[ghc-hetmet.git] / ghc / compiler / deSugar / Match.lhs

%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[Main_match]{The @match@ function}

\begin{code}
module Match ( match, matchWrapper, matchSimply, matchSinglePat ) where

#include "HsVersions.h"

import DynFlags ( DynFlag(..), dopt )
import HsSyn            
import TcHsSyn          ( hsPatType )
import Check            ( check, ExhaustivePat )
import CoreSyn
import CoreUtils        ( bindNonRec, exprType )
import DsMonad
import DsBinds          ( dsHsNestedBinds )
import DsGRHSs          ( dsGRHSs )
import DsUtils
import Id               ( idName, idType, Id )
import DataCon          ( dataConFieldLabels, dataConInstOrigArgTys, isVanillaDataCon )
import MatchCon         ( matchConFamily )
import MatchLit         ( matchLiterals, matchNPlusKPats, matchNPats, tidyLitPat, tidyNPat )
import PrelInfo         ( pAT_ERROR_ID )
import TcType           ( Type, tcTyConAppArgs )
import Type             ( splitFunTysN )
import TysWiredIn       ( consDataCon, mkTupleTy, mkListTy,
                          tupleCon, parrFakeCon, mkPArrTy )
import BasicTypes       ( Boxity(..) )
import ListSetOps       ( runs )
import SrcLoc           ( noSrcSpan, noLoc, unLoc, Located(..) )
import Util             ( lengthExceeds, notNull )
import Name             ( Name )
import Outputable
\end{code}

This function is a wrapper of @match@, it must be called from all the parts where 
it was called match, but only substitutes the firs call, ....
if the associated flags are declared, warnings will be issued.
It can not be called matchWrapper because this name already exists :-(

JJCQ 30-Nov-1997

\begin{code}
matchCheck ::  DsMatchContext
            -> [Id]             -- Vars rep'ing the exprs we're matching with
            -> Type             -- Type of the case expression
            -> [EquationInfo]   -- Info about patterns, etc. (type synonym below)
            -> DsM MatchResult  -- Desugared result!

matchCheck ctx vars ty qs
   = getDOptsDs                                 `thenDs` \ dflags ->
     matchCheck_really dflags ctx vars ty qs

matchCheck_really dflags ctx vars ty qs
  | incomplete && shadow = 
      dsShadowWarn ctx eqns_shadow              `thenDs`   \ () ->
      dsIncompleteWarn ctx pats                 `thenDs`   \ () ->
      match vars ty qs
  | incomplete            = 
      dsIncompleteWarn ctx pats                 `thenDs`   \ () ->
      match vars ty qs
  | shadow                = 
      dsShadowWarn ctx eqns_shadow              `thenDs`   \ () ->
      match vars ty qs
  | otherwise             =
      match vars ty qs
  where (pats, eqns_shadow) = check qs
        incomplete    = want_incomplete && (notNull pats)
        want_incomplete = case ctx of
                              DsMatchContext RecUpd _ _ ->
                                  dopt Opt_WarnIncompletePatternsRecUpd dflags
                              _ ->
                                  dopt Opt_WarnIncompletePatterns       dflags
        shadow        = dopt Opt_WarnOverlappingPatterns dflags
                        && not (null eqns_shadow)
\end{code}

This variable shows the maximum number of lines of output generated for warnings.
It will limit the number of patterns/equations displayed to@ maximum_output@.

(ToDo: add command-line option?)

\begin{code}
maximum_output = 4
\end{code}

The next two functions create the warning message.

\begin{code}
dsShadowWarn :: DsMatchContext -> [EquationInfo] -> DsM ()
dsShadowWarn ctx@(DsMatchContext kind _ _) qs = dsWarn warn 
        where
          warn | qs `lengthExceeds` maximum_output
               = pp_context ctx (ptext SLIT("are overlapped"))
                            (\ f -> vcat (map (ppr_eqn f kind) (take maximum_output qs)) $$
                            ptext SLIT("..."))
               | otherwise
               = pp_context ctx (ptext SLIT("are overlapped"))
                            (\ f -> vcat $ map (ppr_eqn f kind) qs)


