From 4df10a81d9c3294e496ea1cfbcf1871533497630 Mon Sep 17 00:00:00 2001
From: Yihui Xie <xie@yihui.name>
Date: Wed, 26 Feb 2025 11:48:36 -0600
Subject: [PATCH] drop YAML headers in non-index chapters

---
 DESCRIPTION |  2 +-
 NEWS.md     |  2 ++
 R/preview.R | 12 ++++++++----
 R/site.R    |  5 +++--
 R/utils.R   |  7 +++++++
 5 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/DESCRIPTION b/DESCRIPTION
index 3dddbf2..c6f1e34 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,7 +1,7 @@
 Package: litedown
 Type: Package
 Title: A Lightweight Version of R Markdown
-Version: 0.5.16
+Version: 0.5.17
 Authors@R: c(
     person("Yihui", "Xie", role = c("aut", "cre"), email = "xie@yihui.name", comment = c(ORCID = "0000-0003-0645-5666", URL = "https://yihui.org")),
     person("Tim", "Taylor", role = "ctb", comment = c(ORCID = "0000-0002-8587-7113")),
diff --git a/NEWS.md b/NEWS.md
index 9670b01..ab11c98 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -10,6 +10,8 @@
 
 - Fixed a bug that `fuse()` fails to print the error location when the whole input document consists of a single chunk that throws an error (thanks, @kevinushey, yihui/knitr#2387).
 
+- `fuse_book()` will ignore YAML headers in book chapters except for the index chapter.
+
 # CHANGES IN litedown VERSION 0.5
 
 - Added a wizard in `roam()` to create new `.Rmd`/`.md`/`.R` files with selected HTML features.
diff --git a/R/preview.R b/R/preview.R
index ba6b591..8ba6060 100644
--- a/R/preview.R
+++ b/R/preview.R
@@ -219,7 +219,7 @@ file_resp = function(x, preview) {
     list(payload = switch(
       info$type,
       book = {
-        store_book(info$root, x)
+        store_book(info$root, x, info$index)
         fuse_book(if (info$index) info$root else x, full_output, globalenv())
       },
       site = fuse_site(x),
@@ -240,9 +240,13 @@ file_resp = function(x, preview) {
 
 # store book dir for books to resolve number_refs() because the book may be
 # partially rendered (in which case we can't resolve refs to other chapters)
-store_book = function(dir, x) {
-  .env$current_book = dir; .env$current_file = x
-  exit_call(function() .env$current_book <- .env$current_file <- NULL)
+store_book = function(dir, x, index = FALSE) {
+  f = function(...) .mapply(
+    function(n, v) .env[[paste0('current_', n)]] = v,
+    c('book', 'file', 'index'), list(...)
+  )
+  f(dir, x, index)
+  exit_call(function() f(NULL))
 }
 
 # detect project type for a directory (_litedown.yml may be in an upper-level dir)
diff --git a/R/site.R b/R/site.R
index 9818742..bd6827f 100644
--- a/R/site.R
+++ b/R/site.R
@@ -232,8 +232,9 @@ fuse_book = function(input = '.', output = NULL, envir = parent.frame()) {
         fuse(x, fmt, NULL, envir)
       }
     }
-    # remove YAML in the preview mode since we only need the body
-    if (length(preview)) out = xfun::yaml_body(split_lines(out), parse = FALSE)$body
+    # remove YAML in the preview mode or for non-index chapters since we only need the body
+    if (length(preview) || isFALSE(.env$current_index) || x != input[1])
+      out = sans_yaml(out)
 
     if (format != 'html') return(out)
     # add input filenames to the end for HTML output and wrap each file in a div
diff --git a/R/utils.R b/R/utils.R
index 991227d..993aecd 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -31,6 +31,13 @@ sans_p = function(x) gsub('^<p[^>]*>|(</p>)?\n$', '', x)
 # remove ugly single quotes, e.g., 'LaTeX' -> LaTeX
 sans_sq = function(x) gsub("(^|\\W)'([^']+)'(\\W|$)", '\\1\\2\\3', x)
 
+# remove YAML header
+sans_yaml = function(x) {
+  if (length(x) && grepl('^---\\s*?($|\n)', x[1]))
+    x = xfun::yaml_body(split_lines(x), parse = FALSE)$body
+  x
+}
+
 is_lang = function(x) is.symbol(x) || is.language(x)
 
 uapply = function(..., recursive = TRUE) unlist(lapply(...), recursive = recursive)