hsSyn/HsPat.lhs

% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % \section[PatSyntax]{Abstract Haskell syntax---patterns} \begin{code}

    {-# LANGUAGE DeriveDataTypeable #-}

    module HsPat (
            Pat(..), InPat, OutPat, LPat,

            HsConDetails(..),
            HsConPatDetails, hsConPatArgs,
            HsRecFields(..), HsRecField(..), hsRecFields,

            mkPrefixConPat, mkCharLitPat, mkNilPat,

            isStrictHsBind, looksLazyPatBind,
            isStrictLPat, hsPatNeedsParens,
            isIrrefutableHsPat,

            pprParendLPat
        ) where

    import {-# SOURCE #-} HsExpr            (SyntaxExpr, LHsExpr, HsSplice, pprLExpr, pprUntypedSplice)

    -- friends:
    import HsBinds
    import HsLit
    import HsTypes
    import TcEvidence
    import BasicTypes
    -- others:
    import PprCore          ( {- instance OutputableBndr TyVar -} )
    import TysWiredIn
    import Var
    import ConLike
    import DataCon
    import TyCon
    import Outputable
    import Type
    import SrcLoc
    import FastString
    -- libraries:
    import Data.Data hiding (TyCon)
    import Data.Maybe

\end{code} \begin{code}

    type InPat id  = LPat id        -- No 'Out' constructors
    type OutPat id = LPat id        -- No 'In' constructors

    type LPat id = Located (Pat id)

    data Pat id
      =     ------------ Simple patterns ---------------
        WildPat     PostTcType              -- Wild card
            -- The sole reason for a type on a WildPat is to
            -- support hsPatType :: Pat Id -> Type

      | VarPat      id                      -- Variable
      | LazyPat     (LPat id)               -- Lazy pattern
      | AsPat       (Located id) (LPat id)  -- As pattern
      | ParPat      (LPat id)               -- Parenthesised pattern
                                            -- See Note [Parens in HsSyn] in HsExpr
      | BangPat     (LPat id)               -- Bang pattern

            ------------ Lists, tuples, arrays ---------------
      | ListPat     [LPat id]                            -- Syntactic list
                    PostTcType                           -- The type of the elements
                    (Maybe (PostTcType, SyntaxExpr id))  -- For rebindable syntax
                       -- For OverloadedLists a Just (ty,fn) gives
                       -- overall type of the pattern, and the toList
                       -- function to convert the scrutinee to a list value

      | TuplePat    [LPat id]               -- Tuple
                    Boxity                  -- UnitPat is TuplePat []
                    PostTcType
            -- You might think that the PostTcType was redundant, but it's essential
            --      data T a where
            --        T1 :: Int -> T Int
            --      f :: (T a, a) -> Int
            --      f (T1 x, z) = z
            -- When desugaring, we must generate
            --      f = /\a. \v::a.  case v of (t::T a, w::a) ->
            --                       case t of (T1 (x::Int)) ->
            -- Note the (w::a), NOT (w::Int), because we have not yet
            -- refined 'a' to Int.  So we must know that the second component
            -- of the tuple is of type 'a' not Int.  See selectMatchVar

      | PArrPat     [LPat id]               -- Syntactic parallel array
                    PostTcType              -- The type of the elements

            ------------ Constructor patterns ---------------
      | ConPatIn    (Located id)
                    (HsConPatDetails id)

      | ConPatOut {
            pat_con   :: Located ConLike,
            pat_tvs   :: [TyVar],           -- Existentially bound type variables (tyvars only)
            pat_dicts :: [EvVar],           -- Ditto *coercion variables* and *dictionaries*
                                            -- One reason for putting coercion variable here, I think,
                                            --      is to ensure their kinds are zonked
            pat_binds :: TcEvBinds,         -- Bindings involving those dictionaries
            pat_args  :: HsConPatDetails id,
            pat_ty    :: Type,              -- The type of the pattern
            pat_wrap  :: HsWrapper          -- Extra wrapper to pass to the matcher
        }

            ------------ View patterns ---------------
      | ViewPat       (LHsExpr id)
                      (LPat id)
                      PostTcType        -- The overall type of the pattern
                                        -- (= the argument type of the view function)
                                        -- for hsPatType.