dsIncompleteWarn :: DsMatchContext -> [ExhaustivePat] -> DsM ()
dsIncompleteWarn ctx@(DsMatchContext kind _ _) pats = dsWarn warn 
        where
          warn = pp_context ctx (ptext SLIT("are non-exhaustive"))
                            (\f -> hang (ptext SLIT("Patterns not matched:"))
                                   4 ((vcat $ map (ppr_incomplete_pats kind)
                                                  (take maximum_output pats))
                                      $$ dots))

          dots | pats `lengthExceeds` maximum_output = ptext SLIT("...")
               | otherwise                           = empty

pp_context (DsMatchContext kind pats loc) msg rest_of_msg_fun
  = (loc, vcat [ptext SLIT("Pattern match(es)") <+> msg,
                sep [ptext SLIT("In") <+> ppr_match <> char ':', nest 4 (rest_of_msg_fun pref)]])
  where
    (ppr_match, pref)
        = case kind of
             FunRhs fun -> (pprMatchContext kind, \ pp -> ppr fun <+> pp)
             other      -> (pprMatchContext kind, \ pp -> pp)

ppr_pats pats = sep (map ppr pats)

ppr_shadow_pats kind pats
  = sep [ppr_pats pats, matchSeparator kind, ptext SLIT("...")]
    
ppr_incomplete_pats kind (pats,[]) = ppr_pats pats
ppr_incomplete_pats kind (pats,constraints) = 
                         sep [ppr_pats pats, ptext SLIT("with"), 
                              sep (map ppr_constraint constraints)]
    

ppr_constraint (var,pats) = sep [ppr var, ptext SLIT("`notElem`"), ppr pats]

ppr_eqn prefixF kind eqn = prefixF (ppr_shadow_pats kind (eqn_pats eqn))
\end{code}


The function @match@ is basically the same as in the Wadler chapter,
except it is monadised, to carry around the name supply, info about
annotations, etc.

Notes on @match@'s arguments, assuming $m$ equations and $n$ patterns:
\begin{enumerate}
\item
A list of $n$ variable names, those variables presumably bound to the
$n$ expressions being matched against the $n$ patterns.  Using the
list of $n$ expressions as the first argument showed no benefit and
some inelegance.

\item
The second argument, a list giving the ``equation info'' for each of
the $m$ equations:
\begin{itemize}
\item
the $n$ patterns for that equation, and
\item
a list of Core bindings [@(Id, CoreExpr)@ pairs] to be ``stuck on
the front'' of the matching code, as in:
\begin{verbatim}
let <binds>
in  <matching-code>
\end{verbatim}
\item
and finally: (ToDo: fill in)

The right way to think about the ``after-match function'' is that it
is an embryonic @CoreExpr@ with a ``hole'' at the end for the
final ``else expression''.
\end{itemize}

There is a type synonym, @EquationInfo@, defined in module @DsUtils@.

An experiment with re-ordering this information about equations (in
particular, having the patterns available in column-major order)
showed no benefit.

\item
A default expression---what to evaluate if the overall pattern-match
fails.  This expression will (almost?) always be
a measly expression @Var@, unless we know it will only be used once
(as we do in @glue_success_exprs@).

Leaving out this third argument to @match@ (and slamming in lots of
@Var "fail"@s) is a positively {\em bad} idea, because it makes it
impossible to share the default expressions.  (Also, it stands no
chance of working in our post-upheaval world of @Locals@.)
\end{enumerate}
So, the full type signature:
\begin{code}
match :: [Id]             -- Variables rep'ing the exprs we're matching with
      -> Type             -- Type of the case expression
      -> [EquationInfo]   -- Info about patterns, etc. (type synonym below)
      -> DsM MatchResult  -- Desugared result!
\end{code}

Note: @match@ is often called via @matchWrapper@ (end of this module),
a function that does much of the house-keeping that goes with a call
to @match@.

