deSugar/MatchLit.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    Pattern-matching literal patterns
    -}

    {-# LANGUAGE CPP, ScopedTypeVariables #-}

    module MatchLit ( dsLit, dsOverLit, dsOverLit', hsLitKey
                    , tidyLitPat, tidyNPat
                    , matchLiterals, matchNPlusKPats, matchNPats
                    , warnAboutIdentities, warnAboutOverflowedLiterals
                    , warnAboutEmptyEnumerations
                    ) where

    #include "HsVersions.h"

    import {-# SOURCE #-} Match  ( match )
    import {-# SOURCE #-} DsExpr ( dsExpr, dsSyntaxExpr )

    import DsMonad
    import DsUtils

    import HsSyn

    import Id
    import CoreSyn
    import MkCore
    import TyCon
    import DataCon
    import TcHsSyn ( shortCutLit )
    import TcType
    import Name
    import Type
    import PrelNames
    import TysWiredIn
    import Literal
    import SrcLoc
    import Data.Ratio
    import Outputable
    import BasicTypes
    import DynFlags
    import Util
    import FastString
    import qualified GHC.LanguageExtensions as LangExt

    import Control.Monad
    import Data.Int
    import Data.Word

    {-
    ************************************************************************
    *                                                                      *
                    Desugaring literals
            [used to be in DsExpr, but DsMeta needs it,
             and it's nice to avoid a loop]
    *                                                                      *
    ************************************************************************

    We give int/float literals type @Integer@ and @Rational@, respectively.
    The typechecker will (presumably) have put \tr{from{Integer,Rational}s}
    around them.

    ToDo: put in range checks for when converting ``@i@''
    (or should that be in the typechecker?)

    For numeric literals, we try to detect there use at a standard type
    (@Int@, @Float@, etc.) are directly put in the right constructor.
    [NB: down with the @App@ conversion.]

    See also below where we look for @DictApps@ for \tr{plusInt}, etc.
    -}

    dsLit :: HsLit -> DsM CoreExpr
    dsLit (HsStringPrim _ s) = return (Lit (MachStr s))
    dsLit (HsCharPrim   _ c) = return (Lit (MachChar c))
    dsLit (HsIntPrim    _ i) = return (Lit (MachInt i))
    dsLit (HsWordPrim   _ w) = return (Lit (MachWord w))
    dsLit (HsInt64Prim  _ i) = return (Lit (MachInt64 i))
    dsLit (HsWord64Prim _ w) = return (Lit (MachWord64 w))
    dsLit (HsFloatPrim    f) = return (Lit (MachFloat (fl_value f)))
    dsLit (HsDoublePrim   d) = return (Lit (MachDouble (fl_value d)))

    dsLit (HsChar _ c)       = return (mkCharExpr c)
    dsLit (HsString _ str)   = mkStringExprFS str
    dsLit (HsInteger _ i _)  = mkIntegerExpr i
    dsLit (HsInt _ i)        = do dflags <- getDynFlags
                                  return (mkIntExpr dflags i)

    dsLit (HsRat r ty) = do
       num   <- mkIntegerExpr (numerator (fl_value r))
       denom <- mkIntegerExpr (denominator (fl_value r))
       return (mkCoreConApps ratio_data_con [Type integer_ty, num, denom])
      where
        (ratio_data_con, integer_ty)
            = case tcSplitTyConApp ty of
                    (tycon, [i_ty]) -> ASSERT(isIntegerTy i_ty && tycon `hasKey` ratioTyConKey)
                                       (head (tyConDataCons tycon), i_ty)
                    x -> pprPanic "dsLit" (ppr x)

    dsOverLit :: HsOverLit Id -> DsM CoreExpr
    dsOverLit lit = do { dflags <- getDynFlags
                       ; warnAboutOverflowedLiterals dflags lit
                       ; dsOverLit' dflags lit }

    dsOverLit' :: DynFlags -> HsOverLit Id -> DsM CoreExpr
    -- Post-typechecker, the HsExpr field of an OverLit contains
    -- (an expression for) the literal value itself
    dsOverLit' dflags (OverLit { ol_val = val, ol_rebindable = rebindable
                               , ol_witness = witness, ol_type = ty })
      | not rebindable
      , Just expr <- shortCutLit dflags val ty = dsExpr expr        -- Note [Literal short cut]
      | otherwise                              = dsExpr witness

    {-
    Note [Literal short cut]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    The type checker tries to do this short-cutting as early as possible, but
    because of unification etc, more information is available to the desugarer.
    And where it's possible to generate the correct literal right away, it's
    much better to do so.