            ------------ Pattern splices ---------------
      | SplicePat       (HsSplice id)

            ------------ Quasiquoted patterns ---------------
            -- See Note [Quasi-quote overview] in TcSplice
      | QuasiQuotePat   (HsQuasiQuote id)

            ------------ Literal and n+k patterns ---------------
      | LitPat          HsLit               -- Used for *non-overloaded* literal patterns:
                                            -- Int#, Char#, Int, Char, String, etc.

      | NPat                -- Used for all overloaded literals,
                            -- including overloaded strings with -XOverloadedStrings
                        (HsOverLit id)              -- ALWAYS positive
                        (Maybe (SyntaxExpr id))     -- Just (Name of 'negate') for negative
                                                    -- patterns, Nothing otherwise
                        (SyntaxExpr id)             -- Equality checker, of type t->t->Bool

      | NPlusKPat       (Located id)        -- n+k pattern
                        (HsOverLit id)      -- It'll always be an HsIntegral
                        (SyntaxExpr id)     -- (>=) function, of type t->t->Bool
                        (SyntaxExpr id)     -- Name of '-' (see RnEnv.lookupSyntaxName)

            ------------ Pattern type signatures ---------------
      | SigPatIn        (LPat id)                   -- Pattern with a type signature
                        (HsWithBndrs (LHsType id))  -- Signature can bind both kind and type vars

      | SigPatOut       (LPat id)           -- Pattern with a type signature
                        Type

            ------------ Pattern coercions (translation only) ---------------
      | CoPat       HsWrapper               -- If co :: t1 ~ t2, p :: t2,
                                            -- then (CoPat co p) :: t1
                    (Pat id)                -- Why not LPat?  Ans: existing locn will do
                    Type                    -- Type of whole pattern, t1
            -- During desugaring a (CoPat co pat) turns into a cast with 'co' on
            -- the scrutinee, followed by a match on 'pat'
      deriving (Data, Typeable)

\end{code} HsConDetails is use for patterns/expressions *and* for data type declarations \begin{code}

    data HsConDetails arg rec
      = PrefixCon [arg]             -- C p1 p2 p3
      | RecCon    rec               -- C { x = p1, y = p2 }
      | InfixCon  arg arg           -- p1 `C` p2
      deriving (Data, Typeable)

    type HsConPatDetails id = HsConDetails (LPat id) (HsRecFields id (LPat id))

    hsConPatArgs :: HsConPatDetails id -> [LPat id]
    hsConPatArgs (PrefixCon ps)   = ps
    hsConPatArgs (RecCon fs)      = map hsRecFieldArg (rec_flds fs)
    hsConPatArgs (InfixCon p1 p2) = [p1,p2]

\end{code} However HsRecFields is used only for patterns and expressions (not data type declarations) \begin{code}

    data HsRecFields id arg         -- A bunch of record fields
                                    --      { x = 3, y = True }
            -- Used for both expressions and patterns
      = HsRecFields { rec_flds   :: [HsRecField id arg],
                      rec_dotdot :: Maybe Int }  -- Note [DotDot fields]
      deriving (Data, Typeable)

    -- Note [DotDot fields]
    -- ~~~~~~~~~~~~~~~~~~~~
    -- The rec_dotdot field means this:
    --   Nothing => the normal case
    --   Just n  => the group uses ".." notation,
    --
    -- In the latter case:
    --
    --   *before* renamer: rec_flds are exactly the n user-written fields
    --
    --   *after* renamer:  rec_flds includes *all* fields, with
    --                     the first 'n' being the user-written ones
    --                     and the remainder being 'filled in' implicitly

    data HsRecField id arg = HsRecField {
            hsRecFieldId  :: Located id,
            hsRecFieldArg :: arg,           -- Filled in by renamer
            hsRecPun      :: Bool           -- Note [Punning]
      } deriving (Data, Typeable)