It is also worth mentioning the {\em typical} way a block of equations
is desugared with @match@.  At each stage, it is the first column of
patterns that is examined.  The steps carried out are roughly:
\begin{enumerate}
\item
Tidy the patterns in column~1 with @tidyEqnInfo@ (this may add
bindings to the second component of the equation-info):
\begin{itemize}
\item
Remove the `as' patterns from column~1.
\item
Make all constructor patterns in column~1 into @ConPats@, notably
@ListPats@ and @TuplePats@.
\item
Handle any irrefutable (or ``twiddle'') @LazyPats@.
\end{itemize}
\item
Now {\em unmix} the equations into {\em blocks} [w/ local function
@unmix_eqns@], in which the equations in a block all have variable
patterns in column~1, or they all have constructor patterns in ...
(see ``the mixture rule'' in SLPJ).
\item
Call @matchEqnBlock@ on each block of equations; it will do the
appropriate thing for each kind of column-1 pattern, usually ending up
in a recursive call to @match@.
\end{enumerate}

%************************************************************************
%*                                                                      *
%*  match: empty rule                                                   *
%*                                                                      *
%************************************************************************
\subsection[Match-empty-rule]{The ``empty rule''}

We are a little more paranoid about the ``empty rule'' (SLPJ, p.~87)
than the Wadler-chapter code for @match@ (p.~93, first @match@ clause).
And gluing the ``success expressions'' together isn't quite so pretty.

\begin{code}
match [] ty eqns_info
  = ASSERT( not (null eqns_info) )
    returnDs (foldr1 combineMatchResults match_results)
  where
    match_results = [ ASSERT( null (eqn_pats eqn) ) 
                      adjustMatchResult (eqn_wrap eqn) (eqn_rhs eqn)
                    | eqn <- eqns_info ]
\end{code}


%************************************************************************
%*                                                                      *
%*  match: non-empty rule                                               *
%*                                                                      *
%************************************************************************
\subsection[Match-nonempty]{@match@ when non-empty: unmixing}

This (more interesting) clause of @match@ uses @tidy_and_unmix_eqns@
(a)~to get `as'- and `twiddle'-patterns out of the way (tidying), and
(b)~to do ``the mixture rule'' (SLPJ, p.~88) [which really {\em
un}mixes the equations], producing a list of equation-info
blocks, each block having as its first column of patterns either all
constructors, or all variables (or similar beasts), etc.

@match_unmixed_eqn_blks@ simply takes the place of the @foldr@ in the
Wadler-chapter @match@ (p.~93, last clause), and @match_unmixed_blk@
corresponds roughly to @matchVarCon@.

\begin{code}
match vars@(v:_) ty eqns_info
  = do  { tidy_eqns <- mappM (tidyEqnInfo v) eqns_info
        ; let eqns_blks = runs same_family tidy_eqns
        ; match_results <- mappM match_block eqns_blks
        ; ASSERT( not (null match_results) )
          return (foldr1 combineMatchResults match_results) }
  where
    same_family eqn1 eqn2 
      = samePatFamily (firstPat eqn1) (firstPat eqn2)
 
    match_block eqns
      = case firstPat (head eqns) of
          WildPat {}   -> matchVariables  vars ty eqns
          ConPatOut {} -> matchConFamily  vars ty eqns
          NPlusKPat {} -> matchNPlusKPats vars ty eqns
          NPat {}      -> matchNPats      vars ty eqns
          LitPat {}    -> matchLiterals   vars ty eqns

-- After tidying, there are only five kinds of patterns
samePatFamily (WildPat {})   (WildPat {})   = True
samePatFamily (ConPatOut {}) (ConPatOut {}) = True
samePatFamily (NPlusKPat {}) (NPlusKPat {}) = True
samePatFamily (NPat {})      (NPat {})      = True
samePatFamily (LitPat {})    (LitPat {})    = True
samePatFamily _              _              = False

matchVariables :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
-- Real true variables, just like in matchVar, SLPJ p 94
-- No binding to do: they'll all be wildcards by now (done in tidy)
matchVariables (var:vars) ty eqns = match vars ty (shiftEqns eqns)
\end{code}


\end{code}

Tidy up the leftmost pattern in an @EquationInfo@, given the variable @v@
which will be scrutinised.  This means:
\begin{itemize}
\item
Replace variable patterns @x@ (@x /= v@) with the pattern @_@,
together with the binding @x = v@.
\item
Replace the `as' pattern @x@@p@ with the pattern p and a binding @x = v@.
\item
Removing lazy (irrefutable) patterns (you don't want to know...).
\item
Converting explicit tuple-, list-, and parallel-array-pats into ordinary
@ConPats@. 
\item
Convert the literal pat "" to [].
\end{itemize}

The result of this tidying is that the column of patterns will include
{\em only}:
\begin{description}
\item[@WildPats@:]
The @VarPat@ information isn't needed any more after this.

