Desugar.hs 40.3 KB
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{- |
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  Module      :  $Header$
  Description :  Desugaring Curry Expressions
  Copyright   :  (c) 2001 - 2004 Wolfgang Lux
                                 Martin Engelke
                     2011 - 2015 Björn Peemöller
  License     :  OtherLicense

  Maintainer  :  bjp@informatik.uni-kiel.de
  Stability   :  experimental
  Portability :  portable

  The desugaring pass removes all syntactic sugar from the module. In
  particular, the output of the desugarer will have the following
  properties.

  * No guarded right hand sides occur in equations, pattern
    declarations, and case alternatives. In addition, the declaration
    lists of the right hand sides are empty; local declarations are
    transformed into let expressions.

  * Patterns in equations and case alternatives are composed only of
    - literals,
    - variables,
    - constructor applications, and
    - as patterns.

  * Expressions are composed only of
    - literals,
    - variables,
    - constructors,
    - (binary) applications,
    - let expressions, and
    - case expressions.

  * Applications 'N x' in patterns and expressions, where 'N' is a
    newtype constructor, are replaced by a 'x'. Note that neither the
    newtype declaration itself nor partial applications of newtype
    constructors are changed.
    It were possible to replace partial applications of newtype constructor
    by 'Prelude.id'.
    However, our solution yields a more accurate output when the result
    of a computation includes partial applications.

  * Functional patterns are replaced by variables and are integrated
    in a guarded right hand side using the (=:<=) operator

  * Records, which currently must be declared using the keyword 'type',
    are transformed into data types with one constructor.
    Record construction and pattern matching are represented using the
    record constructor. Selection and update are represented using selector
    and update functions which are generated for each record declaration.
    The record constructor must be entered into the type environment as well
    as the selector functions and the update functions.

  As we are going to insert references to real prelude entities,
  all names must be properly qualified before calling this module.
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-}
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{-# LANGUAGE CPP #-}
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module Transformations.Desugar (desugar) where

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#if __GLASGOW_HASKELL__ >= 710
import           Control.Applicative        ((<$>))
#else
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import           Control.Applicative        ((<$>), (<*>))
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#endif
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import           Control.Arrow              (first, second)
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import           Control.Monad              (mplus)
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import qualified Control.Monad.State as S   (State, runState, gets, modify)
import           Data.List                  ((\\), nub, tails)
import           Data.Maybe                 (fromMaybe)
import qualified Data.Set            as Set (Set, empty, member, insert)

import Curry.Base.Ident
import Curry.Base.Position hiding (first)
import Curry.Syntax

import Base.Expr
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import Base.CurryTypes (toType, fromType)
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import Base.Messages   (internalError)
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import Base.Types
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import Base.TypeSubst  (expandAliasType)
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import Base.Typing
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import Base.Utils      (mapAccumM, concatMapM)
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import Env.TypeConstructor (TCEnv, TypeInfo (..), qualLookupTC)
import Env.Value (ValueEnv, ValueInfo (..), bindFun, bindGlobalInfo
  , lookupValue, qualLookupValue)

-- New identifiers may be introduced while desugaring pattern
-- declarations, case and lambda-expressions, and list comprehensions.
-- As usual, we use a state monad transformer for generating unique
-- names. In addition, the state is also used for passing through the
-- type environment, which must be augmented with the types of these new
-- variables.

data DesugarState = DesugarState
  { moduleIdent :: ModuleIdent      -- read-only
  , extensions  :: [KnownExtension] -- read-only
  , tyConsEnv   :: TCEnv            -- read-only
  , valueEnv    :: ValueEnv
  , nextId      :: Integer     -- counter
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  , desugarFP   :: Bool
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  }

type DsM a = S.State DesugarState a

getModuleIdent :: DsM ModuleIdent
getModuleIdent = S.gets moduleIdent

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checkNegativeLitsExtension :: DsM Bool
checkNegativeLitsExtension = S.gets (\s -> NegativeLiterals `elem` extensions s)
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getTyConsEnv :: DsM TCEnv
getTyConsEnv = S.gets tyConsEnv

getValueEnv :: DsM ValueEnv
getValueEnv = S.gets valueEnv

modifyValueEnv :: (ValueEnv -> ValueEnv) -> DsM ()
modifyValueEnv f = S.modify $ \ s -> s { valueEnv = f $ valueEnv s }