    ************************************************************************
    *                                                                      *
                     Warnings about overflowed literals
    *                                                                      *
    ************************************************************************

    Warn about functions like toInteger, fromIntegral, that convert
    between one type and another when the to- and from- types are the
    same.  Then it's probably (albeit not definitely) the identity
    -}

    warnAboutIdentities :: DynFlags -> CoreExpr -> Type -> DsM ()
    warnAboutIdentities dflags (Var conv_fn) type_of_conv
      | wopt Opt_WarnIdentities dflags
      , idName conv_fn `elem` conversionNames
      , Just (arg_ty, res_ty) <- splitFunTy_maybe type_of_conv
      , arg_ty `eqType` res_ty  -- So we are converting  ty -> ty
      = warnDs (Reason Opt_WarnIdentities)
               (vcat [ text "Call of" <+> ppr conv_fn <+> dcolon <+> ppr type_of_conv
                     , nest 2 $ text "can probably be omitted"
               ])
    warnAboutIdentities _ _ _ = return ()

    conversionNames :: [Name]
    conversionNames
      = [ toIntegerName, toRationalName
        , fromIntegralName, realToFracName ]
     -- We can't easily add fromIntegerName, fromRationalName,
     -- because they are generated by literals

    warnAboutOverflowedLiterals :: DynFlags -> HsOverLit Id -> DsM ()
    warnAboutOverflowedLiterals dflags lit
     | wopt Opt_WarnOverflowedLiterals dflags
     , Just (i, tc) <- getIntegralLit lit
      = if      tc == intTyConName    then check i tc (undefined :: Int)
        else if tc == int8TyConName   then check i tc (undefined :: Int8)
        else if tc == int16TyConName  then check i tc (undefined :: Int16)
        else if tc == int32TyConName  then check i tc (undefined :: Int32)
        else if tc == int64TyConName  then check i tc (undefined :: Int64)
        else if tc == wordTyConName   then check i tc (undefined :: Word)
        else if tc == word8TyConName  then check i tc (undefined :: Word8)
        else if tc == word16TyConName then check i tc (undefined :: Word16)
        else if tc == word32TyConName then check i tc (undefined :: Word32)
        else if tc == word64TyConName then check i tc (undefined :: Word64)
        else return ()

      | otherwise = return ()
      where
        check :: forall a. (Bounded a, Integral a) => Integer -> Name -> a -> DsM ()
        check i tc _proxy
          = when (i < minB || i > maxB) $ do
            warnDs (Reason Opt_WarnOverflowedLiterals)
                   (vcat [ text "Literal" <+> integer i
                           <+> text "is out of the" <+> ppr tc <+> ptext (sLit "range")
                           <+> integer minB <> text ".." <> integer maxB
                         , sug ])
          where
            minB = toInteger (minBound :: a)
            maxB = toInteger (maxBound :: a)
            sug | minB == -i   -- Note [Suggest NegativeLiterals]
                , i > 0
                , not (xopt LangExt.NegativeLiterals dflags)
                = text "If you are trying to write a large negative literal, use NegativeLiterals"
                | otherwise = Outputable.empty

