_VERSION -> do
-    Lua.push (versionBranch pandocTypesVersion)
+    Lua.push pandocTypesVersion
     Lua.setglobal "PANDOC_API_VERSION"
   PANDOC_DOCUMENT doc -> do
     Lua.push (LazyPandoc doc)
@@ -67,7 +66,7 @@ setGlobal global = case global of
     Lua.push commonState
     Lua.setglobal "PANDOC_STATE"
   PANDOC_VERSION              -> do
-    Lua.push (versionBranch version)
+    Lua.push version
     Lua.setglobal "PANDOC_VERSION"
 
 -- | Readonly and lazy pandoc objects.
diff --git a/src/Text/Pandoc/Lua/Marshaling.hs b/src/Text/Pandoc/Lua/Marshaling.hs
index cc0451c09..8a1270ab7 100644
--- a/src/Text/Pandoc/Lua/Marshaling.hs
+++ b/src/Text/Pandoc/Lua/Marshaling.hs
@@ -14,3 +14,4 @@ module Text.Pandoc.Lua.Marshaling () where
 import Text.Pandoc.Lua.Marshaling.AST ()
 import Text.Pandoc.Lua.Marshaling.CommonState ()
 import Text.Pandoc.Lua.Marshaling.ReaderOptions ()
+import Text.Pandoc.Lua.Marshaling.Version ()
diff --git a/src/Text/Pandoc/Lua/Marshaling/Version.hs b/src/Text/Pandoc/Lua/Marshaling/Version.hs
new file mode 100644
index 000000000..3c667cbc4
--- /dev/null
+++ b/src/Text/Pandoc/Lua/Marshaling/Version.hs
@@ -0,0 +1,154 @@
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+{-# LANGUAGE LambdaCase           #-}
+{-# LANGUAGE NoImplicitPrelude    #-}
+{- |
+   Module      : Text.Pandoc.Lua.Marshaling.Version
+   Copyright   : © 2019 Albert Krewinkel
+   License     : GNU GPL, version 2 or above
+
+   Maintainer  : Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
+   Stability   : alpha
+
+Marshaling of @'Version'@s. The marshaled elements can be compared using
+default comparison operators (like @>@ and @<=@).
+-}
+module Text.Pandoc.Lua.Marshaling.Version
+  ( peekVersion
+  , pushVersion
+  )
+  where
+
+import Prelude
+import Data.Maybe (fromMaybe)
+import Data.Version (Version (..), makeVersion, parseVersion, showVersion)
+import Foreign.Lua (Lua, Optional (..), NumResults,
+                    Peekable, Pushable, StackIndex)
+import Foreign.Lua.Types.Peekable (reportValueOnFailure)
+import Foreign.Lua.Userdata (ensureUserdataMetatable, pushAnyWithMetatable,
+                             toAnyWithName)
+import Safe (atMay, lastMay)
+import Text.Pandoc.Lua.Marshaling.AnyValue (AnyValue (..))
+import Text.ParserCombinators.ReadP (readP_to_S)
+
+import qualified Foreign.Lua as Lua
+import qualified Text.Pandoc.Lua.Util as LuaUtil
+
+-- | Push a @'Version'@ element to the Lua stack.
+pushVersion :: Version -> Lua ()
+pushVersion version = pushAnyWithMetatable pushVersionMT version
+ where
+  pushVersionMT = ensureUserdataMetatable versionTypeName $ do
+    LuaUtil.addFunction "__eq" __eq
+    LuaUtil.addFunction "__le" __le
+    LuaUtil.addFunction "__lt" __lt
+    LuaUtil.addFunction "__len" __len
+    LuaUtil.addFunction "__index" __index
+    LuaUtil.addFunction "__pairs" __pairs
+    LuaUtil.addFunction "__tostring" __tostring
+
+instance Pushable Version where
+  push = pushVersion
+
+peekVersion :: StackIndex -> Lua Version
+peekVersion idx = Lua.ltype idx >>= \case
+  Lua.TypeString -> do
+    versionStr <- Lua.peek idx
+    let parses = readP_to_S parseVersion versionStr
+    case lastMay parses of
+      Just (v, "") -> return v
+      _  -> Lua.throwException $ "could not parse as Version: " ++ versionStr
+
+  Lua.TypeUserdata ->
+    reportValueOnFailure versionTypeName
+                         (`toAnyWithName` versionTypeName)
+                         idx
+  Lua.TypeNumber -> do
+    n <- Lua.peek idx
+    return (makeVersion [n])
+
+  Lua.TypeTable ->
+    makeVersion <$> Lua.peek idx
+
+  _ ->
+    Lua.throwException "could not peek Version"
+
+instance Peekable Version where
+  peek = peekVersion
+
+-- | Name used by Lua for the @CommonState@ type.
+versionTypeName :: String
+versionTypeName = "HsLua Version"
+
+__eq :: Version -> Version -> Lua Bool
+__eq v1 v2 = return (v1 == v2)
+
+__le :: Version -> Version -> Lua Bool
+__le v1 v2 = return (v1 <= v2)
+
