LaTeX reader: improved parsing of \def, \let.

We now correctly parse:

```
\def\bar{hello}
\let\fooi\bar
\def\fooii{\bar}
\fooi +\fooii

\def\bar{goodbye}
\fooi +\fooii
```

src/Text/Pandoc/Readers/LaTeX.hs

@@ -1506,21 +1506,28 @@ macroDef =
 letmacro :: PandocMonad m => LP m (Text, Macro)
 letmacro = do
   controlSeq "let"
-  Tok _ (CtrlSeq name) _ <- anyControlSeq
-  optional $ symbol '='
-  spaces
-  contents <- bracedOrToken
-  return (name, Macro ExpandWhenDefined [] Nothing contents)
+  (name, contents) <- withVerbatimMode $ do
+    Tok _ (CtrlSeq name) _ <- withVerbatimMode anyControlSeq
+    optional $ symbol '='
+    spaces
+    -- we first parse in verbatim mode, and then expand macros,
+    -- because we don't want \let\foo\bar to turn into
+    -- \let\foo hello if we have previously \def\bar{hello}
+    contents <- bracedOrToken
+    return (name, contents)
+  contents' <- fromMaybe contents <$> doMacros' 0 contents
+  return (name, Macro ExpandWhenDefined [] Nothing contents')
 
 defmacro :: PandocMonad m => LP m (Text, Macro)
-defmacro = try $ do
-  controlSeq "def"
-  Tok _ (CtrlSeq name) _ <- anyControlSeq
-  argspecs <- many (argspecArg <|> argspecPattern)
+defmacro = try $
   -- we use withVerbatimMode, because macros are to be expanded
   -- at point of use, not point of definition
-  contents <- withVerbatimMode bracedOrToken
-  return (name, Macro ExpandWhenUsed argspecs Nothing contents)
+  withVerbatimMode $ do
+    controlSeq "def"
+    Tok _ (CtrlSeq name) _ <- anyControlSeq
+    argspecs <- many (argspecArg <|> argspecPattern)
+    contents <- bracedOrToken
+    return (name, Macro ExpandWhenUsed argspecs Nothing contents)
 
 argspecArg :: PandocMonad m => LP m ArgSpec
 argspecArg = do

src/Text/Pandoc/Readers/LaTeX/Parsing.hs

@@ -49,6 +49,7 @@ module Text.Pandoc.Readers.LaTeX.Parsing
   , toksToString
   , satisfyTok
   , doMacros
+  , doMacros'
   , setpos
   , anyControlSeq
   , anySymbol
@@ -451,22 +452,28 @@ doMacros' n inp = do
       case M.lookup name macros of
            Nothing -> return Nothing
            Just (Macro expansionPoint argspecs optarg newtoks) -> do
-             setInput ts
-             args <- case optarg of
-                          Nothing -> getargs M.empty argspecs
-                          Just o  -> do
-                             x <- option o bracketedToks
-                             getargs (M.singleton 1 x) argspecs
-             -- first boolean param is true if we're tokenizing
-             -- an argument (in which case we don't want to
-             -- expand #1 etc.)
-             ts' <- getInput
-             let result = foldr (addTok False args spos) ts' newtoks
-             case expansionPoint of
-               ExpandWhenUsed    ->
-                  doMacros' (n' + 1) result >>=
-                    maybe (return (Just result)) (return . Just)
-               ExpandWhenDefined -> return $ Just result
+             let getargs' = do
+                   args <- case optarg of
+                             Nothing -> getargs M.empty argspecs
+                             Just o  -> do
+                                x <- option o bracketedToks
+                                getargs (M.singleton 1 x) argspecs
+                   rest <- getInput
+                   return (args, rest)
+             lstate <- getState
+             res <- lift $ runParserT getargs' lstate "args" ts
+             case res of
+               Left _ -> fail $ "Could not parse arguments for " ++
+                                T.unpack name
+               Right (args, rest) -> do
+                 -- first boolean param is true if we're tokenizing
+                 -- an argument (in which case we don't want to
+                 -- expand #1 etc.)
+                 let result = foldr (addTok False args spos) rest newtoks
+                 case expansionPoint of
+                   ExpandWhenUsed    ->
+                      maybe (Just result) Just <$> doMacros' (n' + 1) result
+                   ExpandWhenDefined -> return $ Just result
 
 setpos :: SourcePos -> Tok -> Tok
 setpos spos (Tok _ tt txt) = Tok spos tt txt