% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1997-1998 % % Author: Juan J. Quintela <quintela@krilin.dc.fi.udc.es> \begin{code}

    {-# OPTIONS -fno-warn-incomplete-patterns #-}
    -- The above warning supression flag is a temporary kludge.
    -- While working on this module you are encouraged to remove it and fix
    -- any warnings in the module. See
    --     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
    -- for details

    module Check ( check , ExhaustivePat ) where

    #include "HsVersions.h"

    import HsSyn
    import TcHsSyn
    import DsUtils
    import MatchLit
    import Id
    import DataCon
    import Name
    import TysWiredIn
    import PrelNames
    import TyCon
    import Type
    import Unify( dataConCannotMatch )
    import SrcLoc
    import UniqSet
    import Util
    import Outputable
    import FastString

\end{code} This module performs checks about if one list of equations are: \begin{itemize} \item Overlapped \item Non exhaustive \end{itemize} To discover that we go through the list of equations in a tree-like fashion. If you like theory, a similar algorithm is described in: \begin{quotation} {\em Two Techniques for Compiling Lazy Pattern Matching}, Luc Maranguet, INRIA Rocquencourt (RR-2385, 1994) \end{quotation} The algorithm is based on the first technique, but there are some differences: \begin{itemize} \item We don't generate code \item We have constructors and literals (not only literals as in the article) \item We don't use directions, we must select the columns from left-to-right \end{itemize} (By the way the second technique is really similar to the one used in @Match.lhs@ to generate code) This function takes the equations of a pattern and returns: \begin{itemize} \item The patterns that are not recognized \item The equations that are not overlapped \end{itemize} It simplify the patterns and then call @check'@ (the same semantics), and it needs to reconstruct the patterns again .... The problem appear with things like: \begin{verbatim} f [x,y] = .... f (x:xs) = ..... \end{verbatim} We want to put the two patterns with the same syntax, (prefix form) and then all the constructors are equal: \begin{verbatim} f (: x (: y [])) = .... f (: x xs) = ..... \end{verbatim} (more about that in @tidy_eqns@) We would prefer to have a @WarningPat@ of type @String@, but Strings and the Pretty Printer are not friends. We use @InPat@ in @WarningPat@ instead of @OutPat@ because we need to print the warning messages in the same way they are introduced, i.e. if the user wrote: \begin{verbatim} f [x,y] = .. \end{verbatim} He don't want a warning message written: \begin{verbatim} f (: x (: y [])) ........ \end{verbatim} Then we need to use InPats. \begin{quotation} Juan Quintela 5 JUL 1998\\ User-friendliness and compiler writers are no friends. \end{quotation} \begin{code}

    type WarningPat = InPat Name
    type ExhaustivePat = ([WarningPat], [(Name, [HsLit])])
    type EqnNo  = Int
    type EqnSet = UniqSet EqnNo

    check :: [EquationInfo] -> ([ExhaustivePat], [EquationInfo])
      -- Second result is the shadowed equations
      -- if there are view patterns, just give up - don't know what the function is
    check qs = (untidy_warns, shadowed_eqns)
          where
    	(warns, used_nos) = check' ([1..] `zip` map tidy_eqn qs)
    	untidy_warns = map untidy_exhaustive warns
    	shadowed_eqns = [eqn | (eqn,i) <- qs `zip` [1..],
    				not (i `elementOfUniqSet` used_nos)]

    untidy_exhaustive :: ExhaustivePat -> ExhaustivePat
    untidy_exhaustive ([pat], messages) =
    		  ([untidy_no_pars pat], map untidy_message messages)
    untidy_exhaustive (pats, messages) =
    		  (map untidy_pars pats, map untidy_message messages)

    untidy_message :: (Name, [HsLit]) -> (Name, [HsLit])
    untidy_message (string, lits) = (string, map untidy_lit lits)