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desugarFunPats :: DsM Bool
desugarFunPats = S.gets desugarFP

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getNextId :: DsM Integer
getNextId = do
  nid <- S.gets nextId
  S.modify $ \ s -> s { nextId = succ nid }
  return nid

-- ---------------------------------------------------------------------------
-- Generation of fresh names
-- ---------------------------------------------------------------------------

getTypeOf :: Typeable t => t -> DsM Type
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getTypeOf t = do
  tyEnv <- getValueEnv
  tcEnv <- getTyConsEnv
  return (typeOf tyEnv tcEnv t)
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freshIdent :: String -> Int -> TypeScheme -> DsM Ident
freshIdent prefix arity ty = do
  m <- getModuleIdent
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  x <- freeIdent
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  modifyValueEnv $ bindFun m x arity ty
  return x
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  where
  mkName pre n = mkIdent $ pre ++ show n
  -- TODO: This loop is only necessary because a combination of desugaring,
  -- simplification and a repeated desugaring, as currently needed for
  -- non-linear and functional patterns, may reintroduce identifiers removed
  -- during desugaring. The better solution would be to move the translation
  -- of non-linear and functional pattern into a separate module.
  freeIdent = do
    x <- mkName prefix <$> getNextId
    tyEnv <- getValueEnv
    case lookupValue x tyEnv of
      [] -> return x
      _  -> freeIdent
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freshMonoTypeVar :: Typeable t => String -> t -> DsM Ident
freshMonoTypeVar prefix t = getTypeOf t >>= \ ty ->
  freshIdent prefix (arrowArity ty) (monoType ty)

-- The desugaring phase keeps only the type, function, and value
-- declarations of the module. In the current version, record declarations
-- are transformed into data types. The remaining type declarations are
-- not desugared and cannot occur in local declaration groups.
-- They are filtered out separately.

-- In order to use records within other modules, the export specification
-- of the module has to be extended with the selector and update functions of
-- all exported labels.

-- Actually, the transformation is slightly more general than necessary
-- as it allows value declarations at the top-level of a module.

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desugar :: Bool -> [KnownExtension] -> ValueEnv -> TCEnv -> Module
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        -> (Module, ValueEnv)
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desugar dsFunPats xs tyEnv tcEnv (Module ps m es is ds)
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  = (Module ps m es is ds', valueEnv s')
  where (ds', s') = S.runState (desugarModuleDecls ds)
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                               (DesugarState m xs tcEnv tyEnv 1 dsFunPats)
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desugarModuleDecls :: [Decl] -> DsM [Decl]
desugarModuleDecls ds = do
  ds'  <- concatMapM dsRecordDecl ds -- convert record decls to data decls
  ds'' <- dsDeclGroup ds'
  return $ filter isTypeDecl ds' ++ ds''

-- Within a declaration group, all type signatures and evaluation
-- annotations are discarded. First, the patterns occurring in the left
-- hand sides are desugared. Due to lazy patterns, this may add further
-- declarations to the group that must be desugared as well.

dsDeclGroup :: [Decl] -> DsM [Decl]
dsDeclGroup ds = concatMapM dsDeclLhs valDecls >>= mapM dsDeclRhs
 where valDecls = filter isValueDecl ds

dsDeclLhs :: Decl -> DsM [Decl]
dsDeclLhs (PatternDecl p t rhs) = do
  (ds', t') <- dsPattern p [] t
  dss'      <- mapM dsDeclLhs ds'
  return $ PatternDecl p t' rhs : concat dss'
dsDeclLhs (ExternalDecl   p fs) = mapM (genForeignDecl p) fs
dsDeclLhs d                     = return [d]

genForeignDecl :: Position -> Ident -> DsM Decl
genForeignDecl p f = do
  m     <- getModuleIdent
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  ty    <- fromType <$> (getTypeOf $ Variable $ qual m f)
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  return $ ForeignDecl p CallConvPrimitive (Just $ idName f) f ty
  where qual m f'
         | hasGlobalScope f' = qualifyWith m f'
         | otherwise         = qualify f'

