typecheck/TcArrows.hs

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

    Typecheck arrow notation
    -}

    {-# LANGUAGE RankNTypes, TupleSections #-}

    module TcArrows ( tcProc ) where

    import {-# SOURCE #-}   TcExpr( tcMonoExpr, tcInferRho, tcSyntaxOp, tcCheckId, tcPolyExpr )

    import HsSyn
    import TcMatches
    import TcHsSyn( hsLPatType )
    import TcType
    import TcMType
    import TcBinds
    import TcPat
    import TcUnify
    import TcRnMonad
    import TcEnv
    import TcEvidence
    import Id( mkLocalId )
    import Inst
    import Name
    import TysWiredIn
    import VarSet
    import TysPrim
    import BasicTypes( Arity )
    import SrcLoc
    import Outputable
    import Util

    import Control.Monad

    {-
    Note [Arrow overview]
    ~~~~~~~~~~~~~~~~~~~~~
    Here's a summary of arrows and how they typecheck.  First, here's
    a cut-down syntax:

      expr ::= ....
            |  proc pat cmd

      cmd ::= cmd exp                    -- Arrow application
           |  \pat -> cmd                -- Arrow abstraction
           |  (| exp cmd1 ... cmdn |)    -- Arrow form, n>=0
           |  ... -- If, case in the usual way

      cmd_type ::= carg_type --> type

      carg_type ::= ()
                 |  (type, carg_type)

    Note that
     * The 'exp' in an arrow form can mention only
       "arrow-local" variables

     * An "arrow-local" variable is bound by an enclosing
       cmd binding form (eg arrow abstraction)

     * A cmd_type is here written with a funny arrow "-->",
       The bit on the left is a carg_type (command argument type)
       which itself is a nested tuple, finishing with ()

     * The arrow-tail operator (e1 -< e2) means
           (| e1 <<< arr snd |) e2

    ************************************************************************
    *                                                                      *
                    Proc
    *                                                                      *
    ************************************************************************
    -}

    tcProc :: InPat Name -> LHsCmdTop Name          -- proc pat -> expr
           -> ExpRhoType                            -- Expected type of whole proc expression
           -> TcM (OutPat TcId, LHsCmdTop TcId, TcCoercion)

    tcProc pat cmd exp_ty
      = newArrowScope $
        do  { exp_ty <- expTypeToType exp_ty  -- no higher-rank stuff with arrows
            ; (co, (exp_ty1, res_ty)) <- matchExpectedAppTy exp_ty
            ; (co1, (arr_ty, arg_ty)) <- matchExpectedAppTy exp_ty1
            ; let cmd_env = CmdEnv { cmd_arr = arr_ty }
            ; (pat', cmd') <- tcPat ProcExpr pat (mkCheckExpType arg_ty) $
                              tcCmdTop cmd_env cmd (unitTy, res_ty)
            ; let res_co = mkTcTransCo co
                             (mkTcAppCo co1 (mkTcNomReflCo res_ty))
            ; return (pat', cmd', res_co) }

    {-
    ************************************************************************
    *                                                                      *
                    Commands
    *                                                                      *
    ************************************************************************
    -}

    -- See Note [Arrow overview]
    type CmdType    = (CmdArgType, TcTauType)    -- cmd_type
    type CmdArgType = TcTauType                  -- carg_type, a nested tuple

    data CmdEnv
      = CmdEnv {
            cmd_arr :: TcType -- arrow type constructor, of kind *->*->*
        }

    mkCmdArrTy :: CmdEnv -> TcTauType -> TcTauType -> TcTauType
    mkCmdArrTy env t1 t2 = mkAppTys (cmd_arr env) [t1, t2]

    ---------------------------------------
    tcCmdTop :: CmdEnv
             -> LHsCmdTop Name
             -> CmdType
             -> TcM (LHsCmdTop TcId)

    tcCmdTop env (L loc (HsCmdTop cmd _ _ names)) cmd_ty@(cmd_stk, res_ty)
      = setSrcSpan loc $
        do  { cmd'   <- tcCmd env cmd cmd_ty
            ; names' <- mapM (tcSyntaxName ProcOrigin (cmd_arr env)) names
            ; return (L loc $ HsCmdTop cmd' cmd_stk res_ty names') }
    ----------------------------------------
    tcCmd  :: CmdEnv -> LHsCmd Name -> CmdType -> TcM (LHsCmd TcId)
            -- The main recursive function
    tcCmd env (L loc cmd) res_ty
      = setSrcSpan loc $ do
            { cmd' <- tc_cmd env cmd res_ty
            ; return (L loc cmd') }

    tc_cmd :: CmdEnv -> HsCmd Name  -> CmdType -> TcM (HsCmd TcId)
    tc_cmd env (HsCmdPar cmd) res_ty
      = do  { cmd' <- tcCmd env cmd res_ty
            ; return (HsCmdPar cmd') }