\end{code} The function @untidy@ does the reverse work of the @tidy_pat@ funcion. \begin{code}

    type NeedPars = Bool

    untidy_no_pars :: WarningPat -> WarningPat
    untidy_no_pars p = untidy False p

    untidy_pars :: WarningPat -> WarningPat
    untidy_pars p = untidy True p

    untidy :: NeedPars -> WarningPat -> WarningPat
    untidy b (L loc p) = L loc (untidy' b p)
      where
        untidy' _ p@(WildPat _)          = p
        untidy' _ p@(VarPat _)           = p
        untidy' _ (LitPat lit)           = LitPat (untidy_lit lit)
        untidy' _ p@(ConPatIn _ (PrefixCon [])) = p
        untidy' b (ConPatIn name ps)     = pars b (L loc (ConPatIn name (untidy_con ps)))
        untidy' _ (ListPat pats ty)      = ListPat (map untidy_no_pars pats) ty
        untidy' _ (TuplePat pats box ty) = TuplePat (map untidy_no_pars pats) box ty
        untidy' _ (PArrPat _ _)	     = panic "Check.untidy: Shouldn't get a parallel array here!"
        untidy' _ (SigPatIn _ _) 	     = panic "Check.untidy: SigPat"

    untidy_con :: HsConPatDetails Name -> HsConPatDetails Name
    untidy_con (PrefixCon pats) = PrefixCon (map untidy_pars pats)
    untidy_con (InfixCon p1 p2) = InfixCon  (untidy_pars p1) (untidy_pars p2)
    untidy_con (RecCon (HsRecFields flds dd))
      = RecCon (HsRecFields [ fld { hsRecFieldArg = untidy_pars (hsRecFieldArg fld) }
    			| fld <- flds ] dd)

    pars :: NeedPars -> WarningPat -> Pat Name
    pars True p = ParPat p
    pars _    p = unLoc p

    untidy_lit :: HsLit -> HsLit
    untidy_lit (HsCharPrim c) = HsChar c
    untidy_lit lit 		  = lit

\end{code} This equation is the same that check, the only difference is that the boring work is done, that work needs to be done only once, this is the reason top have two functions, check is the external interface, @check'@ is called recursively. There are several cases: \begin{itemize} \item There are no equations: Everything is OK. \item There are only one equation, that can fail, and all the patterns are variables. Then that equation is used and the same equation is non-exhaustive. \item All the patterns are variables, and the match can fail, there are more equations then the results is the result of the rest of equations and this equation is used also. \item The general case, if all the patterns are variables (here the match can't fail) then the result is that this equation is used and this equation doesn't generate non-exhaustive cases. \item In the general case, there can exist literals ,constructors or only vars in the first column, we actuate in consequence. \end{itemize} \begin{code}

    check' :: [(EqnNo, EquationInfo)]
    	-> ([ExhaustivePat], 	-- Pattern scheme that might not be matched at all
    	    EqnSet)  		-- Eqns that are used (others are overlapped)

    check' [] = ([([],[])],emptyUniqSet)

    check' ((n, EqnInfo { eqn_pats = ps, eqn_rhs = MatchResult can_fail _ }) : rs)
       | first_eqn_all_vars && case can_fail of { CantFail -> True; CanFail -> False }
       = ([], unitUniqSet n)	-- One eqn, which can't fail

       | first_eqn_all_vars && null rs	-- One eqn, but it can fail
       = ([(takeList ps (repeat nlWildPat),[])], unitUniqSet n)

       | first_eqn_all_vars		-- Several eqns, first can fail
       = (pats, addOneToUniqSet indexs n)
      where
        first_eqn_all_vars = all_vars ps
        (pats,indexs) = check' rs

    check' qs
       | some_literals     = split_by_literals qs
       | some_constructors = split_by_constructor qs
       | only_vars         = first_column_only_vars qs
       | otherwise = pprPanic "Check.check': Not implemented :-(" (ppr first_pats)
         	         -- Shouldn't happen
      where
         -- Note: RecPats will have been simplified to ConPats
         --       at this stage.
        first_pats        = ASSERT2( okGroup qs, pprGroup qs ) map firstPatN qs
        some_constructors = any is_con first_pats
        some_literals     = any is_lit first_pats
        only_vars         = all is_var first_pats