    {-
    Note [Suggest NegativeLiterals]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If you write
      x :: Int8
      x = -128
    it'll parse as (negate 128), and overflow.  In this case, suggest NegativeLiterals.
    We get an erroneous suggestion for
      x = 128
    but perhaps that does not matter too much.
    -}

    warnAboutEmptyEnumerations :: DynFlags -> LHsExpr Id -> Maybe (LHsExpr Id) -> LHsExpr Id -> DsM ()
    -- Warns about [2,3 .. 1] which returns the empty list
    -- Only works for integral types, not floating point
    warnAboutEmptyEnumerations dflags fromExpr mThnExpr toExpr
      | wopt Opt_WarnEmptyEnumerations dflags
      , Just (from,tc) <- getLHsIntegralLit fromExpr
      , Just mThn      <- traverse getLHsIntegralLit mThnExpr
      , Just (to,_)    <- getLHsIntegralLit toExpr
      , let check :: forall a. (Enum a, Num a) => a -> DsM ()
            check _proxy
              = when (null enumeration) $
                warnDs (Reason Opt_WarnEmptyEnumerations) (text "Enumeration is empty")
              where
                enumeration :: [a]
                enumeration = case mThn of
                                Nothing      -> [fromInteger from                    .. fromInteger to]
                                Just (thn,_) -> [fromInteger from, fromInteger thn   .. fromInteger to]

      = if      tc == intTyConName    then check (undefined :: Int)
        else if tc == int8TyConName   then check (undefined :: Int8)
        else if tc == int16TyConName  then check (undefined :: Int16)
        else if tc == int32TyConName  then check (undefined :: Int32)
        else if tc == int64TyConName  then check (undefined :: Int64)
        else if tc == wordTyConName   then check (undefined :: Word)
        else if tc == word8TyConName  then check (undefined :: Word8)
        else if tc == word16TyConName then check (undefined :: Word16)
        else if tc == word32TyConName then check (undefined :: Word32)
        else if tc == word64TyConName then check (undefined :: Word64)
        else if tc == integerTyConName then check (undefined :: Integer)
        else return ()

      | otherwise = return ()

    getLHsIntegralLit :: LHsExpr Id -> Maybe (Integer, Name)
    -- See if the expression is an Integral literal
    -- Remember to look through automatically-added tick-boxes! (Trac #8384)
    getLHsIntegralLit (L _ (HsPar e))            = getLHsIntegralLit e
    getLHsIntegralLit (L _ (HsTick _ e))         = getLHsIntegralLit e
    getLHsIntegralLit (L _ (HsBinTick _ _ e))    = getLHsIntegralLit e
    getLHsIntegralLit (L _ (HsOverLit over_lit)) = getIntegralLit over_lit
    getLHsIntegralLit _ = Nothing

    getIntegralLit :: HsOverLit Id -> Maybe (Integer, Name)
    getIntegralLit (OverLit { ol_val = HsIntegral _ i, ol_type = ty })
      | Just tc <- tyConAppTyCon_maybe ty
      = Just (i, tyConName tc)
    getIntegralLit _ = Nothing

    {-
    ************************************************************************
    *                                                                      *
            Tidying lit pats
    *                                                                      *
    ************************************************************************
    -}

    tidyLitPat :: HsLit -> Pat Id
    -- Result has only the following HsLits:
    --      HsIntPrim, HsWordPrim, HsCharPrim, HsFloatPrim
    --      HsDoublePrim, HsStringPrim, HsString
    --  * HsInteger, HsRat, HsInt can't show up in LitPats
    --  * We get rid of HsChar right here
    tidyLitPat (HsChar src c) = unLoc (mkCharLitPat src c)
    tidyLitPat (HsString src s)
      | lengthFS s <= 1     -- Short string literals only
      = unLoc $ foldr (\c pat -> mkPrefixConPat consDataCon
                                                 [mkCharLitPat src c, pat] [charTy])
                      (mkNilPat charTy) (unpackFS s)
            -- The stringTy is the type of the whole pattern, not
            -- the type to instantiate (:) or [] with!
    tidyLitPat lit = LitPat lit