\item[@ConPats@:]
@ListPats@, @TuplePats@, etc., are all converted into @ConPats@.

\item[@LitPats@ and @NPats@:]
@LitPats@/@NPats@ of ``known friendly types'' (Int, Char,
Float,  Double, at least) are converted to unboxed form; e.g.,
\tr{(NPat (HsInt i) _ _)} is converted to:
\begin{verbatim}
(ConPat I# _ _ [LitPat (HsIntPrim i)])
\end{verbatim}
\end{description}

\begin{code}
tidyEqnInfo :: Id -> EquationInfo -> DsM EquationInfo
        -- DsM'd because of internal call to dsHsNestedBinds
        --      and mkSelectorBinds.
        -- "tidy1" does the interesting stuff, looking at
        -- one pattern and fiddling the list of bindings.
        --
        -- POST CONDITION: head pattern in the EqnInfo is
        --      WildPat
        --      ConPat
        --      NPat
        --      LitPat
        --      NPlusKPat
        -- but no other

tidyEqnInfo v eqn@(EqnInfo { eqn_wrap = wrap, eqn_pats = pat : pats })
  = tidy1 v wrap pat    `thenDs` \ (wrap', pat') ->
    returnDs (eqn { eqn_wrap = wrap', eqn_pats = pat' : pats })

tidy1 :: Id                     -- The Id being scrutinised
      -> DsWrapper              -- Previous wrapping bindings
      -> Pat Id                 -- The pattern against which it is to be matched
      -> DsM (DsWrapper,        -- Extra bindings around what to do afterwards
              Pat Id)           -- Equivalent pattern

-- The extra bindings etc are all wrapped around the RHS of the match
-- so they are only available when matching is complete.  But that's ok
-- becuase, for example, in the pattern x@(...), the x can only be
-- used in the RHS, not in the nested pattern, nor subsquent patterns
--
-- However this does have an awkward consequence.  The bindings in 
-- a VarPatOut get wrapped around the result in right to left order,
-- rather than left to right.  This only matters if one set of 
-- bindings can mention things used in another, and that can happen
-- if we allow equality dictionary bindings of form d1=d2.  
-- bindIInstsOfLocalFuns is now careful not to do this, but it's a wart.
-- (Without this care in bindInstsOfLocalFuns, compiling 
-- Data.Generics.Schemes.hs fails in function everywhereBut.)

-------------------------------------------------------
--      (pat', mr') = tidy1 v pat mr
-- tidies the *outer level only* of pat, giving pat'
-- It eliminates many pattern forms (as-patterns, variable patterns,
-- list patterns, etc) yielding one of:
--      WildPat
--      ConPatOut
--      LitPat
--      NPat
--      NPlusKPat

tidy1 v wrap (ParPat pat)      = tidy1 v wrap (unLoc pat) 
tidy1 v wrap (SigPatOut pat _) = tidy1 v wrap (unLoc pat) 
tidy1 v wrap (WildPat ty)      = returnDs (wrap, WildPat ty)

        -- case v of { x -> mr[] }
        -- = case v of { _ -> let x=v in mr[] }
tidy1 v wrap (VarPat var)
  = returnDs (wrap . wrapBind var v, WildPat (idType var)) 

tidy1 v wrap (VarPatOut var binds)
  = do  { prs <- dsHsNestedBinds binds
        ; return (wrap . wrapBind var v . mkDsLet (Rec prs),
                  WildPat (idType var)) }

        -- case v of { x@p -> mr[] }
        -- = case v of { p -> let x=v in mr[] }
tidy1 v wrap (AsPat (L _ var) pat)
  = tidy1 v (wrap . wrapBind var v) (unLoc pat)


{- now, here we handle lazy patterns:
    tidy1 v ~p bs = (v, v1 = case v of p -> v1 :
                        v2 = case v of p -> v2 : ... : bs )

    where the v_i's are the binders in the pattern.