    tc_cmd env (HsCmdLet (L l binds) (L body_loc body)) res_ty
      = do  { (binds', body') <- tcLocalBinds binds         $
                                 setSrcSpan body_loc        $
                                 tc_cmd env body res_ty
            ; return (HsCmdLet (L l binds') (L body_loc body')) }

    tc_cmd env in_cmd@(HsCmdCase scrut matches) (stk, res_ty)
      = addErrCtxt (cmdCtxt in_cmd) $ do
          (scrut', scrut_ty) <- tcInferRho scrut
          matches' <- tcMatchesCase match_ctxt scrut_ty matches (mkCheckExpType res_ty)
          return (HsCmdCase scrut' matches')
      where
        match_ctxt = MC { mc_what = CaseAlt,
                          mc_body = mc_body }
        mc_body body res_ty' = do { res_ty' <- expTypeToType res_ty'
                                  ; tcCmd env body (stk, res_ty') }

    tc_cmd env (HsCmdIf Nothing pred b1 b2) res_ty    -- Ordinary 'if'
      = do  { pred' <- tcMonoExpr pred (mkCheckExpType boolTy)
            ; b1'   <- tcCmd env b1 res_ty
            ; b2'   <- tcCmd env b2 res_ty
            ; return (HsCmdIf Nothing pred' b1' b2')
        }

    tc_cmd env (HsCmdIf (Just fun) pred b1 b2) res_ty -- Rebindable syntax for if
      = do  { pred_ty <- newOpenFlexiTyVarTy
            -- For arrows, need ifThenElse :: forall r. T -> r -> r -> r
            -- because we're going to apply it to the environment, not
            -- the return value.
            ; (_, [r_tv]) <- tcInstSkolTyVars [alphaTyVar]
            ; let r_ty = mkTyVarTy r_tv
            ; checkTc (not (r_tv `elemVarSet` tyCoVarsOfType pred_ty))
                      (text "Predicate type of `ifThenElse' depends on result type")
            ; (pred', fun')
                <- tcSyntaxOp IfOrigin fun (map synKnownType [pred_ty, r_ty, r_ty])
                                           (mkCheckExpType r_ty) $ \ _ ->
                   tcMonoExpr pred (mkCheckExpType pred_ty)

            ; b1'   <- tcCmd env b1 res_ty
            ; b2'   <- tcCmd env b2 res_ty
            ; return (HsCmdIf (Just fun') pred' b1' b2')
        }

    -------------------------------------------
    --              Arrow application
    --          (f -< a)   or   (f -<< a)
    --
    --   D   |- fun :: a t1 t2
    --   D,G |- arg :: t1
    --  ------------------------
    --   D;G |-a  fun -< arg :: stk --> t2
    --
    --   D,G |- fun :: a t1 t2
    --   D,G |- arg :: t1
    --  ------------------------
    --   D;G |-a  fun -<< arg :: stk --> t2
    --
    -- (plus -<< requires ArrowApply)

    tc_cmd env cmd@(HsCmdArrApp fun arg _ ho_app lr) (_, res_ty)
      = addErrCtxt (cmdCtxt cmd)    $
        do  { arg_ty <- newOpenFlexiTyVarTy
            ; let fun_ty = mkCmdArrTy env arg_ty res_ty
            ; fun' <- select_arrow_scope (tcMonoExpr fun (mkCheckExpType fun_ty))

            ; arg' <- tcMonoExpr arg (mkCheckExpType arg_ty)

            ; return (HsCmdArrApp fun' arg' fun_ty ho_app lr) }
      where
           -- Before type-checking f, use the environment of the enclosing
           -- proc for the (-<) case.
           -- Local bindings, inside the enclosing proc, are not in scope
           -- inside f.  In the higher-order case (-<<), they are.
           -- See Note [Escaping the arrow scope] in TcRnTypes
        select_arrow_scope tc = case ho_app of
            HsHigherOrderApp -> tc
            HsFirstOrderApp  -> escapeArrowScope tc

    -------------------------------------------
    --              Command application
    --
    -- D,G |-  exp : t
    -- D;G |-a cmd : (t,stk) --> res
    -- -----------------------------
    -- D;G |-a cmd exp : stk --> res

    tc_cmd env cmd@(HsCmdApp fun arg) (cmd_stk, res_ty)
      = addErrCtxt (cmdCtxt cmd)    $
        do  { arg_ty <- newOpenFlexiTyVarTy
            ; fun'   <- tcCmd env fun (mkPairTy arg_ty cmd_stk, res_ty)
            ; arg'   <- tcMonoExpr arg (mkCheckExpType arg_ty)
            ; return (HsCmdApp fun' arg') }