\end{code} Here begins the code to deal with literals, we need to split the matrix in different matrix beginning by each literal and a last matrix with the rest of values. \begin{code}

    split_by_literals :: [(EqnNo, EquationInfo)] -> ([ExhaustivePat], EqnSet)
    split_by_literals qs = process_literals used_lits qs
               where
                 used_lits = get_used_lits qs

\end{code} @process_explicit_literals@ is a function that process each literal that appears in the column of the matrix. \begin{code}

    process_explicit_literals :: [HsLit] -> [(EqnNo, EquationInfo)] -> ([ExhaustivePat],EqnSet)
    process_explicit_literals lits qs = (concat pats, unionManyUniqSets indexs)
        where
          pats_indexs   = map (\x -> construct_literal_matrix x qs) lits
          (pats,indexs) = unzip pats_indexs

\end{code} @process_literals@ calls @process_explicit_literals@ to deal with the literals that appears in the matrix and deal also with the rest of the cases. It must be one Variable to be complete. \begin{code}

    process_literals :: [HsLit] -> [(EqnNo, EquationInfo)] -> ([ExhaustivePat],EqnSet)
    process_literals used_lits qs
      | null default_eqns  = ASSERT( not (null qs) ) ([make_row_vars used_lits (head qs)] ++ pats,indexs)
      | otherwise          = (pats_default,indexs_default)
         where
           (pats,indexs)   = process_explicit_literals used_lits qs
           default_eqns    = ASSERT2( okGroup qs, pprGroup qs )
    			 [remove_var q | q <- qs, is_var (firstPatN q)]
           (pats',indexs') = check' default_eqns
           pats_default    = [(nlWildPat:ps,constraints) | (ps,constraints) <- (pats')] ++ pats
           indexs_default  = unionUniqSets indexs' indexs

\end{code} Here we have selected the literal and we will select all the equations that begins for that literal and create a new matrix. \begin{code}

    construct_literal_matrix :: HsLit -> [(EqnNo, EquationInfo)] -> ([ExhaustivePat],EqnSet)
    construct_literal_matrix lit qs =
        (map (\ (xs,ys) -> (new_lit:xs,ys)) pats,indexs)
      where
        (pats,indexs) = (check' (remove_first_column_lit lit qs))
        new_lit = nlLitPat lit

    remove_first_column_lit :: HsLit
                            -> [(EqnNo, EquationInfo)]
                            -> [(EqnNo, EquationInfo)]
    remove_first_column_lit lit qs
      = ASSERT2( okGroup qs, pprGroup qs )
        [(n, shift_pat eqn) | q@(n,eqn) <- qs, is_var_lit lit (firstPatN q)]
      where
         shift_pat eqn@(EqnInfo { eqn_pats = _:ps}) = eqn { eqn_pats = ps }
         shift_pat _                                = panic "Check.shift_var: no patterns"

\end{code} This function splits the equations @qs@ in groups that deal with the same constructor. \begin{code}

    split_by_constructor :: [(EqnNo, EquationInfo)] -> ([ExhaustivePat], EqnSet)
    split_by_constructor qs
      | notNull unused_cons = need_default_case used_cons unused_cons qs
      | otherwise           = no_need_default_case used_cons qs
                           where
                              used_cons   = get_used_cons qs
                              unused_cons = get_unused_cons used_cons

\end{code} The first column of the patterns matrix only have vars, then there is nothing to do. \begin{code}

    first_column_only_vars :: [(EqnNo, EquationInfo)] -> ([ExhaustivePat],EqnSet)
    first_column_only_vars qs = (map (\ (xs,ys) -> (nlWildPat:xs,ys)) pats,indexs)
                              where
                                (pats, indexs) = check' (map remove_var qs)