    ToDo: in "v_i = ... -> v_i", are the v_i's really the same thing?

    The case expr for v_i is just: match [v] [(p, [], \ x -> Var v_i)] any_expr
-}

tidy1 v wrap (LazyPat pat)
  = do  { v' <- newSysLocalDs (idType v)
        ; sel_prs <- mkSelectorBinds pat (Var v)
        ; let sel_binds =  [NonRec b rhs | (b,rhs) <- sel_prs]
        ; returnDs (wrap . wrapBind v' v . mkDsLets sel_binds,
                    WildPat (idType v)) }

-- re-express <con-something> as (ConPat ...) [directly]

tidy1 v wrap (ConPatOut (L loc con) ex_tvs dicts binds ps pat_ty)
  = returnDs (wrap, ConPatOut (L loc con) ex_tvs dicts binds tidy_ps pat_ty)
  where
    tidy_ps = PrefixCon (tidy_con con pat_ty ps)

tidy1 v wrap (ListPat pats ty)
  = returnDs (wrap, unLoc list_ConPat)
  where
    list_ty     = mkListTy ty
    list_ConPat = foldr (\ x y -> mkPrefixConPat consDataCon [x, y] list_ty)
                        (mkNilPat list_ty)
                        pats

-- Introduce fake parallel array constructors to be able to handle parallel
-- arrays with the existing machinery for constructor pattern
tidy1 v wrap (PArrPat pats ty)
  = returnDs (wrap, unLoc parrConPat)
  where
    arity      = length pats
    parrConPat = mkPrefixConPat (parrFakeCon arity) pats (mkPArrTy ty)

tidy1 v wrap (TuplePat pats boxity)
  = returnDs (wrap, unLoc tuple_ConPat)
  where
    arity = length pats
    tuple_ConPat = mkPrefixConPat (tupleCon boxity arity) pats
                                  (mkTupleTy boxity arity (map hsPatType pats))

tidy1 v wrap (DictPat dicts methods)
  = case num_of_d_and_ms of
        0 -> tidy1 v wrap (TuplePat [] Boxed) 
        1 -> tidy1 v wrap (unLoc (head dict_and_method_pats))
        _ -> tidy1 v wrap (TuplePat dict_and_method_pats Boxed)
  where
    num_of_d_and_ms      = length dicts + length methods
    dict_and_method_pats = map nlVarPat (dicts ++ methods)

-- LitPats: we *might* be able to replace these w/ a simpler form
tidy1 v wrap pat@(LitPat lit)
  = returnDs (wrap, unLoc (tidyLitPat lit (noLoc pat)))

-- NPats: we *might* be able to replace these w/ a simpler form
tidy1 v wrap pat@(NPat lit mb_neg _ lit_ty)
  = returnDs (wrap, unLoc (tidyNPat lit mb_neg lit_ty (noLoc pat)))

-- and everything else goes through unchanged...

tidy1 v wrap non_interesting_pat
  = returnDs (wrap, non_interesting_pat)


tidy_con data_con pat_ty (PrefixCon ps)   = ps
tidy_con data_con pat_ty (InfixCon p1 p2) = [p1,p2]
tidy_con data_con pat_ty (RecCon rpats)
  | null rpats
  =     -- Special case for C {}, which can be used for 
        -- a constructor that isn't declared to have
        -- fields at all
    map (noLoc . WildPat) con_arg_tys'

  | otherwise
  = ASSERT( isVanillaDataCon data_con )
        -- We're in a record case, so the data con must be vanilla
        -- and hence no existentials to worry about
    map mk_pat tagged_arg_tys
  where
        -- Boring stuff to find the arg-tys of the constructor
        
    inst_tys         = tcTyConAppArgs pat_ty    -- Newtypes must be opaque
    con_arg_tys'     = dataConInstOrigArgTys data_con inst_tys
    tagged_arg_tys   = con_arg_tys' `zip` dataConFieldLabels data_con

