feat(inputters): Add AST inputter for reading back SILE's AST outputter

core/sile.lua

@@ -431,7 +431,7 @@ function SILE.process (ast)
    end
 end
 
-local preloadedinputters = { "xml", "lua", "sil" }
+local preloadedinputters = { "xml", "lua", "ast", "sil" }
 
 local function detectFormat (doc, filename)
    -- Preload default reader types so content detection has something to work with

inputters/ast.lua

@@ -0,0 +1,42 @@
+local base = require("inputters.base")
+local serpent = require("serpent")
+
+local inputter = pl.class(base)
+inputter._name = "ast"
+
+inputter.order = 35
+
+function inputter.appropriate (round, filename, doc)
+   if round == 1 then
+      return filename:match(".ast$")
+   elseif round == 2 then
+      local sniff = doc:sub(1, 100)
+      local promising = sniff:match("^{\n   [^ ]") or sniff:match("command =") or sniff:match("loadstring or load")
+      return promising and inputter.appropriate(3, filename, doc) or false
+   elseif round == 3 then
+      local status, _ = serpent.load(doc, { safe = true })
+      return status and true or false
+   end
+end
+
+function inputter.parse (_, doc)
+   local status, result = serpent.load(doc, { safe = true })
+   if not status then
+      SU.error(result)
+   end
+   return result
+end
+
+function inputter:process (doc)
+   local tree = self:parse(doc)
+   if not tree.type then
+      -- hoping tree is an AST
+      self:requireClass(tree)
+      return SILE.process(tree)
+   else
+      SILE.use(tree, self.options)
+   end
+end
+
+return inputter
+

outputters/ast.lua

@@ -29,6 +29,8 @@ function outputter:preProcess (ast)
       }
    )
    outfile:write(serialized)
+   -- Only dump the *first* AST we're given dodging SILE generated ones (like folios) later
+   self.preProcess = function () end
 end
 
 function outputter:finish ()