Beamer writer: support exampleblock and alertblock.

A block will be rendered as an exampleblock if the heading
has class `example` and alertblock if it has class `alert`.

Closes #7278.

MANUAL.txt

@@ -5666,7 +5666,11 @@ rules:
   * A heading at the slide level always starts a new slide.
 
   * Headings *below* the slide level in the hierarchy create
-    headings *within* a slide.
+    headings *within* a slide.  (In beamer, a "block" will be
+    created.  If the heading has the class `example`, an
+    `exampleblock` environment will be used; if it has the class
+    `alert`, an `alertblock` will be used; otherwise a regular
+    `block` will be used.)
 
   * Headings *above* the slide level in the hierarchy create
     "title slides," which just contain the section title

src/Text/Pandoc/Writers/LaTeX.hs

@@ -290,7 +290,12 @@ blockToLaTeX :: PandocMonad m
              => Block     -- ^ Block to convert
              -> LW m (Doc Text)
 blockToLaTeX Null = return empty
-blockToLaTeX (Div attr@(identifier,"block":_,_) (Header _ _ ils : bs)) = do
+blockToLaTeX (Div attr@(identifier,"block":dclasses,_)
+             (Header _ _ ils : bs)) = do
+  let blockname
+        | "example" `elem` dclasses = "exampleblock"
+        | "alert" `elem` dclasses = "alertblock"
+        | otherwise = "block"
   ref <- toLabel identifier
   let anchor = if T.null identifier
                   then empty
@@ -298,8 +303,8 @@ blockToLaTeX (Div attr@(identifier,"block":_,_) (Header _ _ ils : bs)) = do
                        braces (literal ref) <> braces empty
   title' <- inlineListToLaTeX ils
   contents <- blockListToLaTeX bs
-  wrapDiv attr $ ("\\begin{block}" <> braces title' <> anchor) $$
-                 contents $$ "\\end{block}"
+  wrapDiv attr $ ("\\begin" <> braces blockname <> braces title' <> anchor) $$
+                 contents $$ "\\end" <> braces blockname
 blockToLaTeX (Div (identifier,"slide":dclasses,dkvs)
                (Header _ (_,hclasses,hkvs) ils : bs)) = do
   -- note: [fragile] is required or verbatim breaks

test/command/7278.md

@@ -0,0 +1,38 @@
+```
+% pandoc -t beamer
+# Slide
+
+Some blocks:
+
+## example block title {.example}
+
+text in block
+
+## alert block title {.alert}
+
+text in block
+
+## block title
+
+text in block
+^D
+\begin{frame}{Slide}
+\protect\hypertarget{slide}{}
+Some blocks:
+
+\begin{exampleblock}{example block title}
+\protect\hypertarget{example-block-title}{}
+text in block
+\end{exampleblock}
+
+\begin{alertblock}{alert block title}
+\protect\hypertarget{alert-block-title}{}
+text in block
+\end{alertblock}
+
+\begin{block}{block title}
+\protect\hypertarget{block-title}{}
+text in block
+\end{block}
+\end{frame}
+```