Default files: combine with prior values when appropriate.

Certain command-line arguments can be repeated:
`--metadata-file`, `--css`, `--include-in-header`,
`--include-before-body`, `--include-after-body`, `--variable`,
`--metadata`, `--syntax-definition`.  In these cases, values
specified in default files should be added to the list rather
than replacing values specified earlier on the command line
(perhaps in other default files).

So, for example, if one does

    pandoc --variable foo=3 --defaults d1 --defaults d2

and `d1` sets the variable `bar` and `d2` sets `baz`,
all three variables will be set.

Closes #5894.

2 files changed, 54 insertions, 21 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index b20344ab0..ef737c452 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -1591,6 +1591,15 @@ of the user data directory, and then invoke these defaults
 from any directory using `pandoc --defaults letter`
 or `pandoc -dletter`.
 
+When multiple defaults are used, their contents will be combined.
+
+Note that, where command-line arguments may be repeated
+(`--metadata-file`, `--css`, `--include-in-header`,
+`--include-before-body`, `--include-after-body`, `--variable`,
+`--metadata`, `--syntax-definition`), the values specified on
+the command line will combine with values specified in the
+defaults file, rather than replacing them.
+
 # Templates
 
 When the `-s/--standalone` option is used, pandoc uses a template to
diff --git a/src/Text/Pandoc/App/Opt.hs b/src/Text/Pandoc/App/Opt.hs
index b5807a19c..402d09bb7 100644
--- a/src/Text/Pandoc/App/Opt.hs
+++ b/src/Text/Pandoc/App/Opt.hs
@@ -179,25 +179,37 @@ doOpt (k',v) = do
     "template" ->
       parseYAML v >>= \x -> return (\o -> o{ optTemplate = unpack <$> x })
     "variables" ->
-      parseYAML v >>= \x -> return (\o -> o{ optVariables = x })
+      parseYAML v >>= \x -> return (\o -> o{ optVariables =
+                                               optVariables o <> x })
     "metadata" ->
-      parseYAML v >>= \x -> return (\o -> o{ optMetadata = contextToMeta x })
+      parseYAML v >>= \x -> return (\o -> o{ optMetadata = optMetadata o <>
+                                               contextToMeta x })
     "metadata-files" ->
       (parseYAML v >>= \x ->
-                        return (\o -> o{ optMetadataFiles = map unpack x }))
+                        return (\o -> o{ optMetadataFiles =
+                                           optMetadataFiles o <>
+                                           map unpack x }))
     "metadata-file" -> -- allow either a list or a single value
-      (parseYAML v >>= \x -> return (\o -> o{ optMetadataFiles = map unpack x }))
+      (parseYAML v >>= \x -> return (\o -> o{ optMetadataFiles =
+                                                optMetadataFiles o <>
+                                                map unpack x }))
       <|>
       (parseYAML v >>= \x ->
-                        return (\o -> o{ optMetadataFiles = [unpack x] }))
+                        return (\o -> o{ optMetadataFiles =
+                                           optMetadataFiles o <>[unpack x] }))
     "output-file" ->
       parseYAML v >>= \x -> return (\o -> o{ optOutputFile = unpack <$> x })
     "input-files" ->
-      parseYAML v >>= \x -> return (\o -> o{ optInputFiles = map unpack x })
+      parseYAML v >>= \x -> return (\o -> o{ optInputFiles = optInputFiles o <>
+                                              map unpack x })
     "input-file" -> -- allow either a list or a single value
-      (parseYAML v >>= \x -> return (\o -> o{ optInputFiles = map unpack x }))
+      (parseYAML v >>= \x -> return (\o -> o{ optInputFiles =
+                                                optInputFiles o <>
+                                                map unpack x }))
       <|>
-      (parseYAML v >>= \x -> return (\o -> o{ optInputFiles = [unpack x] }))
+      (parseYAML v >>= \x -> return (\o -> o{ optInputFiles =
+                                                optInputFiles o <>
+                                                [unpack x] }))
     "number-sections" ->
       parseYAML v >>= \x -> return (\o -> o{ optNumberSections = x })