\end{code} This equation takes a matrix of patterns and split the equations by constructor, using all the constructors that appears in the first column of the pattern matching. We can need a default clause or not ...., it depends if we used all the constructors or not explicitly. The reasoning is similar to @process_literals@, the difference is that here the default case is not always needed. \begin{code}

    no_need_default_case :: [Pat Id] -> [(EqnNo, EquationInfo)] -> ([ExhaustivePat],EqnSet)
    no_need_default_case cons qs = (concat pats, unionManyUniqSets indexs)
        where
          pats_indexs   = map (\x -> construct_matrix x qs) cons
          (pats,indexs) = unzip pats_indexs

    need_default_case :: [Pat Id] -> [DataCon] -> [(EqnNo, EquationInfo)] -> ([ExhaustivePat],EqnSet)
    need_default_case used_cons unused_cons qs
      | null default_eqns  = (pats_default_no_eqns,indexs)
      | otherwise          = (pats_default,indexs_default)
         where
           (pats,indexs)   = no_need_default_case used_cons qs
           default_eqns    = ASSERT2( okGroup qs, pprGroup qs )
    			 [remove_var q | q <- qs, is_var (firstPatN q)]
           (pats',indexs') = check' default_eqns
           pats_default    = [(make_whole_con c:ps,constraints) |
                              c <- unused_cons, (ps,constraints) <- pats'] ++ pats
           new_wilds       = ASSERT( not (null qs) ) make_row_vars_for_constructor (head qs)
           pats_default_no_eqns =  [(make_whole_con c:new_wilds,[]) | c <- unused_cons] ++ pats
           indexs_default  = unionUniqSets indexs' indexs

    construct_matrix :: Pat Id -> [(EqnNo, EquationInfo)] -> ([ExhaustivePat],EqnSet)
    construct_matrix con qs =
        (map (make_con con) pats,indexs)
      where
        (pats,indexs) = (check' (remove_first_column con qs))

\end{code} Here remove first column is more difficult that with literals due to the fact that constructors can have arguments. For instance, the matrix \begin{verbatim} (: x xs) y z y \end{verbatim} is transformed in: \begin{verbatim} x xs y _ _ y \end{verbatim} \begin{code}

    remove_first_column :: Pat Id                -- Constructor
                        -> [(EqnNo, EquationInfo)]
                        -> [(EqnNo, EquationInfo)]
    remove_first_column (ConPatOut{ pat_con = L _ con, pat_args = PrefixCon con_pats }) qs
      = ASSERT2( okGroup qs, pprGroup qs )
        [(n, shift_var eqn) | q@(n, eqn) <- qs, is_var_con con (firstPatN q)]
      where
         new_wilds = [WildPat (hsLPatType arg_pat) | arg_pat <- con_pats]
         shift_var eqn@(EqnInfo { eqn_pats = ConPatOut{ pat_args = PrefixCon ps' } : ps})
     	= eqn { eqn_pats = map unLoc ps' ++ ps }
         shift_var eqn@(EqnInfo { eqn_pats = WildPat _ : ps })
      	= eqn { eqn_pats = new_wilds ++ ps }
         shift_var _ = panic "Check.Shift_var:No done"

    make_row_vars :: [HsLit] -> (EqnNo, EquationInfo) -> ExhaustivePat
    make_row_vars used_lits (_, EqnInfo { eqn_pats = pats})
       = (nlVarPat new_var:takeList (tail pats) (repeat nlWildPat),[(new_var,used_lits)])
      where
         new_var = hash_x

    hash_x :: Name
    hash_x = mkInternalName unboundKey {- doesn't matter much -}
    		     (mkVarOccFS (fsLit "#x"))
    		     noSrcSpan