    -- Note [Punning]
    -- ~~~~~~~~~~~~~~
    -- If you write T { x, y = v+1 }, the HsRecFields will be
    --      HsRecField x x True ...
    --      HsRecField y (v+1) False ...
    -- That is, for "punned" field x is expanded (in the renamer)
    -- to x=x; but with a punning flag so we can detect it later
    -- (e.g. when pretty printing)
    --
    -- If the original field was qualified, we un-qualify it, thus
    --    T { A.x } means T { A.x = x }

    hsRecFields :: HsRecFields id arg -> [id]
    hsRecFields rbinds = map (unLoc . hsRecFieldId) (rec_flds rbinds)

\end{code} %************************************************************************ %* * %* Printing patterns %* * %************************************************************************ \begin{code}

    instance (OutputableBndr name) => Outputable (Pat name) where
        ppr = pprPat

    pprPatBndr :: OutputableBndr name => name -> SDoc
    pprPatBndr var                  -- Print with type info if -dppr-debug is on
      = getPprStyle $ \ sty ->
        if debugStyle sty then
            parens (pprBndr LambdaBind var)         -- Could pass the site to pprPat
                                                    -- but is it worth it?
        else
            pprPrefixOcc var

    pprParendLPat :: (OutputableBndr name) => LPat name -> SDoc
    pprParendLPat (L _ p) = pprParendPat p

    pprParendPat :: (OutputableBndr name) => Pat name -> SDoc
    pprParendPat p | hsPatNeedsParens p = parens (pprPat p)
                   | otherwise          = pprPat p

    pprPat :: (OutputableBndr name) => Pat name -> SDoc
    pprPat (VarPat var)       = pprPatBndr var
    pprPat (WildPat _)        = char '_'
    pprPat (LazyPat pat)      = char '~' <> pprParendLPat pat
    pprPat (BangPat pat)      = char '!' <> pprParendLPat pat
    pprPat (AsPat name pat)   = hcat [pprPrefixOcc (unLoc name), char '@', pprParendLPat pat]
    pprPat (ViewPat expr pat _) = hcat [pprLExpr expr, text " -> ", ppr pat]
    pprPat (ParPat pat)         = parens (ppr pat)
    pprPat (ListPat pats _ _)     = brackets (interpp'SP pats)
    pprPat (PArrPat pats _)     = paBrackets (interpp'SP pats)
    pprPat (TuplePat pats bx _) = tupleParens (boxityNormalTupleSort bx) (interpp'SP pats)

    pprPat (ConPatIn con details) = pprUserCon (unLoc con) details
    pprPat (ConPatOut { pat_con = con, pat_tvs = tvs, pat_dicts = dicts,
                        pat_binds = binds, pat_args = details })
      = getPprStyle $ \ sty ->      -- Tiresome; in TcBinds.tcRhs we print out a
        if debugStyle sty then      -- typechecked Pat in an error message,
                                    -- and we want to make sure it prints nicely
            ppr con
              <> braces (sep [ hsep (map pprPatBndr (tvs ++ dicts))
                             , ppr binds])
              <+> pprConArgs details
        else pprUserCon (unLoc con) details

    pprPat (LitPat s)           = ppr s
    pprPat (NPat l Nothing  _)  = ppr l
    pprPat (NPat l (Just _) _)  = char '-' <> ppr l
    pprPat (NPlusKPat n k _ _)  = hcat [ppr n, char '+', ppr k]
    pprPat (SplicePat splice)   = pprUntypedSplice splice
    pprPat (QuasiQuotePat qq)   = ppr qq
    pprPat (CoPat co pat _)     = pprHsWrapper (ppr pat) co
    pprPat (SigPatIn pat ty)    = ppr pat <+> dcolon <+> ppr ty
    pprPat (SigPatOut pat ty)   = ppr pat <+> dcolon <+> ppr ty

    pprUserCon :: (OutputableBndr con, OutputableBndr id) => con -> HsConPatDetails id -> SDoc
    pprUserCon c (InfixCon p1 p2) = ppr p1 <+> pprInfixOcc c <+> ppr p2
    pprUserCon c details          = pprPrefixOcc c <+> pprConArgs details

