{-# LANGUAGE TupleSections #-}
{-|
Module      : Parsley.Internal.Backend.Machine.LetRecBuilder
Description : Function for building recursive groups.
License     : BSD-3-Clause
Maintainer  : Jamie Willis
Stability   : experimental

Exposes the `letRec` function, used to provide a recursive /group/ of bindings
for the top level of a parser.

@since 1.0.0.0
-}
module Parsley.Internal.Backend.Machine.LetRecBuilder (letRec) where

import Data.Dependent.Sum                           (DSum((:=>)))
import Data.Functor.Const                           (Const(..))
import Data.GADT.Compare                            (GCompare)
import Data.Some                                    (Some(Some))
import Language.Haskell.TH                          (newName, Name)
import Language.Haskell.TH.Syntax                   (Exp(VarE, LetE), Dec(FunD), Clause(Clause), Body(NormalB))
import Parsley.Internal.Backend.Machine.LetBindings (LetBinding(..), Metadata, Binding, Regs)
import Parsley.Internal.Backend.Machine.THUtils     (unsafeCodeCoerce, unTypeCode)
import Parsley.Internal.Backend.Machine.Types       (QSubroutine, qSubroutine, Func)
import Parsley.Internal.Common.Utils                (Code)

import Data.Dependent.Map as DMap (DMap, (!), map, toList, traverseWithKey)

{-|
Given a collection of bindings, generates a recursive binding group where each is allowed to
refer to every other. These are then in scope for the top-level parser.

@since 1.5.0.0
-}
letRec :: GCompare key
       => {-bindings-}  DMap key (LetBinding o a)   -- ^ The bindings that should form part of the recursive group
      -> {-nameof-}     (forall x. key x -> String) -- ^ A function which can give a name to a key in the map
      -> {-genBinding-} (forall x rs. key x -> Binding o a x -> Regs rs -> DMap key (QSubroutine s o a) -> Metadata -> Code (Func rs s o a x))
      -- ^ How a binding - and their free registers - should be converted into code
      -> {-expr-}       (DMap key (QSubroutine s o a) -> Code b)
      -- ^ How to produce the top-level binding given the compiled bindings, i.e. the @in@ for the @let@
      -> Code b
letRec bindings nameOf genBinding expr = unsafeCodeCoerce $
  do -- Make a bunch of names
     names <- traverseWithKey (\k (LetBinding _ rs meta) -> Const . (, rs, meta) <$> newName (nameOf k)) bindings
     -- Wrap them up so that they are valid typed template haskell names
     let typedNames = DMap.map makeTypedName names
     -- Generate each binding providing them with the names
     let makeDecl (k :=> LetBinding body (Some frees) _) =
          do let Const (name, _, meta) = names ! k
             func <- unTypeCode (genBinding k body frees typedNames meta)
             return (FunD name [Clause [] (NormalB func) []])
     decls <- traverse makeDecl (toList bindings)
     -- Generate the main expression using the same names
     exp <- unTypeCode (expr typedNames)
     -- Construct the let expression
     return (LetE decls exp)
  where
     makeTypedName :: Const (Name, Some Regs, Metadata) x -> QSubroutine s o a x
     makeTypedName (Const (name, Some frees, meta)) = qSubroutine (unsafeCodeCoerce (return (VarE name))) frees meta