    make_row_vars_for_constructor :: (EqnNo, EquationInfo) -> [WarningPat]
    make_row_vars_for_constructor (_, EqnInfo { eqn_pats = pats})
      = takeList (tail pats) (repeat nlWildPat)

    compare_cons :: Pat Id -> Pat Id -> Bool
    compare_cons (ConPatOut{ pat_con = L _ id1 }) (ConPatOut { pat_con = L _ id2 }) = id1 == id2

    remove_dups :: [Pat Id] -> [Pat Id]
    remove_dups []     = []
    remove_dups (x:xs) | or (map (\y -> compare_cons x y) xs) = remove_dups  xs
                       | otherwise                            = x : remove_dups xs

    get_used_cons :: [(EqnNo, EquationInfo)] -> [Pat Id]
    get_used_cons qs = remove_dups [pat | q <- qs, let pat = firstPatN q,
    				      isConPatOut pat]

    isConPatOut :: Pat Id -> Bool
    isConPatOut (ConPatOut {}) = True
    isConPatOut _              = False

    remove_dups' :: [HsLit] -> [HsLit]
    remove_dups' []                   = []
    remove_dups' (x:xs) | x `elem` xs = remove_dups' xs
                        | otherwise   = x : remove_dups' xs

    get_used_lits :: [(EqnNo, EquationInfo)] -> [HsLit]
    get_used_lits qs = remove_dups' all_literals
    	         where
    	           all_literals = get_used_lits' qs

    get_used_lits' :: [(EqnNo, EquationInfo)] -> [HsLit]
    get_used_lits' [] = []
    get_used_lits' (q:qs)
      | Just lit <- get_lit (firstPatN q) = lit : get_used_lits' qs
      | otherwise		              = get_used_lits qs

    get_lit :: Pat id -> Maybe HsLit
    -- Get a representative HsLit to stand for the OverLit
    -- It doesn't matter which one, because they will only be compared
    -- with other HsLits gotten in the same way
    get_lit (LitPat lit)				          = Just lit
    get_lit (NPat (OverLit { ol_val = HsIntegral i})    mb _) = Just (HsIntPrim   (mb_neg mb i))
    get_lit (NPat (OverLit { ol_val = HsFractional f }) mb _) = Just (HsFloatPrim (mb_neg mb f))
    get_lit (NPat (OverLit { ol_val = HsIsString s })   _  _) = Just (HsStringPrim s)
    get_lit _                                	          = Nothing

    mb_neg :: Num a => Maybe b -> a -> a
    mb_neg Nothing  v = v
    mb_neg (Just _) v = -v

    get_unused_cons :: [Pat Id] -> [DataCon]
    get_unused_cons used_cons = ASSERT( not (null used_cons) ) unused_cons
         where
           used_set :: UniqSet DataCon
           used_set = mkUniqSet [d | ConPatOut{ pat_con = L _ d} <- used_cons]
           (ConPatOut { pat_ty = ty }) = head used_cons
           Just (ty_con, inst_tys) = splitTyConApp_maybe ty
           unused_cons = filterOut is_used (tyConDataCons ty_con)
           is_used con = con `elementOfUniqSet` used_set
    		     || dataConCannotMatch inst_tys con

    all_vars :: [Pat Id] -> Bool
    all_vars []             = True
    all_vars (WildPat _:ps) = all_vars ps
    all_vars _              = False

    remove_var :: (EqnNo, EquationInfo) -> (EqnNo, EquationInfo)
    remove_var (n, eqn@(EqnInfo { eqn_pats = WildPat _ : ps})) = (n, eqn { eqn_pats = ps })
    remove_var _  = panic "Check.remove_var: equation does not begin with a variable"

    -----------------------
    eqnPats :: (EqnNo, EquationInfo) -> [Pat Id]
    eqnPats (_, eqn) = eqn_pats eqn

    okGroup :: [(EqnNo, EquationInfo)] -> Bool
    -- True if all equations have at least one pattern, and
    -- all have the same number of patterns
    okGroup [] = True
    okGroup (e:es) = n_pats > 0 && and [length (eqnPats e) == n_pats | e <- es]
    	       where
    		 n_pats = length (eqnPats e)