    pprConArgs ::  OutputableBndr id => HsConPatDetails id -> SDoc
    pprConArgs (PrefixCon pats) = sep (map pprParendLPat pats)
    pprConArgs (InfixCon p1 p2) = sep [pprParendLPat p1, pprParendLPat p2]
    pprConArgs (RecCon rpats)   = ppr rpats

    instance (OutputableBndr id, Outputable arg)
          => Outputable (HsRecFields id arg) where
      ppr (HsRecFields { rec_flds = flds, rec_dotdot = Nothing })
            = braces (fsep (punctuate comma (map ppr flds)))
      ppr (HsRecFields { rec_flds = flds, rec_dotdot = Just n })
            = braces (fsep (punctuate comma (map ppr (take n flds) ++ [dotdot])))
            where
              dotdot = ptext (sLit "..") <+> ifPprDebug (ppr (drop n flds))

    instance (OutputableBndr id, Outputable arg)
          => Outputable (HsRecField id arg) where
      ppr (HsRecField { hsRecFieldId = f, hsRecFieldArg = arg,
                        hsRecPun = pun })
        = ppr f <+> (ppUnless pun $ equals <+> ppr arg)

\end{code} %************************************************************************ %* * %* Building patterns %* * %************************************************************************ \begin{code}

    mkPrefixConPat :: DataCon -> [OutPat id] -> Type -> OutPat id
    -- Make a vanilla Prefix constructor pattern
    mkPrefixConPat dc pats ty
      = noLoc $ ConPatOut { pat_con = noLoc (RealDataCon dc), pat_tvs = [], pat_dicts = [],
                            pat_binds = emptyTcEvBinds, pat_args = PrefixCon pats,
                            pat_ty = ty, pat_wrap = idHsWrapper }

    mkNilPat :: Type -> OutPat id
    mkNilPat ty = mkPrefixConPat nilDataCon [] ty

    mkCharLitPat :: Char -> OutPat id
    mkCharLitPat c = mkPrefixConPat charDataCon [noLoc $ LitPat (HsCharPrim c)] charTy

\end{code} %************************************************************************ %* * %* Predicates for checking things about pattern-lists in EquationInfo * %* * %************************************************************************ \subsection[Pat-list-predicates]{Look for interesting things in patterns} Unlike in the Wadler chapter, where patterns are either ``variables'' or ``constructors,'' here we distinguish between: \begin{description} \item[unfailable:] Patterns that cannot fail to match: variables, wildcards, and lazy patterns. These are the irrefutable patterns; the two other categories are refutable patterns. \item[constructor:] A non-literal constructor pattern (see next category). \item[literal patterns:] At least the numeric ones may be overloaded. \end{description} A pattern is in {\em exactly one} of the above three categories; `as' patterns are treated specially, of course. The 1.3 report defines what ``irrefutable'' and ``failure-free'' patterns are. \begin{code}

    isStrictLPat :: LPat id -> Bool
    isStrictLPat (L _ (ParPat p))             = isStrictLPat p
    isStrictLPat (L _ (BangPat {}))           = True
    isStrictLPat (L _ (TuplePat _ Unboxed _)) = True
    isStrictLPat _                            = False

    isStrictHsBind :: HsBind id -> Bool
    -- A pattern binding with an outermost bang or unboxed tuple must be matched strictly
    -- Defined in this module because HsPat is above HsBinds in the import graph
    isStrictHsBind (PatBind { pat_lhs = p }) = isStrictLPat p
    isStrictHsBind _                         = False

    looksLazyPatBind :: HsBind id -> Bool
    -- Returns True of anything *except*
    --     a StrictHsBind (as above) or
    --     a VarPat
    -- In particular, returns True of a pattern binding with a compound pattern, like (I# x)
    looksLazyPatBind (PatBind { pat_lhs = p }) = looksLazyLPat p
    looksLazyPatBind _                         = False