+__lt :: Version -> Version -> Lua Bool
+__lt v1 v2 = return (v1 < v2)
+
+-- | Get number of version components.
+__len :: Version -> Lua Int
+__len = return . length . versionBranch
+
+-- | Access fields.
+__index :: Version -> AnyValue -> Lua NumResults
+__index v (AnyValue k) = do
+  ty <- Lua.ltype k
+  case ty of
+    Lua.TypeNumber -> do
+      n <- Lua.peek k
+      let versionPart = atMay (versionBranch v) (n - 1)
+      Lua.push (Lua.Optional versionPart)
+      return 1
+    Lua.TypeString -> do
+      str <- Lua.peek k
+      if str == "must_be_at_least"
+        then 1 <$ Lua.pushHaskellFunction must_be_at_least
+        else 1 <$ Lua.pushnil
+    _ -> 1 <$ Lua.pushnil
+
+-- | Create iterator.
+__pairs :: Version -> Lua NumResults
+__pairs v = do
+  Lua.pushHaskellFunction nextFn
+  Lua.pushnil
+  Lua.pushnil
+  return 3
+ where
+  nextFn :: AnyValue -> Optional Int -> Lua Lua.NumResults
+  nextFn _ (Optional key) =
+    case key of
+      Nothing -> case versionBranch v of
+                   []  -> 2 <$ (Lua.pushnil *> Lua.pushnil)
+                   n:_ -> 2 <$ (Lua.push (1 :: Int) *> Lua.push n)
+      Just n  -> case atMay (versionBranch v) n of
+                   Nothing -> 2 <$ (Lua.pushnil *> Lua.pushnil)
+                   Just b  -> 2 <$ (Lua.push (n + 1) *> Lua.push b)
+
+-- | Convert to string.
+__tostring :: Version -> Lua String
+__tostring v = return (showVersion v)
+
+-- | Default error message when a version is too old. This message is
+-- formatted in Lua with the expected and actual versions as arguments.
+versionTooOldMessage :: String
+versionTooOldMessage = "version too old: expected version %s or newer, got %s"
+
+-- | Throw an error if this version is older than the given version.
+-- FIXME: This function currently requires the string library to be
+-- loaded.
+must_be_at_least :: Version -> Version -> Optional String -> Lua NumResults
+must_be_at_least actual expected optMsg = do
+  let msg = fromMaybe versionTooOldMessage (fromOptional optMsg)
+  if expected <= actual
+    then return 0
+    else do
+      Lua.getglobal' "string.format"
+      Lua.push msg
+      Lua.push (showVersion expected)
+      Lua.push (showVersion actual)
+      Lua.call 3 1
+      Lua.error
+
diff --git a/src/Text/Pandoc/Lua/Module/Types.hs b/src/Text/Pandoc/Lua/Module/Types.hs
new file mode 100644
index 000000000..641bde7d6
--- /dev/null
+++ b/src/Text/Pandoc/Lua/Module/Types.hs
@@ -0,0 +1,28 @@
+{- |
+   Module      : Text.Pandoc.Lua.Module.Types
+   Copyright   : © 2019 Albert Krewinkel
+   License     : GNU GPL, version 2 or above
+
+   Maintainer  : Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
+   Stability   : alpha
+
+Pandoc data type constructors.
+-}
+module Text.Pandoc.Lua.Module.Types
+  ( pushModule
+  ) where
+
+import Prelude
+import Data.Version (Version)
+import Foreign.Lua (Lua, NumResults)
+import Text.Pandoc.Lua.Marshaling.Version ()
+import Text.Pandoc.Lua.Util (addFunction)
+
+import qualified Foreign.Lua as Lua
+
+-- | Push the pandoc.system module on the Lua stack.
+pushModule :: Lua NumResults
+pushModule = do
+  Lua.newtable
+  addFunction "Version" (return :: Version -> Lua Version)
+  return 1
diff --git a/src/Text/Pandoc/Lua/Module/Utils.hs b/src/Text/Pandoc/Lua/Module/Utils.hs
index 7f201a4b2..21e3f5674 100644
--- a/src/Text/Pandoc/Lua/Module/Utils.hs
+++ b/src/Text/Pandoc/Lua/Module/Utils.hs
@@ -17,6 +17,7 @@ import Prelude
 import Control.Applicative ((<|>))
 import Data.Char (toLower)
 import Data.Default (def)
+import Data.Version (Version)
 import Foreign.Lua (Peekable, Lua, NumResults)
 import Text.Pandoc.Class (runIO, setUserDataDir)
 import Text.Pandoc.Definition ( Pandoc, Meta, MetaValue (..), Block, Inline
@@ -43,6 +44,7 @@ pushModule mbDatadir = do
   addFunction "sha1" sha1
   addFunction "stringify" stringify
   addFunction "to_roman_numeral" toRomanNumeral
+  addFunction "Version" (return :: Version -> Lua Version)
   return 1
 
