GHC/Types/FieldLabel.hs

    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE FlexibleContexts   #-}
    {-# LANGUAGE UndecidableInstances #-}

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

    Note [FieldLabel]
    ~~~~~~~~~~~~~~~~~

    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 and NoFieldSelectors extensions.

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

        data T = MkT { foo :: Int }

    has

        FieldLabel { flLabel                    = "foo"
                   , flHasDuplicateRecordFields = NoDuplicateRecordFields
                   , flHasFieldSelector         = FieldSelectors
                   , 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"
                   , flHasDuplicateRecordFields = DuplicateRecordFields
                   , flHasFieldSelector         = FieldSelectors
                   , flSelector                 = $sel:foo:MkT }.

    Similarly, the selector name will be mangled if NoFieldSelectors is used
    (whether or not DuplicateRecordFields is enabled).  See Note [NoFieldSelectors]
    in GHC.Rename.Env.

    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 GHC.Types.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.

    -}

    module GHC.Types.FieldLabel
       ( FieldLabelString
       , FieldLabelEnv
       , FieldLabel(..)
       , fieldSelectorOccName
       , fieldLabelPrintableName
       , DuplicateRecordFields(..)
       , FieldSelectors(..)
       , flIsOverloaded
       )
    where

    import GHC.Prelude

    import {-# SOURCE #-} GHC.Types.Name.Occurrence
    import {-# SOURCE #-} GHC.Types.Name

    import GHC.Data.FastString
    import GHC.Data.FastString.Env
    import GHC.Utils.Outputable
    import GHC.Utils.Binary

    import Data.Bool
    import Data.Data

    -- | 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 = DFastStringEnv FieldLabel

    -- | Fields in an algebraic record type; see Note [FieldLabel].
    data FieldLabel = FieldLabel {
          flLabel :: FieldLabelString,
          -- ^ User-visible label of the field
          flHasDuplicateRecordFields :: DuplicateRecordFields,
          -- ^ Was @DuplicateRecordFields@ on in the defining module for this datatype?
          flHasFieldSelector :: FieldSelectors,
          -- ^ Was @FieldSelectors@ enabled in the defining module for this datatype?
          -- See Note [NoFieldSelectors] in GHC.Rename.Env
          flSelector :: Name
          -- ^ Record selector function
        }
      deriving (Data, Eq)

    instance HasOccName FieldLabel where
      occName = mkVarOccFS . flLabel

    instance Outputable FieldLabel where
        ppr fl = ppr (flLabel fl) <> whenPprDebug (braces (ppr (flSelector fl))
                                                    <> ppr (flHasDuplicateRecordFields fl)
                                                    <> ppr (flHasFieldSelector fl))

    -- | Flag to indicate whether the DuplicateRecordFields extension is enabled.
    data DuplicateRecordFields
        = DuplicateRecordFields   -- ^ Fields may be duplicated in a single module
        | NoDuplicateRecordFields -- ^ Fields must be unique within a module (the default)
      deriving (Show, Eq, Typeable, Data)

    instance Binary DuplicateRecordFields where
        put_ bh f = put_ bh (f == DuplicateRecordFields)
        get bh = bool NoDuplicateRecordFields DuplicateRecordFields <$> get bh

    instance Outputable DuplicateRecordFields where
        ppr DuplicateRecordFields   = text "+dup"
        ppr NoDuplicateRecordFields = text "-dup"

    -- | Flag to indicate whether the FieldSelectors extension is enabled.
    data FieldSelectors
        = FieldSelectors   -- ^ Selector functions are available (the default)
        | NoFieldSelectors -- ^ Selector functions are not available
      deriving (Show, Eq, Typeable, Data)

    instance Binary FieldSelectors where
        put_ bh f = put_ bh (f == FieldSelectors)
        get bh = bool NoFieldSelectors FieldSelectors <$> get bh

    instance Outputable FieldSelectors where
        ppr FieldSelectors   = text "+sel"
        ppr NoFieldSelectors = text "-sel"

    -- | We need the @Binary Name@ constraint here even though there is an instance
    -- defined in "GHC.Types.Name", because the we have a SOURCE import, so the
    -- instance is not in scope.  And the instance cannot be added to Name.hs-boot
    -- because "GHC.Utils.Binary" itself depends on "GHC.Types.Name".
    instance Binary Name => Binary FieldLabel where
        put_ bh (FieldLabel aa ab ac ad) = do
            put_ bh aa
            put_ bh ab
            put_ bh ac
            put_ bh ad
        get bh = do
            aa <- get bh
            ab <- get bh
            ac <- get bh
            ad <- get bh
            return (FieldLabel aa 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].
    fieldSelectorOccName :: FieldLabelString -> OccName -> DuplicateRecordFields -> FieldSelectors -> OccName
    fieldSelectorOccName lbl dc dup_fields_ok has_sel
      | shouldMangleSelectorNames dup_fields_ok has_sel = mkRecFldSelOcc str
      | otherwise     = mkVarOccFS lbl
      where
        str     = ":" ++ unpackFS lbl ++ ":" ++ occNameString dc

    -- | Undo the name mangling described in Note [FieldLabel] to produce a Name
    -- that has the user-visible OccName (but the selector's unique).  This should
    -- be used only when generating output, when we want to show the label, but may
    -- need to qualify it with a module prefix.
    fieldLabelPrintableName :: FieldLabel -> Name
    fieldLabelPrintableName fl
      | flIsOverloaded fl = tidyNameOcc (flSelector fl) (mkVarOccFS (flLabel fl))
      | otherwise         = flSelector fl

    -- | Selector name mangling should be used if either DuplicateRecordFields or
    -- NoFieldSelectors is enabled, so that the OccName of the field can be used for
    -- something else.  See Note [FieldLabel], and Note [NoFieldSelectors] in
    -- GHC.Rename.Env.
    shouldMangleSelectorNames :: DuplicateRecordFields -> FieldSelectors -> Bool
    shouldMangleSelectorNames dup_fields_ok has_sel
        = dup_fields_ok == DuplicateRecordFields || has_sel == NoFieldSelectors

    flIsOverloaded :: FieldLabel -> Bool
    flIsOverloaded fl =
        shouldMangleSelectorNames (flHasDuplicateRecordFields fl) (flHasFieldSelector fl)