    -- Half-baked print
    pprGroup :: [(EqnNo, EquationInfo)] -> SDoc
    pprEqnInfo :: (EqnNo, EquationInfo) -> SDoc
    pprGroup es = vcat (map pprEqnInfo es)
    pprEqnInfo e = ppr (eqnPats e)

    firstPatN :: (EqnNo, EquationInfo) -> Pat Id
    firstPatN (_, eqn) = firstPat eqn

    is_con :: Pat Id -> Bool
    is_con (ConPatOut {}) = True
    is_con _              = False

    is_lit :: Pat Id -> Bool
    is_lit (LitPat _)      = True
    is_lit (NPat _ _ _)  = True
    is_lit _               = False

    is_var :: Pat Id -> Bool
    is_var (WildPat _) = True
    is_var _           = False

    is_var_con :: DataCon -> Pat Id -> Bool
    is_var_con _   (WildPat _)                                 = True
    is_var_con con (ConPatOut{ pat_con = L _ id }) | id == con = True
    is_var_con _   _                                           = False

    is_var_lit :: HsLit -> Pat Id -> Bool
    is_var_lit _   (WildPat _)   = True
    is_var_lit lit pat
      | Just lit' <- get_lit pat = lit == lit'
      | otherwise		     = False

\end{code} The difference beteewn @make_con@ and @make_whole_con@ is that @make_wole_con@ creates a new constructor with all their arguments, and @make_con@ takes a list of argumntes, creates the contructor getting their arguments from the list. See where \fbox{\ ???\ } are used for details. We need to reconstruct the patterns (make the constructors infix and similar) at the same time that we create the constructors. You can tell tuple constructors using \begin{verbatim} Id.isTupleCon \end{verbatim} You can see if one constructor is infix with this clearer code :-)))))))))) \begin{verbatim} Lex.isLexConSym (Name.occNameString (Name.getOccName con)) \end{verbatim} Rather clumsy but it works. (Simon Peyton Jones) We don't mind the @nilDataCon@ because it doesn't change the way to print the messsage, we are searching only for things like: @[1,2,3]@, not @x:xs@ .... In @reconstruct_pat@ we want to ``undo'' the work that we have done in @tidy_pat@. In particular: \begin{tabular}{lll} @((,) x y)@ & returns to be & @(x, y)@ \\ @((:) x xs)@ & returns to be & @(x:xs)@ \\ @(x:(...:[])@ & returns to be & @[x,...]@ \end{tabular} % The difficult case is the third one becouse we need to follow all the contructors until the @[]@ to know that we need to use the second case, not the second. \fbox{\ ???\ } % \begin{code}

    isInfixCon :: DataCon -> Bool
    isInfixCon con = isDataSymOcc (getOccName con)

    is_nil :: Pat Name -> Bool
    is_nil (ConPatIn con (PrefixCon [])) = unLoc con == getName nilDataCon
    is_nil _               		     = False

    is_list :: Pat Name -> Bool
    is_list (ListPat _ _) = True
    is_list _             = False

    return_list :: DataCon -> Pat Name -> Bool
    return_list id q = id == consDataCon && (is_nil q || is_list q)

    make_list :: LPat Name -> Pat Name -> Pat Name
    make_list p q | is_nil q    = ListPat [p] placeHolderType
    make_list p (ListPat ps ty) = ListPat (p:ps) ty
    make_list _ _               = panic "Check.make_list: Invalid argument"

    make_con :: Pat Id -> ExhaustivePat -> ExhaustivePat
    make_con (ConPatOut{ pat_con = L _ id }) (lp:lq:ps, constraints)
         | return_list id q = (noLoc (make_list lp q) : ps, constraints)
         | isInfixCon id    = (nlInfixConPat (getName id) lp lq : ps, constraints)
       where q  = unLoc lq

