basicTypes/FieldLabel.hs

    {-
    %
    % (c) Adam Gundry 2013-2015
    %

    This module defines the representation of FieldLabels as stored in
    TyCons.  As well as a selector name, these have some extra structure
    to support the DuplicateRecordFields extension.

    In the normal case (with NoDuplicateRecordFields), a datatype like

        data T = MkT { foo :: Int }

    has

        FieldLabel { flLabel        = "foo"
                   , flIsOverloaded = False
                   , flSelector     = foo }.

    In particular, the Name of the selector has the same string
    representation as the label.  If DuplicateRecordFields
    is enabled, however, the same declaration instead gives

        FieldLabel { flLabel        = "foo"
                   , flIsOverloaded = True
                   , flSelector     = $sel:foo:MkT }.

    Now the name of the selector ($sel:foo:MkT) does not match the label of
    the field (foo).  We must be careful not to show the selector name to
    the user!  The point of mangling the selector name is to allow a
    module to define the same field label in different datatypes:

        data T = MkT { foo :: Int }
        data U = MkU { foo :: Bool }

    Now there will be two FieldLabel values for 'foo', one in T and one in
    U.  They share the same label (FieldLabelString), but the selector
    functions differ.

    See also Note [Representing fields in AvailInfo] in Avail.

    Note [Why selector names include data constructors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    As explained above, a selector name includes the name of the first
    data constructor in the type, so that the same label can appear
    multiple times in the same module.  (This is irrespective of whether
    the first constructor has that field, for simplicity.)

    We use a data constructor name, rather than the type constructor name,
    because data family instances do not have a representation type
    constructor name generated until relatively late in the typechecking
    process.

    Of course, datatypes with no constructors cannot have any fields.

    -}

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE DeriveFunctor #-}
    {-# LANGUAGE DeriveFoldable #-}
    {-# LANGUAGE DeriveTraversable #-}
    {-# LANGUAGE StandaloneDeriving #-}

    module FieldLabel ( FieldLabelString
                      , FieldLabelEnv
                      , FieldLbl(..)
                      , FieldLabel
                      , mkFieldLabelOccs
                      ) where

    import OccName
    import Name

    import FastString
    import Outputable
    import Binary

    import Data.Data

    #if __GLASGOW_HASKELL__ < 709
    import Data.Foldable ( Foldable )
    import Data.Traversable ( Traversable )
    #endif

    -- | Field labels are just represented as strings;
    -- they are not necessarily unique (even within a module)
    type FieldLabelString = FastString

    -- | A map from labels to all the auxiliary information
    type FieldLabelEnv = FastStringEnv FieldLabel

    type FieldLabel = FieldLbl Name

    -- | Fields in an algebraic record type
    data FieldLbl a = FieldLabel {
          flLabel        :: FieldLabelString, -- ^ User-visible label of the field
          flIsOverloaded :: Bool,             -- ^ Was DuplicateRecordFields on
                                              --   in the defining module for this datatype?
          flSelector     :: a                 -- ^ Record selector function
        }
      deriving (Eq, Functor, Foldable, Traversable, Typeable)
    deriving instance Data a => Data (FieldLbl a)

    instance Outputable a => Outputable (FieldLbl a) where
        ppr fl = ppr (flLabel fl) <> braces (ppr (flSelector fl))

    instance Binary a => Binary (FieldLbl a) where
        put_ bh (FieldLabel aa ab ac) = do
            put_ bh aa
            put_ bh ab
            put_ bh ac
        get bh = do
            ab <- get bh
            ac <- get bh
            ad <- get bh
            return (FieldLabel ab ac ad)

    -- | Record selector OccNames are built from the underlying field name
    -- and the name of the first data constructor of the type, to support
    -- duplicate record field names.
    -- See Note [Why selector names include data constructors].
    mkFieldLabelOccs :: FieldLabelString -> OccName -> Bool -> FieldLbl OccName
    mkFieldLabelOccs lbl dc is_overloaded
      = FieldLabel lbl is_overloaded sel_occ
      where
        str     = ":" ++ unpackFS lbl ++ ":" ++ occNameString dc
        sel_occ | is_overloaded = mkRecFldSelOcc str
                | otherwise     = mkVarOccFS lbl