feat(outputters): Promote pseudo AST backend to full-blow outputter

This also comes with the ability for *any* outputter to pre-process the
AST tree if it wanted to (e.g. for remove leaf types it would not be
able to output. If the outputter doesn't do anything to the AST it is
just a pass-through step.

Note this does *not* yet walk the AST and do any AST munging that
functions might engage in, it only catches the top level tree as parsed
by the inputter.

Additionally it is not a full serialization of the AST that could be
read back in with an input driver. We'd need a new dependency on
something like serpent for that, which would be a lovely next step.

core/sile.lua

@@ -208,10 +208,6 @@ function SILE.init ()
    if not SILE.backend then
       SILE.backend = "libtexpdf"
    end
-   if SILE.backend == "ast" then
-      runEvals(SILE.input.evaluates, "evaluate")
-      return -- Short circuit for AST processing only
-   end
    if SILE.backend == "libtexpdf" then
       SILE.shaper = SILE.shapers.harfbuzz()
       SILE.outputter = SILE.outputters.libtexpdf()
@@ -227,6 +223,9 @@ function SILE.init ()
    elseif SILE.backend == "dummy" then
       SILE.shaper = SILE.shapers.dummy()
       SILE.outputter = SILE.outputters.dummy()
+   elseif SILE.backend == "ast" then
+      SILE.shaper = SILE.shapers.dummy()
+      SILE.outputter = SILE.outputters.ast()
    end
    SILE.pagebuilder = SILE.pagebuilders.base()
    io.stdout:setvbuf("no")
@@ -399,6 +398,7 @@ end
 -- iterate through the tree handling each item in the order encountered.
 -- @tparam table ast SILE content in abstract syntax tree format (a table of strings, functions, or more AST trees).
 function SILE.process (ast)
+   ast = SILE.outputter:preProcess(ast)
    if not ast then
       return
    end
@@ -699,9 +699,7 @@ end
 -- 5. Close out the Lua profiler if it was running.
 -- 6. Output version information if versions debug flag is set.
 function SILE.finish ()
-   if SILE.documentState.documentClass then
-      SILE.documentState.documentClass:finish()
-   end
+   SILE.documentState.documentClass:finish()
    SILE.font.finish()
    runEvals(SILE.input.evaluateAfters, "evaluate-after")
    if SILE.makeDeps then

inputters/base.lua

@@ -49,9 +49,6 @@ end
 function inputter:process (doc)
    -- Input parsers can already return multiple ASTs, but so far we only process one
    local tree = self:parse(doc)[1]
-   if not SILE.outputter then
-      return pl.pretty.dump(tree)
-   end
    self:requireClass(tree)
    return SILE.process(tree)
 end

outputters/ast.lua

@@ -0,0 +1,27 @@
+local base = require("outputters.base")
+
+local outputter = pl.class(base)
+outputter._name = "ast"
+outputter.extension = "ast"
+
+local outfile
+
+function outputter:_ensureInit ()
+   if not outfile then
+      local fname = self:getOutputFilename()
+      outfile = fname == "-" and io.stdout or io.open(fname, "w+")
+   end
+end
+
+function outputter:preProcess (ast)
+   self:_ensureInit()
+   local prettyprinted = pl.pretty.write(ast, "   ")
+   outfile:write(prettyprinted)
+end
+
+function outputter:finish ()
+   self:runHooks("prefinish")
+   outfile:close()
+end
+
+return outputter

outputters/base.lua

@@ -27,6 +27,10 @@ function outputter:runHooks (category, data)
    return data
 end
 
+function outputter:preProcess (ast)
+   return ast
+end
+
 function outputter.newPage () end
 
 function outputter:finish ()