    make_con (ConPatOut{ pat_con = L _ id, pat_args = PrefixCon pats, pat_ty = ty }) (ps, constraints)
          | isTupleTyCon tc  = (noLoc (TuplePat pats_con (tupleTyConBoxity tc) ty) : rest_pats, constraints)
          | isPArrFakeCon id = (noLoc (PArrPat pats_con placeHolderType)           : rest_pats, constraints)
          | otherwise        = (nlConPat name pats_con      : rest_pats, constraints)
        where
    	name     	      = getName id
    	(pats_con, rest_pats) = splitAtList pats ps
    	tc	    	      = dataConTyCon id

    -- reconstruct parallel array pattern
    --
    --  * don't check for the type only; we need to make sure that we are really
    --   dealing with one of the fake constructors and not with the real
    --   representation

    make_whole_con :: DataCon -> WarningPat
    make_whole_con con | isInfixCon con = nlInfixConPat name nlWildPat nlWildPat
                       | otherwise      = nlConPat name pats
                    where
                      name   = getName con
                      pats   = [nlWildPat | _ <- dataConOrigArgTys con]

\end{code} ------------------------------------------------------------------------ Tidying equations ------------------------------------------------------------------------ tidy_eqn does more or less the same thing as @tidy@ in @Match.lhs@; that is, it removes syntactic sugar, reducing the number of cases that must be handled by the main checking algorithm. One difference is that here we can do *all* the tidying at once (recursively), rather than doing it incrementally. \begin{code}

    tidy_eqn :: EquationInfo -> EquationInfo
    tidy_eqn eqn = eqn { eqn_pats = map tidy_pat (eqn_pats eqn),
    		     eqn_rhs  = tidy_rhs (eqn_rhs eqn) }
      where
    	-- Horrible hack.  The tidy_pat stuff converts "might-fail" patterns to
            -- WildPats which of course loses the info that they can fail to match.
    	-- So we stick in a CanFail as if it were a guard.
        tidy_rhs (MatchResult can_fail body)
    	| any might_fail_pat (eqn_pats eqn) = MatchResult CanFail body
    	| otherwise			    = MatchResult can_fail body

    --------------
    might_fail_pat :: Pat Id -> Bool
    -- Returns True of patterns that might fail (i.e. fall through) in a way
    -- that is not covered by the checking algorithm.  Specifically:
    -- 	   NPlusKPat
    --	   ViewPat (if refutable)

    -- First the two special cases
    might_fail_pat (NPlusKPat {}) 	     	     = True
    might_fail_pat (ViewPat _ p _) 	     	     = not (isIrrefutableHsPat p)

    -- Now the recursive stuff
    might_fail_pat (ParPat p)  	   	     = might_fail_lpat p
    might_fail_pat (AsPat _ p) 	   	     = might_fail_lpat p
    might_fail_pat (SigPatOut p _ )    	     = might_fail_lpat p
    might_fail_pat (ListPat ps _)  	   	     = any might_fail_lpat ps
    might_fail_pat (TuplePat ps _ _)   	     = any might_fail_lpat ps
    might_fail_pat (PArrPat ps _)  	   	     = any might_fail_lpat ps
    might_fail_pat (BangPat p)     	   	     = might_fail_lpat p
    might_fail_pat (ConPatOut { pat_args = ps }) = any might_fail_lpat (hsConPatArgs ps)

    -- Finally the ones that are sure to succeed, or which are covered by the checking algorithm
    might_fail_pat (LazyPat _)                   = False -- Always succeeds
    might_fail_pat _                             = False -- VarPat, VarPatOut, WildPat, LitPat, NPat, TypePat

    --------------
    might_fail_lpat :: LPat Id -> Bool
    might_fail_lpat (L _ p) = might_fail_pat p

    --------------
    tidy_lpat :: LPat Id -> LPat Id
    tidy_lpat p = fmap tidy_pat p