    looksLazyLPat :: LPat id -> Bool
    looksLazyLPat (L _ (ParPat p))             = looksLazyLPat p
    looksLazyLPat (L _ (AsPat _ p))            = looksLazyLPat p
    looksLazyLPat (L _ (BangPat {}))           = False
    looksLazyLPat (L _ (TuplePat _ Unboxed _)) = False
    looksLazyLPat (L _ (VarPat {}))            = False
    looksLazyLPat (L _ (WildPat {}))           = False
    looksLazyLPat _                            = True

    isIrrefutableHsPat :: OutputableBndr id => LPat id -> Bool
    -- (isIrrefutableHsPat p) is true if matching against p cannot fail,
    -- in the sense of falling through to the next pattern.
    --      (NB: this is not quite the same as the (silly) defn
    --      in 3.17.2 of the Haskell 98 report.)
    --
    -- isIrrefutableHsPat returns False if it's in doubt; specifically
    -- on a ConPatIn it doesn't know the size of the constructor family
    -- But if it returns True, the pattern is definitely irrefutable
    isIrrefutableHsPat pat
      = go pat
      where
        go (L _ pat) = go1 pat

        go1 (WildPat {})        = True
        go1 (VarPat {})         = True
        go1 (LazyPat {})        = True
        go1 (BangPat pat)       = go pat
        go1 (CoPat _ pat _)     = go1 pat
        go1 (ParPat pat)        = go pat
        go1 (AsPat _ pat)       = go pat
        go1 (ViewPat _ pat _)   = go pat
        go1 (SigPatIn pat _)    = go pat
        go1 (SigPatOut pat _)   = go pat
        go1 (TuplePat pats _ _) = all go pats
        go1 (ListPat {}) = False
        go1 (PArrPat {})        = False     -- ?

        go1 (ConPatIn {})       = False     -- Conservative
        go1 (ConPatOut{ pat_con = L _ (RealDataCon con), pat_args = details })
            =  isJust (tyConSingleDataCon_maybe (dataConTyCon con))
               -- NB: tyConSingleDataCon_maybe, *not* isProductTyCon, because
               -- the latter is false of existentials. See Trac #4439
            && all go (hsConPatArgs details)
        go1 (ConPatOut{ pat_con = L _ (PatSynCon _pat) })
            = False -- Conservative

        go1 (LitPat {})    = False
        go1 (NPat {})      = False
        go1 (NPlusKPat {}) = False

        -- Both should be gotten rid of by renamer before
        -- isIrrefutablePat is called
        go1 (SplicePat {})     = urk pat
        go1 (QuasiQuotePat {}) = urk pat

        urk pat = pprPanic "isIrrefutableHsPat:" (ppr pat)

    hsPatNeedsParens :: Pat a -> Bool
    hsPatNeedsParens (NPlusKPat {})      = True
    hsPatNeedsParens (SplicePat {})      = False
    hsPatNeedsParens (QuasiQuotePat {})  = True
    hsPatNeedsParens (ConPatIn _ ds)     = conPatNeedsParens ds
    hsPatNeedsParens p@(ConPatOut {})    = conPatNeedsParens (pat_args p)
    hsPatNeedsParens (SigPatIn {})       = True
    hsPatNeedsParens (SigPatOut {})      = True
    hsPatNeedsParens (ViewPat {})        = True
    hsPatNeedsParens (CoPat {})          = True
    hsPatNeedsParens (WildPat {})        = False
    hsPatNeedsParens (VarPat {})         = False
    hsPatNeedsParens (LazyPat {})        = False
    hsPatNeedsParens (BangPat {})        = False
    hsPatNeedsParens (ParPat {})         = False
    hsPatNeedsParens (AsPat {})          = False
    hsPatNeedsParens (TuplePat {})       = False
    hsPatNeedsParens (ListPat {})        = False
    hsPatNeedsParens (PArrPat {})        = False
    hsPatNeedsParens (LitPat {})         = False
    hsPatNeedsParens (NPat {})           = False

    conPatNeedsParens :: HsConDetails a b -> Bool
    conPatNeedsParens (PrefixCon args) = not (null args)
    conPatNeedsParens (InfixCon {})    = True
    conPatNeedsParens (RecCon {})      = True

\end{code}