    -------------------------------------------
    --              Lambda
    --
    -- D;G,x:t |-a cmd : stk --> res
    -- ------------------------------
    -- D;G |-a (\x.cmd) : (t,stk) --> res

    tc_cmd env
           (HsCmdLam (MG { mg_alts = L l [L mtch_loc
                                       (match@(Match _ pats _maybe_rhs_sig grhss))],
                           mg_origin = origin }))
           (cmd_stk, res_ty)
      = addErrCtxt (pprMatchInCtxt match)        $
        do  { (co, arg_tys, cmd_stk') <- matchExpectedCmdArgs n_pats cmd_stk

                    -- Check the patterns, and the GRHSs inside
            ; (pats', grhss') <- setSrcSpan mtch_loc                                 $
                                 tcPats LambdaExpr pats (map mkCheckExpType arg_tys) $
                                 tc_grhss grhss cmd_stk' (mkCheckExpType res_ty)

            ; let match' = L mtch_loc (Match LambdaExpr pats' Nothing grhss')
                  arg_tys = map hsLPatType pats'
                  cmd' = HsCmdLam (MG { mg_alts = L l [match'], mg_arg_tys = arg_tys
                                      , mg_res_ty = res_ty, mg_origin = origin })
            ; return (mkHsCmdWrap (mkWpCastN co) cmd') }
      where
        n_pats     = length pats
        match_ctxt = (LambdaExpr :: HsMatchContext Name)    -- Maybe KappaExpr?
        pg_ctxt    = PatGuard match_ctxt

        tc_grhss (GRHSs grhss (L l binds)) stk_ty res_ty
            = do { (binds', grhss') <- tcLocalBinds binds $
                                       mapM (wrapLocM (tc_grhs stk_ty res_ty)) grhss
                 ; return (GRHSs grhss' (L l binds')) }

        tc_grhs stk_ty res_ty (GRHS guards body)
            = do { (guards', rhs') <- tcStmtsAndThen pg_ctxt tcGuardStmt guards res_ty $
                                      \ res_ty -> tcCmd env body
                                                    (stk_ty, checkingExpType "tc_grhs" res_ty)
                 ; return (GRHS guards' rhs') }

    -------------------------------------------
    --              Do notation

    tc_cmd env (HsCmdDo (L l stmts) _) (cmd_stk, res_ty)
      = do  { co <- unifyType noThing unitTy cmd_stk  -- Expecting empty argument stack
            ; stmts' <- tcStmts ArrowExpr (tcArrDoStmt env) stmts res_ty
            ; return (mkHsCmdWrap (mkWpCastN co) (HsCmdDo (L l stmts') res_ty)) }

    -----------------------------------------------------------------
    --      Arrow ``forms''       (| e c1 .. cn |)
    --
    --      D; G |-a1 c1 : stk1 --> r1
    --      ...
    --      D; G |-an cn : stkn --> rn
    --      D |-  e :: forall e. a1 (e, stk1) t1
    --                                ...
    --                        -> an (e, stkn) tn
    --                        -> a  (e, stk) t
    --      e \not\in (stk, stk1, ..., stkm, t, t1, ..., tn)
    --      ----------------------------------------------
    --      D; G |-a  (| e c1 ... cn |)  :  stk --> t

    tc_cmd env cmd@(HsCmdArrForm expr f fixity cmd_args) (cmd_stk, res_ty)
      = addErrCtxt (cmdCtxt cmd)    $
        do  { (cmd_args', cmd_tys) <- mapAndUnzipM tc_cmd_arg cmd_args
                                  -- We use alphaTyVar for 'w'
            ; let e_ty = mkInvForAllTy alphaTyVar $
                         mkFunTys cmd_tys $
                         mkCmdArrTy env (mkPairTy alphaTy cmd_stk) res_ty
            ; expr' <- tcPolyExpr expr e_ty
            ; return (HsCmdArrForm expr' f fixity cmd_args') }

      where
        tc_cmd_arg :: LHsCmdTop Name -> TcM (LHsCmdTop TcId, TcType)
        tc_cmd_arg cmd
           = do { arr_ty <- newFlexiTyVarTy arrowTyConKind
                ; stk_ty <- newFlexiTyVarTy liftedTypeKind
                ; res_ty <- newFlexiTyVarTy liftedTypeKind
                ; let env' = env { cmd_arr = arr_ty }
                ; cmd' <- tcCmdTop env' cmd (stk_ty, res_ty)
                ; return (cmd',  mkCmdArrTy env' (mkPairTy alphaTy stk_ty) res_ty) }