-- After desugaring its right hand side, each equation is eta-expanded
-- by adding as many variables as necessary to the argument list and
-- applying the right hand side to those variables (Note: eta-expansion
-- is disabled in the version for PAKCS).
-- Furthermore every occurrence of a record type within the type of a function
-- is simplified to the corresponding type constructor from the record
-- declaration. This is possible because currently records must not be empty
-- and a record label belongs to only one record declaration.

dsDeclRhs :: Decl -> DsM Decl
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dsDeclRhs (FunctionDecl     p f eqs) = FunctionDecl p f <$> mapM dsEquation eqs
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dsDeclRhs (PatternDecl      p t rhs) = PatternDecl  p t <$> dsRhs p id rhs
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dsDeclRhs (ForeignDecl p cc ie f ty) = return $ ForeignDecl p cc ie' f ty
  where ie' = ie `mplus` Just (idName f)
dsDeclRhs fs@(FreeDecl          _ _) = return fs
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dsDeclRhs _ = error "Desugar.dsDeclRhs: no pattern match"

dsEquation :: Equation -> DsM Equation
dsEquation (Equation p lhs rhs) = do
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  funpats        <- desugarFunPats
  (ds1, cs, ts1) <- if funpats then do
                                  (     cs1, ts1) <- dsNonLinearity         ts
                                  (ds2, cs2, ts2) <- dsFunctionalPatterns p ts1
                                  return (ds2, cs2 ++ cs1, ts2)
                                else return ([], [], ts)
  (ds2    , ts2) <- mapAccumM (dsPattern p) [] ts1
  rhs'           <- dsRhs p (addConstraints cs) $ addDecls (ds1 ++ ds2) $ rhs
  return $ Equation p (FunLhs f ts2) rhs'
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  where (f, ts) = flatLhs lhs

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-- -----------------------------------------------------------------------------
-- Desugaring of non-linear patterns
-- -----------------------------------------------------------------------------

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-- The desugaring traverses a pattern in depth-first order and collects
-- all variables. If it encounters a variable which has been previously
-- introduced, the second occurrence is changed to a fresh variable
-- and a new pair (newvar, oldvar) is saved to generate constraints later.
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-- Non-linear patterns inside single functional patterns are not desugared,
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-- as this special case is handled later.
dsNonLinearity :: [Pattern] -> DsM ([Expression], [Pattern])
dsNonLinearity ts = do
  ((_, cs), ts') <- mapAccumM dsNonLinear (Set.empty, []) ts
  return (reverse cs, ts')

type NonLinearEnv = (Set.Set Ident, [Expression])

dsNonLinear :: NonLinearEnv -> Pattern -> DsM (NonLinearEnv, Pattern)
dsNonLinear env l@(LiteralPattern        _) = return (env, l)
dsNonLinear env n@(NegativePattern     _ _) = return (env, n)
dsNonLinear env t@(VariablePattern       v)
  | v `Set.member` vis = do
    v' <- freshMonoTypeVar "_#nonlinear" t
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    return ((vis, mkStrictEquality v v' : eqs), VariablePattern v')
  | otherwise          = return ((Set.insert v vis, eqs), t)
  where (vis, eqs) = env
dsNonLinear env (ConstructorPattern   c ts) = second (ConstructorPattern c)
                                              <$> mapAccumM dsNonLinear env ts
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dsNonLinear env (InfixPattern     t1 op t2) = do
  (env1, t1') <- dsNonLinear env  t1
  (env2, t2') <- dsNonLinear env1 t2
  return (env2, InfixPattern t1' op t2')
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dsNonLinear env (ParenPattern            t) = second ParenPattern
                                              <$> dsNonLinear env t
dsNonLinear env (TuplePattern       pos ts) = second (TuplePattern pos)
                                              <$> mapAccumM dsNonLinear env ts
dsNonLinear env (ListPattern        pos ts) = second (ListPattern pos)
                                              <$> mapAccumM dsNonLinear env ts
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dsNonLinear env (AsPattern             v t) = do
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  (env1, VariablePattern v') <- dsNonLinear env  (VariablePattern v)
  (env2, t'                ) <- dsNonLinear env1 t
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  return (env2, AsPattern v' t')
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dsNonLinear env (LazyPattern           r t) = second (LazyPattern r)
                                          <$> dsNonLinear env t
dsNonLinear env fp@(FunctionPattern    _ _) = dsNonLinearFuncPat env fp
dsNonLinear env fp@(InfixFuncPattern _ _ _) = dsNonLinearFuncPat env fp
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dsNonLinear env (RecordPattern        fs r) = do
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  (env1, fs') <- mapAccumM dsField env  fs
  (env2, r' ) <- case r of
    Nothing -> return (env1, Nothing)
    Just r0 -> second Just <$> dsNonLinear env1 r0
  return (env2, RecordPattern fs' r')
  where dsField e (Field p i t) = second (Field p i) <$> dsNonLinear e t

