Add lowmem flag.

This disables optimizations in select modules that cause
problems compiling in lower memory environments.

(LaTeX reader, LaTeX and Markdown writers)

Performance is 10x slower for reading LaTeX, 20% slower for
writing Markdown and LaTeX.

INSTALL.md

@@ -296,8 +296,12 @@ You will need cabal version 2.0 or higher.
     - `embed_data_files`: embed all data files into the binary (default no).
       This is helpful if you want to create a relocatable binary.
 
-    - `https`:  enable support for downloading resources over https
-      (using the `http-client` and `http-client-tls` libraries).
+    - `lowmem`: disable optimization on a few modules that take
+      a lot of memory to compile.  Use this flag if compiling
+      pandoc fails with out of memory errors.
+
+    - `trypandoc`:  build the `trypandoc` executable (a CGI web
+      app).
 
 3.  Build:
 

pandoc.cabal

@@ -392,6 +392,12 @@ flag embed_data_files
   Description:   Embed data files in binary for relocatable executable.
   Default:       False
 
+flag lowmem
+  Description:   Disable optimization on three modules that need a lot
+                 of memory to compile.  Set this flag if your pandoc
+                 build is failing due to out of memory errors.
+  Default:       False
+
 flag trypandoc
   Description:   Build trypandoc cgi executable.
   Default:       False
@@ -505,6 +511,8 @@ library
   if flag(embed_data_files)
      cpp-options:   -DEMBED_DATA_FILES
      other-modules: Text.Pandoc.Data
+  if flag(lowmem)
+     cpp-options:   -DLOWMEM
   hs-source-dirs:  src
 
   exposed-modules: Text.Pandoc,

src/Text/Pandoc/Readers/LaTeX.hs

@@ -1,4 +1,7 @@
 {-# LANGUAGE CPP                   #-}
+#ifdef LOWMEM
+{-# OPTIONS_GHC -O0                #-}
+#endif
 {-# LANGUAGE OverloadedStrings     #-}
 {-# LANGUAGE PatternGuards         #-}
 {-# LANGUAGE ScopedTypeVariables   #-}

src/Text/Pandoc/Writers/LaTeX.hs

@@ -1,3 +1,7 @@
+{-# LANGUAGE CPP                 #-}
+#ifdef LOWMEM
+{-# OPTIONS_GHC -O0              #-}
+#endif
 {-# LANGUAGE OverloadedStrings   #-}
 {-# LANGUAGE TupleSections       #-}
 {-# LANGUAGE PatternGuards       #-}

src/Text/Pandoc/Writers/Markdown.hs

@@ -1,3 +1,7 @@
+{-# LANGUAGE CPP                 #-}
+#ifdef LOWMEM
+{-# OPTIONS_GHC -O0              #-}
+#endif
 {-# LANGUAGE MultiWayIf          #-}
 {-# LANGUAGE OverloadedStrings   #-}
 {-# LANGUAGE ScopedTypeVariables #-}