    --------------
    tidy_pat :: Pat Id -> Pat Id
    tidy_pat pat@(WildPat _)  = pat
    tidy_pat (VarPat id)      = WildPat (idType id)
    tidy_pat (VarPatOut id _) = WildPat (idType id) 	-- Ignore the bindings
    tidy_pat (ParPat p)       = tidy_pat (unLoc p)
    tidy_pat (LazyPat p)      = WildPat (hsLPatType p)	-- For overlap and exhaustiveness checking
    							-- purposes, a ~pat is like a wildcard
    tidy_pat (BangPat p)      = tidy_pat (unLoc p)
    tidy_pat (AsPat _ p)      = tidy_pat (unLoc p)
    tidy_pat (SigPatOut p _)  = tidy_pat (unLoc p)
    tidy_pat (CoPat _ pat _)  = tidy_pat pat

    -- These two are might_fail patterns, so we map them to
    -- WildPats.  The might_fail_pat stuff arranges that the
    -- guard says "this equation might fall through".
    tidy_pat (NPlusKPat id _ _ _) = WildPat (idType (unLoc id))
    tidy_pat (ViewPat _ _ ty)     = WildPat ty

    tidy_pat (NPat lit mb_neg eq) = tidyNPat lit mb_neg eq

    tidy_pat pat@(ConPatOut { pat_con = L _ id, pat_args = ps })
      = pat { pat_args = tidy_con id ps }

    tidy_pat (ListPat ps ty)
      = unLoc $ foldr (\ x y -> mkPrefixConPat consDataCon [x,y] list_ty)
    	                          (mkNilPat list_ty)
    	                          (map tidy_lpat ps)
      where list_ty = mkListTy ty

    -- introduce fake parallel array constructors to be able to handle parallel
    -- arrays with the existing machinery for constructor pattern
    --
    tidy_pat (PArrPat ps ty)
      = unLoc $ mkPrefixConPat (parrFakeCon (length ps))
    			   (map tidy_lpat ps)
    			   (mkPArrTy ty)

    tidy_pat (TuplePat ps boxity ty)
      = unLoc $ mkPrefixConPat (tupleCon boxity arity)
    		           (map tidy_lpat ps) ty
      where
        arity = length ps

    -- Unpack string patterns fully, so we can see when they overlap with
    -- each other, or even explicit lists of Chars.
    tidy_pat (LitPat lit)
      | HsString s <- lit
      = unLoc $ foldr (\c pat -> mkPrefixConPat consDataCon [mk_char_lit c, pat] stringTy)
    		  (mkPrefixConPat nilDataCon [] stringTy) (unpackFS s)
      | otherwise
      = tidyLitPat lit
      where
        mk_char_lit c = mkPrefixConPat charDataCon [nlLitPat (HsCharPrim c)] charTy

    -----------------
    tidy_con :: DataCon -> HsConPatDetails Id -> HsConPatDetails Id
    tidy_con _   (PrefixCon ps)   = PrefixCon (map tidy_lpat ps)
    tidy_con _   (InfixCon p1 p2) = PrefixCon [tidy_lpat p1, tidy_lpat p2]
    tidy_con con (RecCon (HsRecFields fs _))
      | null fs   = PrefixCon [nlWildPat | _ <- dataConOrigArgTys con]
    		-- Special case for null patterns; maybe not a record at all
      | otherwise = PrefixCon (map (tidy_lpat.snd) all_pats)
      where
         -- pad out all the missing fields with WildPats.
        field_pats = map (\ f -> (f, nlWildPat)) (dataConFieldLabels con)
        all_pats = foldr (\(HsRecField id p _) acc -> insertNm (getName (unLoc id)) p acc)
    		     field_pats fs

        insertNm nm p [] = [(nm,p)]
        insertNm nm p (x@(n,_):xs)
          | nm == n    = (nm,p):xs
          | otherwise  = x : insertNm nm p xs

\end{code}