    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE ConstraintKinds #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE StandaloneDeriving #-}

    module PlaceHolder where

    import GhcPrelude ()

    import Type       ( Type )
    import Outputable
    import Name
    import NameSet
    import RdrName
    import Var

    import Data.Data hiding ( Fixity )

    {-
    %************************************************************************
    %*                                                                      *
    \subsection{Annotating the syntax}
    %*                                                                      *
    %************************************************************************
    -}

    -- NB: These are intentionally open, allowing API consumers (like Haddock)
    -- to declare new instances

    -- | used as place holder in PostTc and PostRn values
    data PlaceHolder = PlaceHolder
      deriving (Data)

    placeHolderKind :: PlaceHolder
    placeHolderKind = PlaceHolder

    placeHolderFixity :: PlaceHolder
    placeHolderFixity = PlaceHolder

    placeHolderType :: PlaceHolder
    placeHolderType = PlaceHolder

    placeHolderTypeTc :: Type
    placeHolderTypeTc = panic "Evaluated the place holder for a PostTcType"

    placeHolderNames :: PlaceHolder
    placeHolderNames = PlaceHolder

    placeHolderNamesTc :: NameSet
    placeHolderNamesTc = emptyNameSet

    placeHolderHsWrapper :: PlaceHolder
    placeHolderHsWrapper = PlaceHolder

    {-

    Note [Pass sensitive types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Since the same AST types are re-used through parsing,renaming and type
    checking there are naturally some places in the AST that do not have
    any meaningful value prior to the pass they are assigned a value.

    Historically these have been filled in with place holder values of the form

      panic "error message"

    This has meant the AST is difficult to traverse using standard generic
    programming techniques. The problem is addressed by introducing
    pass-specific data types, implemented as a pair of open type families,
    one for PostTc and one for PostRn. These are then explicitly populated
    with a PlaceHolder value when they do not yet have meaning.

    In terms of actual usage, we have the following

      PostTc id Kind
      PostTc id Type

      PostRn id Fixity
      PostRn id NameSet

    TcId and Var are synonyms for Id

    Unfortunately the type checker termination checking conditions fail for the
    DataId constraint type based on this, so even though it is safe the
    UndecidableInstances pragma is required where this is used.
    -}

    -- |Follow the @id@, but never beyond Name. This is used in a 'HsMatchContext',
    -- for printing messages related to a 'Match'
    type family NameOrRdrName id where
      NameOrRdrName Id      = Name
      NameOrRdrName Name    = Name
      NameOrRdrName RdrName = RdrName