    -----------------------------------------------------------------
    --              Base case for illegal commands
    -- This is where expressions that aren't commands get rejected

    tc_cmd _ cmd _
      = failWithTc (vcat [text "The expression", nest 2 (ppr cmd),
                          text "was found where an arrow command was expected"])

    matchExpectedCmdArgs :: Arity -> TcType -> TcM (TcCoercionN, [TcType], TcType)
    matchExpectedCmdArgs 0 ty
      = return (mkTcNomReflCo ty, [], ty)
    matchExpectedCmdArgs n ty
      = do { (co1, [ty1, ty2]) <- matchExpectedTyConApp pairTyCon ty
           ; (co2, tys, res_ty) <- matchExpectedCmdArgs (n-1) ty2
           ; return (mkTcTyConAppCo Nominal pairTyCon [co1, co2], ty1:tys, res_ty) }

    {-
    ************************************************************************
    *                                                                      *
                    Stmts
    *                                                                      *
    ************************************************************************
    -}

    --------------------------------
    --      Mdo-notation
    -- The distinctive features here are
    --      (a) RecStmts, and
    --      (b) no rebindable syntax

    tcArrDoStmt :: CmdEnv -> TcCmdStmtChecker
    tcArrDoStmt env _ (LastStmt rhs noret _) res_ty thing_inside
      = do  { rhs' <- tcCmd env rhs (unitTy, res_ty)
            ; thing <- thing_inside (panic "tcArrDoStmt")
            ; return (LastStmt rhs' noret noSyntaxExpr, thing) }

    tcArrDoStmt env _ (BodyStmt rhs _ _ _) res_ty thing_inside
      = do  { (rhs', elt_ty) <- tc_arr_rhs env rhs
            ; thing          <- thing_inside res_ty
            ; return (BodyStmt rhs' noSyntaxExpr noSyntaxExpr elt_ty, thing) }

    tcArrDoStmt env ctxt (BindStmt pat rhs _ _ _) res_ty thing_inside
      = do  { (rhs', pat_ty) <- tc_arr_rhs env rhs
            ; (pat', thing)  <- tcPat (StmtCtxt ctxt) pat (mkCheckExpType pat_ty) $
                                thing_inside res_ty
            ; return (mkTcBindStmt pat' rhs', thing) }

    tcArrDoStmt env ctxt (RecStmt { recS_stmts = stmts, recS_later_ids = later_names
                                , recS_rec_ids = rec_names }) res_ty thing_inside
      = do  { let tup_names = rec_names ++ filterOut (`elem` rec_names) later_names
            ; tup_elt_tys <- newFlexiTyVarTys (length tup_names) liftedTypeKind
            ; let tup_ids = zipWith mkLocalId tup_names tup_elt_tys
            ; tcExtendIdEnv tup_ids $ do
            { (stmts', tup_rets)
                    <- tcStmtsAndThen ctxt (tcArrDoStmt env) stmts res_ty   $ \ _res_ty' ->
                            -- ToDo: res_ty not really right
                       zipWithM tcCheckId tup_names (map mkCheckExpType tup_elt_tys)

            ; thing <- thing_inside res_ty
                    -- NB:  The rec_ids for the recursive things
                    --      already scope over this part. This binding may shadow
                    --      some of them with polymorphic things with the same Name
                    --      (see note [RecStmt] in HsExpr)

            ; let rec_ids = takeList rec_names tup_ids
            ; later_ids <- tcLookupLocalIds later_names

            ; let rec_rets = takeList rec_names tup_rets
            ; let ret_table = zip tup_ids tup_rets
            ; let later_rets = [r | i <- later_ids, (j, r) <- ret_table, i == j]

            ; return (emptyRecStmtId { recS_stmts = stmts'
                                     , recS_later_ids = later_ids
                                     , recS_later_rets = later_rets
                                     , recS_rec_ids = rec_ids
                                     , recS_rec_rets = rec_rets
                                     , recS_ret_ty = res_ty }, thing)
            }}

    tcArrDoStmt _ _ stmt _ _
      = pprPanic "tcArrDoStmt: unexpected Stmt" (ppr stmt)

    tc_arr_rhs :: CmdEnv -> LHsCmd Name -> TcM (LHsCmd TcId, TcType)
    tc_arr_rhs env rhs = do { ty <- newFlexiTyVarTy liftedTypeKind
                            ; rhs' <- tcCmd env rhs (unitTy, ty)
                            ; return (rhs', ty) }

    {-
    ************************************************************************
    *                                                                      *
                    Helpers
    *                                                                      *
    ************************************************************************
    -}

    mkPairTy :: Type -> Type -> Type
    mkPairTy t1 t2 = mkTyConApp pairTyCon [t1,t2]

    arrowTyConKind :: Kind          --  *->*->*
    arrowTyConKind = mkFunTys [liftedTypeKind, liftedTypeKind] liftedTypeKind

    {-
    ************************************************************************
    *                                                                      *
                    Errors
    *                                                                      *
    ************************************************************************
    -}

    cmdCtxt :: HsCmd Name -> SDoc
    cmdCtxt cmd = text "In the command:" <+> ppr cmd