     "number-offset" ->
@@ -214,13 +226,16 @@ doOpt (k',v) = do
       parseYAML v >>= \x -> return (\o -> o{ optHighlightStyle = x })
     "syntax-definition" ->
       (parseYAML v >>= \x ->
-                return (\o -> o{ optSyntaxDefinitions = map unpack x }))
+                return (\o -> o{ optSyntaxDefinitions =
+                                   optSyntaxDefinitions o <> map unpack x }))
       <|>
       (parseYAML v >>= \x ->
-             return (\o -> o{ optSyntaxDefinitions = [unpack x] }))
+             return (\o -> o{ optSyntaxDefinitions =
+                                 optSyntaxDefinitions o <> [unpack x] }))
     "syntax-definitions" ->
       parseYAML v >>= \x ->
-             return (\o -> o{ optSyntaxDefinitions = map unpack x })
+             return (\o -> o{ optSyntaxDefinitions =
+                                optSyntaxDefinitions o <> map unpack x })
     "top-level-division" ->
       parseYAML v >>= \x -> return (\o -> o{ optTopLevelDivision = x })
     "html-math-method" ->
@@ -238,7 +253,8 @@ doOpt (k',v) = do
       parseYAML v >>= \x ->
              return (\o -> o{ optEpubMetadata = unpack <$> x })
     "epub-fonts" ->
-      parseYAML v >>= \x -> return (\o -> o{ optEpubFonts = map unpack x })
+      parseYAML v >>= \x -> return (\o -> o{ optEpubFonts = optEpubFonts o <>
+                                               map unpack x })
     "epub-chapter-level" ->
       parseYAML v >>= \x -> return (\o -> o{ optEpubChapterLevel = x })
     "epub-cover-image" ->
@@ -269,7 +285,7 @@ doOpt (k',v) = do
     "columns" ->
       parseYAML v >>= \x -> return (\o -> o{ optColumns = x })
     "filters" ->
-      parseYAML v >>= \x -> return (\o -> o{ optFilters = x })
+      parseYAML v >>= \x -> return (\o -> o{ optFilters = optFilters o <> x })
     "email-obfuscation" ->
       parseYAML v >>= \x -> return (\o -> o{ optEmailObfuscation = x })
     "identifier-prefix" ->
@@ -316,29 +332,37 @@ doOpt (k',v) = do
     "title-prefix" ->
       parseYAML v >>= \x -> return (\o -> o{ optTitlePrefix = x })
     "css" ->
-      (parseYAML v >>= \x -> return (\o -> o{ optCss = map unpack x }))
+      (parseYAML v >>= \x -> return (\o -> o{ optCss = optCss o <>
+                                                 map unpack x }))
       <|>
-      (parseYAML v >>= \x -> return (\o -> o{ optCss = [unpack x] }))
+      (parseYAML v >>= \x -> return (\o -> o{ optCss = optCss o <>
+                                                [unpack x] }))
     "ipynb-output" ->
       parseYAML v >>= \x -> return (\o -> o{ optIpynbOutput = x })
     "include-before-body" ->
       (parseYAML v >>= \x ->
-             return (\o -> o{ optIncludeBeforeBody = map unpack x }))
+             return (\o -> o{ optIncludeBeforeBody =
+                                optIncludeBeforeBody o <> map unpack x }))
       <|>
       (parseYAML v >>= \x ->
-             return (\o -> o{ optIncludeBeforeBody = [unpack x] }))
+             return (\o -> o{ optIncludeBeforeBody =
+                                optIncludeBeforeBody o <> [unpack x] }))
     "include-after-body" ->
       (parseYAML v >>= \x ->
-             return (\o -> o{ optIncludeAfterBody = map unpack x }))
+             return (\o -> o{ optIncludeAfterBody =
+                                optIncludeAfterBody o <> map unpack x }))
       <|>
       (parseYAML v >>= \x ->
-             return (\o -> o{ optIncludeAfterBody = [unpack x] }))
+             return (\o -> o{ optIncludeAfterBody =
+                                optIncludeAfterBody o <> [unpack x] }))
     "include-in-header" ->
       (parseYAML v >>= \x ->
-             return (\o -> o{ optIncludeInHeader = map unpack x }))
+             return (\o -> o{ optIncludeInHeader =
+                                optIncludeInHeader o <> map unpack x }))
       <|>
       (parseYAML v >>= \x ->
-             return (\o -> o{ optIncludeInHeader = [unpack x] }))
+             return (\o -> o{ optIncludeInHeader =
+                                optIncludeInHeader o <> [unpack x] }))
     "resource-path" ->
       parseYAML v >>= \x ->
              return (\o -> o{ optResourcePath = map unpack x })