        -- mk_pat picks a WildPat of the appropriate type for absent fields,
        -- and the specified pattern for present fields
    mk_pat (arg_ty, lbl) = 
        case [ pat | (sel_id,pat) <- rpats, idName (unLoc sel_id) == lbl] of
          (pat:pats) -> ASSERT( null pats ) pat
          []         -> noLoc (WildPat arg_ty)
\end{code}

\noindent
{\bf Previous @matchTwiddled@ stuff:}

Now we get to the only interesting part; note: there are choices for
translation [from Simon's notes]; translation~1:
\begin{verbatim}
deTwiddle [s,t] e
\end{verbatim}
returns
\begin{verbatim}
[ w = e,
  s = case w of [s,t] -> s
  t = case w of [s,t] -> t
]
\end{verbatim}

Here \tr{w} is a fresh variable, and the \tr{w}-binding prevents multiple
evaluation of \tr{e}.  An alternative translation (No.~2):
\begin{verbatim}
[ w = case e of [s,t] -> (s,t)
  s = case w of (s,t) -> s
  t = case w of (s,t) -> t
]
\end{verbatim}

%************************************************************************
%*                                                                      *
\subsubsection[improved-unmixing]{UNIMPLEMENTED idea for improved unmixing}
%*                                                                      *
%************************************************************************

We might be able to optimise unmixing when confronted by
only-one-constructor-possible, of which tuples are the most notable
examples.  Consider:
\begin{verbatim}
f (a,b,c) ... = ...
f d ... (e:f) = ...
f (g,h,i) ... = ...
f j ...       = ...
\end{verbatim}
This definition would normally be unmixed into four equation blocks,
one per equation.  But it could be unmixed into just one equation
block, because if the one equation matches (on the first column),
the others certainly will.

You have to be careful, though; the example
\begin{verbatim}
f j ...       = ...
-------------------
f (a,b,c) ... = ...
f d ... (e:f) = ...
f (g,h,i) ... = ...
\end{verbatim}
{\em must} be broken into two blocks at the line shown; otherwise, you
are forcing unnecessary evaluation.  In any case, the top-left pattern
always gives the cue.  You could then unmix blocks into groups of...
\begin{description}
\item[all variables:]
As it is now.
\item[constructors or variables (mixed):]
Need to make sure the right names get bound for the variable patterns.
\item[literals or variables (mixed):]
Presumably just a variant on the constructor case (as it is now).
\end{description}

%************************************************************************
%*                                                                      *
%*  matchWrapper: a convenient way to call @match@                      *
%*                                                                      *
%************************************************************************
\subsection[matchWrapper]{@matchWrapper@: a convenient interface to @match@}

Calls to @match@ often involve similar (non-trivial) work; that work
is collected here, in @matchWrapper@.  This function takes as
arguments:
\begin{itemize}
\item
Typchecked @Matches@ (of a function definition, or a case or lambda
expression)---the main input;
\item
An error message to be inserted into any (runtime) pattern-matching
failure messages.
\end{itemize}

As results, @matchWrapper@ produces:
\begin{itemize}
\item
A list of variables (@Locals@) that the caller must ``promise'' to
bind to appropriate values; and
\item
a @CoreExpr@, the desugared output (main result).
\end{itemize}

The main actions of @matchWrapper@ include:
\begin{enumerate}
\item
Flatten the @[TypecheckedMatch]@ into a suitable list of
@EquationInfo@s.
\item
Create as many new variables as there are patterns in a pattern-list
(in any one of the @EquationInfo@s).
\item
Create a suitable ``if it fails'' expression---a call to @error@ using
the error-string input; the {\em type} of this fail value can be found
by examining one of the RHS expressions in one of the @EquationInfo@s.
\item
Call @match@ with all of this information!
\end{enumerate}

\begin{code}
matchWrapper :: HsMatchContext Name     -- For shadowing warning messages
             -> MatchGroup Id           -- Matches being desugared
             -> DsM ([Id], CoreExpr)    -- Results
\end{code}

 There is one small problem with the Lambda Patterns, when somebody
 writes something similar to:
\begin{verbatim}
    (\ (x:xs) -> ...)
\end{verbatim}
 he/she don't want a warning about incomplete patterns, that is done with 
 the flag @opt_WarnSimplePatterns@.
 This problem also appears in the:
\begin{itemize}
\item @do@ patterns, but if the @do@ can fail
      it creates another equation if the match can fail
      (see @DsExpr.doDo@ function)
\item @let@ patterns, are treated by @matchSimply@
   List Comprension Patterns, are treated by @matchSimply@ also
\end{itemize}

We can't call @matchSimply@ with Lambda patterns,
due to the fact that lambda patterns can have more than
one pattern, and match simply only accepts one pattern.