 -- | Squashes a list of blocks into inlines.
diff --git a/src/Text/Pandoc/Lua/Packages.hs b/src/Text/Pandoc/Lua/Packages.hs
index 5f2751f52..ca1779e27 100644
--- a/src/Text/Pandoc/Lua/Packages.hs
+++ b/src/Text/Pandoc/Lua/Packages.hs
@@ -26,6 +26,7 @@ import qualified Foreign.Lua as Lua
 import Text.Pandoc.Lua.Module.Pandoc as Pandoc
 import Text.Pandoc.Lua.Module.MediaBag as MediaBag
 import Text.Pandoc.Lua.Module.System as System
+import Text.Pandoc.Lua.Module.Types as Types
 import Text.Pandoc.Lua.Module.Utils as Utils
 
 -- | Parameters used to create lua packages/modules.
@@ -54,6 +55,7 @@ pandocPackageSearcher pkgParams pkgName =
                          in pushWrappedHsFun (Pandoc.pushModule datadir)
     "pandoc.mediabag" -> pushWrappedHsFun MediaBag.pushModule
     "pandoc.system"   -> pushWrappedHsFun System.pushModule
+    "pandoc.types"    -> pushWrappedHsFun Types.pushModule
     "pandoc.utils"    -> let datadir = luaPkgDataDir pkgParams
                          in pushWrappedHsFun (Utils.pushModule datadir)
     _ -> searchPureLuaLoader
diff --git a/test/Tests/Lua.hs b/test/Tests/Lua.hs
index c585182e4..7a1261eb2 100644
--- a/test/Tests/Lua.hs
+++ b/test/Tests/Lua.hs
@@ -15,7 +15,6 @@ module Tests.Lua ( runLuaTest, tests ) where
 
 import Prelude
 import Control.Monad (when)
-import Data.Version (Version (versionBranch))
 import System.FilePath ((</>))
 import Test.Tasty (TestTree, localOption)
 import Test.Tasty.HUnit (Assertion, assertEqual, testCase)
@@ -34,6 +33,7 @@ import Text.Pandoc.Options (def)
 import Text.Pandoc.Shared (pandocVersion)
 
 import qualified Foreign.Lua as Lua
+import qualified Data.ByteString.Char8 as BS
 
 tests :: [TestTree]
 tests = map (localOption (QuickCheckTests 20))
@@ -135,17 +135,14 @@ tests = map (localOption (QuickCheckTests 20))
       (doc $ para (str $ "lua" </> "script-name.lua"))
 
   , testCase "Pandoc version is set" . runLuaTest $ do
-      Lua.getglobal' "table.concat"
       Lua.getglobal "PANDOC_VERSION"
-      Lua.push ("." :: String) -- separator
-      Lua.call 2 1
-      Lua.liftIO . assertEqual "pandoc version is wrong" pandocVersion
-        =<< Lua.peek Lua.stackTop
+      Lua.liftIO .
+        assertEqual "pandoc version is wrong" (BS.pack pandocVersion)
+        =<< Lua.tostring' Lua.stackTop
 
   , testCase "Pandoc types version is set" . runLuaTest $ do
-      let versionNums = versionBranch pandocTypesVersion
       Lua.getglobal "PANDOC_API_VERSION"
-      Lua.liftIO . assertEqual "pandoc-types version is wrong" versionNums
+      Lua.liftIO . assertEqual "pandoc-types version is wrong" pandocTypesVersion
         =<< Lua.peek Lua.stackTop
 
   , testCase "Allow singleton inline in constructors" . runLuaTest $ do
diff --git a/test/Tests/Lua/Module.hs b/test/Tests/Lua/Module.hs
index 82c9330e5..324acce04 100644
--- a/test/Tests/Lua/Module.hs
+++ b/test/Tests/Lua/Module.hs
@@ -20,7 +20,8 @@ import Tests.Lua (runLuaTest)
 tests :: [TestTree]
 tests =
   [ testPandocLua "pandoc" ("lua" </> "module" </> "pandoc.lua")
-  , testPandocLua "pandoc.util" ("lua" </> "module" </> "pandoc.utils.lua")
+  , testPandocLua "pandoc.types" ("lua" </> "module" </> "pandoc-types.lua")
+  , testPandocLua "pandoc.util" ("lua" </> "module" </> "pandoc-utils.lua")
   ]
 
