run-liquid.lua: Add error checking and file output parameter

When running `make epub`, Make would try to satisfy the target
'src/plfa/acknowledgements_epub.md' by calling run-liquid.lua.  But due
to the use of shell redirection, Make would always create the file
acknowledgements_epub.md, even if run-liquid.lua failed, in which case
acknowledgements_epub.md would be empty.  This empty file would still
satisfy the 'src/plfa/acknowledgements_epub.md' target, allowing the
pandoc command line in the 'out/plfa.epub' target to proceed---and later
fail.

To avoid this, we now (a) add a bunch of error checking to run-liquid.lua,
and (b) write directly to acknowledgements_epub.md instead of relying on
shell redirection, so that acknowledgements_epub.md will only be created
(or modified) if run-liquid.lua succeeds.

Makefile

@@ -70,7 +70,7 @@ out/plfa.epub: out/ $(AGDA) $(LUA) epub/main.css src/plfa/acknowledgements_epub.
 		epub/index.md
 
 src/plfa/acknowledgements_epub.md: src/plfa/acknowledgements.md _config.yml 
-	lua epub/run-liquid.lua _config.yml $< > $@
+	lua epub/run-liquid.lua _config.yml $< $@
 
 
 # Convert literal Agda to Markdown

epub/run-liquid.lua

@@ -1,19 +1,52 @@
 local yaml = require 'tinyyaml'
 local liquid = require 'liquid'
 
-if #arg ~= 2 then
-    print('usage: ' .. arg[0] .. ' [yaml_file] [markdown_file]')
+-- Given a file name, return its file descriptor.
+-- Throws an error on failure.
+local function errOpen (fname, mode)
+    local fd = io.open(fname, mode)
+    if fd == nil then
+        error('could not open file: ' .. fname)
+    end
+    return fd
+end
+
+-- Given a file name and some data, overwrite the file with the data.
+-- Throws an error on failure.
+local function errWrite (fname, data)
+    local fd = errOpen(fname, 'w')
+    local status, errstr = fd:write(data)
+    if status == nil then
+        error(fname .. ': ' .. errstr)
+    end
+end
+
+-- Given a file name, return that file's entire contents.
+-- Throws an error on failure.
+local function errReadAll (fname)
+    local data = errOpen(fname, 'r'):read('a')
+    if data == nil then
+        error('could not read from open file: ' .. fname)
+    end
+    return data
+end
+
+
+-- We must have exactly three arguments.
+if #arg ~= 3 then
+    print('usage: ' .. arg[0] .. ' [yaml_file] [markdown_in_file] [markdown_out_file]')
     os.exit(1)
 end
 
 -- 1. Read YAML metadata from file, which we nest under the key 'site'.
-local metadata = { ['site'] = yaml.parse(io.open(arg[1]):read('a')) }
+local metadata = { ['site'] = yaml.parse(errReadAll(arg[1])) }
 
 -- 2. Read markdown document from file.
-local document = io.open(arg[2]):read('a')
+local document = errReadAll(arg[2])
 
 -- 3. Render the markdown document with the YAML metadata as context.
 local template = liquid.Template:parse(document)
 local result = template:render(liquid.InterpreterContext:new(metadata))
 
-print(result)
+-- 4. Write rendered document to output file.
+errWrite(arg[3], result)