    ----------------
    tidyNPat :: (HsLit -> Pat Id)   -- How to tidy a LitPat
                     -- We need this argument because tidyNPat is called
                     -- both by Match and by Check, but they tidy LitPats
                     -- slightly differently; and we must desugar
                     -- literals consistently (see Trac #5117)
             -> HsOverLit Id -> Maybe (SyntaxExpr Id) -> SyntaxExpr Id -> Type
             -> Pat Id
    tidyNPat tidy_lit_pat (OverLit val False _ ty) mb_neg _eq outer_ty
            -- False: Take short cuts only if the literal is not using rebindable syntax
            --
            -- Once that is settled, look for cases where the type of the
            -- entire overloaded literal matches the type of the underlying literal,
            -- and in that case take the short cut
            -- NB: Watch out for weird cases like Trac #3382
            --        f :: Int -> Int
            --        f "blah" = 4
            --     which might be ok if we have 'instance IsString Int'
            --
      | not type_change, isIntTy ty,    Just int_lit <- mb_int_lit
                     = mk_con_pat intDataCon    (HsIntPrim    NoSourceText int_lit)
      | not type_change, isWordTy ty,   Just int_lit <- mb_int_lit
                     = mk_con_pat wordDataCon   (HsWordPrim   NoSourceText int_lit)
      | not type_change, isStringTy ty, Just str_lit <- mb_str_lit
                     = tidy_lit_pat (HsString NoSourceText str_lit)
         -- NB: do /not/ convert Float or Double literals to F# 3.8 or D# 5.3
         -- If we do convert to the constructor form, we'll generate a case
         -- expression on a Float# or Double# and that's not allowed in Core; see
         -- Trac #9238 and Note [Rules for floating-point comparisons] in PrelRules
      where
        -- Sometimes (like in test case
        -- overloadedlists/should_run/overloadedlistsrun04), the SyntaxExprs include
        -- type-changing wrappers (for example, from Id Int to Int, for the identity
        -- type family Id). In these cases, we can't do the short-cut.
        type_change = not (outer_ty `eqType` ty)

        mk_con_pat :: DataCon -> HsLit -> Pat Id
        mk_con_pat con lit = unLoc (mkPrefixConPat con [noLoc $ LitPat lit] [])

        mb_int_lit :: Maybe Integer
        mb_int_lit = case (mb_neg, val) of
                       (Nothing, HsIntegral _ i) -> Just i
                       (Just _,  HsIntegral _ i) -> Just (-i)
                       _ -> Nothing

        mb_str_lit :: Maybe FastString
        mb_str_lit = case (mb_neg, val) of
                       (Nothing, HsIsString _ s) -> Just s
                       _ -> Nothing

    tidyNPat _ over_lit mb_neg eq outer_ty
      = NPat (noLoc over_lit) mb_neg eq outer_ty

    {-
    ************************************************************************
    *                                                                      *
                    Pattern matching on LitPat
    *                                                                      *
    ************************************************************************
    -}

    matchLiterals :: [Id]
                  -> Type                   -- Type of the whole case expression
                  -> [[EquationInfo]]       -- All PgLits
                  -> DsM MatchResult

    matchLiterals (var:vars) ty sub_groups
      = ASSERT( notNull sub_groups && all notNull sub_groups )
        do  {       -- Deal with each group
            ; alts <- mapM match_group sub_groups

                    -- Combine results.  For everything except String
                    -- we can use a case expression; for String we need
                    -- a chain of if-then-else
            ; if isStringTy (idType var) then
                do  { eq_str <- dsLookupGlobalId eqStringName
                    ; mrs <- mapM (wrap_str_guard eq_str) alts
                    ; return (foldr1 combineMatchResults mrs) }
              else
                return (mkCoPrimCaseMatchResult var ty alts)
            }
      where
        match_group :: [EquationInfo] -> DsM (Literal, MatchResult)
        match_group eqns
            = do dflags <- getDynFlags
                 let LitPat hs_lit = firstPat (head eqns)
                 match_result <- match vars ty (shiftEqns eqns)
                 return (hsLitKey dflags hs_lit, match_result)

        wrap_str_guard :: Id -> (Literal,MatchResult) -> DsM MatchResult
            -- Equality check for string literals
        wrap_str_guard eq_str (MachStr s, mr)
            = do { -- We now have to convert back to FastString. Perhaps there
                   -- should be separate MachBytes and MachStr constructors?
                   let s'  = mkFastStringByteString s
                 ; lit    <- mkStringExprFS s'
                 ; let pred = mkApps (Var eq_str) [Var var, lit]
                 ; return (mkGuardedMatchResult pred mr) }
        wrap_str_guard _ (l, _) = pprPanic "matchLiterals/wrap_str_guard" (ppr l)

    matchLiterals [] _ _ = panic "matchLiterals []"