JJQC 30-Nov-1997

\begin{code}
matchWrapper ctxt (MatchGroup matches match_ty)
  = do  { eqns_info <- mapM mk_eqn_info matches
        ; dflags <- getDOptsDs
        ; locn <- getSrcSpanDs
        ; let   ds_ctxt      = DsMatchContext ctxt arg_pats locn
                error_string = matchContextErrString ctxt

        ; new_vars     <- selectMatchVars arg_pats pat_tys
        ; match_result <- match_fun dflags ds_ctxt new_vars rhs_ty eqns_info

        ; fail_expr <- mkErrorAppDs pAT_ERROR_ID rhs_ty error_string
        ; result_expr <- extractMatchResult match_result fail_expr
        ; return (new_vars, result_expr) }
  where 
    arg_pats          = map unLoc (hsLMatchPats (head matches))
    n_pats            = length arg_pats
    (pat_tys, rhs_ty) = splitFunTysN n_pats match_ty

    mk_eqn_info (L _ (Match pats _ grhss))
      = do { let upats = map unLoc pats
           ; match_result <- dsGRHSs ctxt upats grhss rhs_ty
           ; return (EqnInfo { eqn_wrap = idWrapper,
                               eqn_pats = upats, 
                               eqn_rhs = match_result}) }

    match_fun dflags ds_ctxt
       = case ctxt of 
           LambdaExpr | dopt Opt_WarnSimplePatterns dflags -> matchCheck ds_ctxt
                      | otherwise                          -> match
           _                                               -> matchCheck ds_ctxt
\end{code}

%************************************************************************
%*                                                                      *
\subsection[matchSimply]{@matchSimply@: match a single expression against a single pattern}
%*                                                                      *
%************************************************************************

@mkSimpleMatch@ is a wrapper for @match@ which deals with the
situation where we want to match a single expression against a single
pattern. It returns an expression.

\begin{code}
matchSimply :: CoreExpr                 -- Scrutinee
            -> HsMatchContext Name      -- Match kind
            -> LPat Id                  -- Pattern it should match
            -> CoreExpr                 -- Return this if it matches
            -> CoreExpr                 -- Return this if it doesn't
            -> DsM CoreExpr

matchSimply scrut hs_ctx pat result_expr fail_expr
  = let
      match_result = cantFailMatchResult result_expr
      rhs_ty       = exprType fail_expr
        -- Use exprType of fail_expr, because won't refine in the case of failure!
    in 
    matchSinglePat scrut hs_ctx pat rhs_ty match_result `thenDs` \ match_result' ->
    extractMatchResult match_result' fail_expr


matchSinglePat :: CoreExpr -> HsMatchContext Name -> LPat Id
               -> Type -> MatchResult -> DsM MatchResult
matchSinglePat (Var var) hs_ctx (L _ pat) ty match_result
  = getDOptsDs                          `thenDs` \ dflags ->
    getSrcSpanDs                        `thenDs` \ locn ->
    let
        match_fn dflags
           | dopt Opt_WarnSimplePatterns dflags = matchCheck ds_ctx
           | otherwise                          = match
           where
             ds_ctx = DsMatchContext hs_ctx [pat] locn
    in
    match_fn dflags [var] ty [EqnInfo { eqn_wrap = idWrapper,
                                        eqn_pats = [pat],
                                        eqn_rhs  = match_result }]

matchSinglePat scrut hs_ctx pat ty match_result
  = selectSimpleMatchVarL pat                           `thenDs` \ var ->
    matchSinglePat (Var var) hs_ctx pat ty match_result `thenDs` \ match_result' ->
    returnDs (adjustMatchResult (bindNonRec var scrut) match_result')
\end{code}