 testPandocLua :: TestName -> FilePath -> TestTree
diff --git a/test/lua/module/pandoc-types.lua b/test/lua/module/pandoc-types.lua
new file mode 100644
index 000000000..8c8d903d9
--- /dev/null
+++ b/test/lua/module/pandoc-types.lua
@@ -0,0 +1,112 @@
+local tasty = require 'tasty'
+local types = require 'pandoc.types'
+local Version = types.Version
+
+local assert = tasty.assert
+local test = tasty.test_case
+local group = tasty.test_group
+
+return {
+  group 'Version' {
+
+    group 'constructor' {
+      test('has type `userdata`', function ()
+        assert.are_same(type(Version {2}), 'userdata')
+      end),
+      test('accepts list of integers', function ()
+        assert.are_same(type(Version {2, 7, 3}), 'userdata')
+      end),
+      test('accepts a single integer', function ()
+        assert.are_same(Version(5), Version {5})
+      end),
+      test('accepts version as string', function ()
+        assert.are_same(
+          Version '4.45.1',
+          Version {4, 45, 1}
+        )
+      end),
+      test('non-version string is rejected', function ()
+        assert.error_matches(
+          function () Version '11friends' end,
+          '11friends'
+        )
+      end)
+    },
+
+    group 'comparison' {
+      test('smaller (equal) than', function ()
+        assert.is_truthy(Version {2, 58, 3} < Version {2, 58, 4})
+        assert.is_falsy(Version {2, 60, 1} < Version {2, 59, 2})
+        assert.is_truthy(Version {0, 14, 3} < Version {0, 14, 3, 1})
+        assert.is_truthy(Version {3, 58, 3} <= Version {4})
+        assert.is_truthy(Version {0, 14, 3} <= Version {0, 14, 3, 1})
+      end),
+      test('larger (equal) than', function ()
+        assert.is_truthy(Version{2,58,3} > Version {2, 57, 4})
+        assert.is_truthy(Version{2,58,3} > Version {2, 58, 2})
+        assert.is_truthy(Version {0, 8} >= Version {0, 8})
+        assert.is_falsy(Version {0, 8} >= Version {0, 8, 2})
+      end),
+      test('equality', function ()
+        assert.is_truthy(Version '8.8', Version {8, 8})
+      end),
+      test('second argument can be a version string', function ()
+        assert.is_truthy(Version '8' < '9.1')
+        assert.is_falsy(Version '8.8' < '8.7')
+      end),
+    },
+
+    group 'list-like behavior' {
+      test('can access version component numbers', function ()
+        local version = Version '2.7.3'
+        assert.is_nil(version[0])
+        assert.are_equal(version[1], 2)
+        assert.are_equal(version[2], 7)
+        assert.are_equal(version[3], 3)
+      end),
+      test('can be iterated over', function ()
+        local version_list = {2, 7, 3}
+        local final_index = 0
+        for i, v in pairs(Version(version_list)) do
+          assert.are_equal(v, version_list[i])
+          final_index = i
+        end
+        assert.are_equal(final_index, 3)
+      end),
+      test('length is the number of components', function ()
+        assert.are_equal(#(Version '0'), 1)
+        assert.are_equal(#(Version '1.6'), 2)
+        assert.are_equal(#(Version '8.7.5'), 3)
+        assert.are_equal(#(Version '2.9.1.5'), 4)
+      end)
+    },
+
+    group 'conversion to string' {
+      test('converting from and to string is a noop', function ()
+        local version_string = '1.19.4'
+        assert.are_equal(tostring(Version(version_string)), version_string)
+      end)
+    },
+
+    group 'convenience functions' {
+      test('throws error if version is too old', function ()
+        local actual = Version {2, 8}
+        local expected = Version {2, 9}
+        assert.error_matches(
+          function () actual:must_be_at_least(expected) end,
+          'version too old: expected version 2.9 or newer, got 2.8'
+        )
+      end),
+      test('does nothing if expected version is older than actual', function ()
+        local actual = Version '2.9'
+        local expected = Version '2.8'
+        actual:must_be_at_least(expected)
+      end),
+      test('does nothing if expected version equals to actual', function ()
+        local actual = Version '2.8'
+        local expected = Version '2.8'
+        actual:must_be_at_least(expected)
+      end)
+    }
+  }
+}
diff --git a/test/lua/module/pandoc.utils.lua b/test/lua/module/pandoc-utils.lua
index dc37ec354..dc37ec354 100644
--- a/test/lua/module/pandoc.utils.lua
+++ b/test/lua/module/pandoc-utils.lua