    ---------------------------
    hsLitKey :: DynFlags -> HsLit -> Literal
    -- Get the Core literal corresponding to a HsLit.
    -- It only works for primitive types and strings;
    -- others have been removed by tidy
    -- For HsString, it produces a MachStr, which really represents an _unboxed_
    -- string literal; and we deal with it in matchLiterals above. Otherwise, it
    -- produces a primitive Literal of type matching the original HsLit.
    -- In the case of the fixed-width numeric types, we need to wrap here
    -- because Literal has an invariant that the literal is in range, while
    -- HsLit does not.
    hsLitKey dflags (HsIntPrim    _ i) = mkMachIntWrap  dflags i
    hsLitKey dflags (HsWordPrim   _ w) = mkMachWordWrap dflags w
    hsLitKey _      (HsInt64Prim  _ i) = mkMachInt64Wrap       i
    hsLitKey _      (HsWord64Prim _ w) = mkMachWord64Wrap      w
    hsLitKey _      (HsCharPrim   _ c) = mkMachChar            c
    hsLitKey _      (HsFloatPrim    f) = mkMachFloat           (fl_value f)
    hsLitKey _      (HsDoublePrim   d) = mkMachDouble          (fl_value d)
    hsLitKey _      (HsString _ s)     = MachStr (fastStringToByteString s)
    hsLitKey _      l                  = pprPanic "hsLitKey" (ppr l)

    {-
    ************************************************************************
    *                                                                      *
                    Pattern matching on NPat
    *                                                                      *
    ************************************************************************
    -}

    matchNPats :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
    matchNPats (var:vars) ty (eqn1:eqns)    -- All for the same literal
      = do  { let NPat (L _ lit) mb_neg eq_chk _ = firstPat eqn1
            ; lit_expr <- dsOverLit lit
            ; neg_lit <- case mb_neg of
                                Nothing  -> return lit_expr
                                Just neg -> dsSyntaxExpr neg [lit_expr]
            ; pred_expr <- dsSyntaxExpr eq_chk [Var var, neg_lit]
            ; match_result <- match vars ty (shiftEqns (eqn1:eqns))
            ; return (mkGuardedMatchResult pred_expr match_result) }
    matchNPats vars _ eqns = pprPanic "matchOneNPat" (ppr (vars, eqns))

    {-
    ************************************************************************
    *                                                                      *
                    Pattern matching on n+k patterns
    *                                                                      *
    ************************************************************************

    For an n+k pattern, we use the various magic expressions we've been given.
    We generate:
    \begin{verbatim}
        if ge var lit then
            let n = sub var lit
            in  <expr-for-a-successful-match>
        else
            <try-next-pattern-or-whatever>
    \end{verbatim}
    -}

    matchNPlusKPats :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
    -- All NPlusKPats, for the *same* literal k
    matchNPlusKPats (var:vars) ty (eqn1:eqns)
      = do  { let NPlusKPat (L _ n1) (L _ lit1) lit2 ge minus _ = firstPat eqn1
            ; lit1_expr   <- dsOverLit lit1
            ; lit2_expr   <- dsOverLit lit2
            ; pred_expr   <- dsSyntaxExpr ge    [Var var, lit1_expr]
            ; minusk_expr <- dsSyntaxExpr minus [Var var, lit2_expr]
            ; let (wraps, eqns') = mapAndUnzip (shift n1) (eqn1:eqns)
            ; match_result <- match vars ty eqns'
            ; return  (mkGuardedMatchResult pred_expr               $
                       mkCoLetMatchResult (NonRec n1 minusk_expr)   $
                       adjustMatchResult (foldr1 (.) wraps)         $
                       match_result) }
      where
        shift n1 eqn@(EqnInfo { eqn_pats = NPlusKPat (L _ n) _ _ _ _ _ : pats })
            = (wrapBind n n1, eqn { eqn_pats = pats })
            -- The wrapBind is a no-op for the first equation
        shift _ e = pprPanic "matchNPlusKPats/shift" (ppr e)

    matchNPlusKPats vars _ eqns = pprPanic "matchNPlusKPats" (ppr (vars, eqns))