luaforwindows/files/lua/metalua/extension/trycatch.mlua

-{ extension 'match' }

--------------------------------------------------------------------------------
--
-- TODO:
--
-- * Hygienize calls to pcall()
--
--------------------------------------------------------------------------------

-{ extension 'H' }
-{ extension 'log' }

-- Get match parsers and builder, for catch cases handling:
local match_alpha = require 'metalua.extension.match'
local H = H:new{side='inside', alpha = match_alpha }

-- We'll need to track rogue return statements:
require 'metalua.walk'

-- Put a block AST into a pcall():
local mkpcall = |block| +{pcall(function() -{block} end)}

-- The statement builder:
function trycatch_builder(x)
   --$log ("trycatch_builder", x, 'nohash', 60)
   local try_code, catch_cases, finally_code = unpack(x)
   local insert_return_catcher = false

   -- Can't be hygienize automatically by the current version of H, as
   -- it must bridge from inside user code (hacjed return statements)
   -- to outside macro code.
   local caught_return = !mlp.gensym 'caught_return'
   local saved_args

   !try_code; !(finally_code or { })
   -- FIXME: Am I sure there's no need to hygienize inside?
   --[[if catch_cases then
      for case in ivalues(catch_cases) do
         --$log(case,'nohash')
         local patterns, guard, block = unpack(case)
         ! block
      end
   end]]


   ----------------------------------------------------------------
   -- Returns in the try-block must be transformed:
   -- from the user's PoV, the code in the try-block isn't
   -- a function, therefore a return in it must not merely
   -- end the execution of the try block, but:
   --  * not cause any error to be caught;
   --  * let the finally-block be executed;
   --  * only then, let the enclosing function return with the
   --    appropraite values.
   -- The way to handle that is that any returned value is stored
   -- into the runtime variable caught_return, then a return with
   -- no value is sent, to stop the execution of the try-code.
   --
   -- Similarly, a return in a catch case code must not prevent
   -- the finally-code from being run.
   --
   -- This walker catches return statements and perform the relevant
   -- transformation into caught_return setting + empty return.
   --
   -- There is an insert_return_catcher compile-time flag, which
   -- allows to avoid inserting return-handling code in the result
   -- when not needed.
   ----------------------------------------------------------------
   local replace_returns_and_dots do
      local function f(x)
         match x with
         | `Return{...} ->
            insert_return_catcher = true
            -- Setvar's 'caught_return' code can't be hygienize by H currently.
            local setvar = `Set{ {caught_return}, { `Table{ unpack(x) } } }
            x <- { setvar; `Return }; x.tag = nil;
            --$log('transformed return stat:', x, 60)
            return 'break'
         | `Function{...} -> return 'break'
            -- inside this, returns would be the nested function's, not ours.
         | `Dots ->
            if not saved_args then saved_args = mlp.gensym 'args' end
            x <- `Call{ `Id 'unpack', saved_args }
         | _ -> -- pass
         end
      end
      local cfg = { stat = {down=f}, expr = {down=f} }
      replace_returns_and_dots = |x| walk.block(cfg, x)
   end

   -- parse returns in the try-block:
   replace_returns_and_dots (try_code)

   -- code handling the error catching process:
   local catch_result do
      if catch_cases and #catch_cases>0 then
         ----------------------------------------------------------
         -- Protect catch code against failures: they run in a pcall(), and
         -- the result is kept in catch_* vars so that it can be used to
         -- relaunch the error after the finally code has been executed.
         ----------------------------------------------------------
         for x in ivalues (catch_cases) do
            local case_code = x[3]
            -- handle rogue returns:
            replace_returns_and_dots (case_code)
            -- in case of error in the catch, we still need to run "finally":
            x[3] = +{block: catch_success, catch_error = -{mkpcall(case_code)}}
         end
         ----------------------------------------------------------
         -- Uncaught exceptions must not cause a mismatch,
         -- so we introduce a catch-all do-nothing last case:
         ----------------------------------------------------------
         table.insert (catch_cases, { { { `Id '_' } }, false, { } })
         catch_result = spmatch.match_builder{ {+{user_error}}, catch_cases }
      else
         catch_result = { }
      end
   end

   ----------------------------------------------------------------
   -- Build the bits of code that will handle return statements
   -- in the user code (try-block and catch-blocks).
   ----------------------------------------------------------------
   local caught_return_init, caught_return_rethrow do
      if insert_return_catcher then
         caught_return_init    = `Local{{caught_return}}
         caught_return_rethrow =
            +{stat: if -{caught_return} then return unpack(-{caught_return}) end}
      else
         caught_return_init, caught_return_rethrow = { }, { }
      end
   end

   local saved_args_init =
      saved_args and `Local{ {saved_args}, { `Table{`Dots} } } or { }

   -- The finally code, to execute no matter what:
   local finally_result = finally_code or { }

   -- And the whole statement, gluing all taht together:
   local result = +{stat:
      do
         -{ saved_args_init }
         -{ caught_return_init }
         local user_success,  user_error  = -{mkpcall(try_code)}
         local catch_success, catch_error = false, user_error
         if not user_success then -{catch_result} end
         -{finally_result}
         if not user_success and not catch_success then error(catch_error) end
         -{ caught_return_rethrow }
      end }

   H(result)

   return result
end

function catch_case_builder(x)
   --$log ("catch_case_builder", x, 'nohash', 60)
   local patterns, guard, _, code = unpack(x)
   -- patterns ought to be a pattern_group, but each expression must
   -- be converted into a single-element pattern_seq.
   for i = 1, #patterns do patterns[i] = {patterns[i]} end
   return { patterns, guard, code }
end

mlp.lexer:add{ 'try', 'catch', 'finally', '->' }
mlp.block.terminators:add{ 'catch', 'finally' }

mlp.stat:add{
   'try',
   mlp.block,
   gg.onkeyword{ 'catch',
      gg.list{
         gg.sequence{
            mlp.expr_list,
            gg.onkeyword{ 'if', mlp.expr },
            gg.optkeyword 'then',
            mlp.block,
            builder = catch_case_builder },
         separators = 'catch' } },
   gg.onkeyword{ 'finally', mlp.block },
   'end',
   builder = trycatch_builder }

return H.alpha