dsNonLinearFuncPat :: NonLinearEnv -> Pattern -> DsM (NonLinearEnv, Pattern)
dsNonLinearFuncPat (vis, eqs) fp = do
  let fpVars = bv fp
      vs     = filter (`Set.member` vis) fpVars
  vs' <- mapM (freshMonoTypeVar "_#nonlinear" . VariablePattern) vs
  let vis' = foldr Set.insert vis fpVars
      fp'  = substPat (zip vs vs') fp
  return ((vis', zipWith mkStrictEquality vs vs' ++ eqs), fp')

mkStrictEquality :: Ident -> Ident -> Expression
mkStrictEquality x y = mkVar x =:= mkVar y

substPat :: [(Ident, Ident)] -> Pattern -> Pattern
substPat _ l@(LiteralPattern        _) = l
substPat _ n@(NegativePattern     _ _) = n
substPat s (VariablePattern         v) = VariablePattern
                                       $ fromMaybe v (lookup v s)
substPat s (ConstructorPattern   c ps) = ConstructorPattern c
                                       $ map (substPat s) ps
substPat s (InfixPattern     p1 op p2) = InfixPattern (substPat s p1) op
                                                      (substPat s p2)
substPat s (ParenPattern            p) = ParenPattern (substPat s p)
substPat s (TuplePattern       pos ps) = TuplePattern pos $ map (substPat s) ps
substPat s (ListPattern        pos ps) = ListPattern  pos $ map (substPat s) ps
substPat s (AsPattern             v p) = AsPattern    (fromMaybe v (lookup v s))
                                                      (substPat s p)
substPat s (LazyPattern           r p) = LazyPattern r (substPat s p)
substPat s (FunctionPattern      f ps) = FunctionPattern f $ map (substPat s) ps
substPat s (InfixFuncPattern p1 op p2) = InfixFuncPattern (substPat s p1) op
                                                          (substPat s p2)
substPat s (RecordPattern        fs p) = RecordPattern    (map substField fs)
                                                          (substPat s <$> p)
  where substField (Field pos i t) = Field pos i (substPat s t)

-- -----------------------------------------------------------------------------
-- Desugaring of functional patterns
-- -----------------------------------------------------------------------------

-- Desugaring of functional patterns works in the following way:
--  1. The patterns are recursively traversed from left to right
--     to extract every functional pattern (note that functional patterns
--     can not be nested).
--     Each pattern is replaced by a fresh variable and a pair
--     (variable, functional pattern) is generated.
--  2. The variable-pattern pairs of the form @(v, p)@ are collected and
--     transformed into additional constraints of the form @p =:<= v@,
--     where the pattern @p@ is converted to the corresponding expression.
--     In addition, any variable occurring in @p@ is declared as a fresh
--     free variable.
--     Multiple constraints will later be combined using the @&>@-operator
--     such that the patterns are evaluated from left to right.

dsFunctionalPatterns :: Position -> [Pattern]
                     -> DsM ([Decl], [Expression], [Pattern])
dsFunctionalPatterns p ts = do
  -- extract functional patterns
  (bs, ts') <- mapAccumM elimFP [] ts
  -- generate declarations of free variables and constraints
  let (ds, cs) = genFPExpr p (bv ts') (reverse bs)
  -- return (declarations, constraints, desugared patterns)
  return (ds, cs, ts')

type LazyBinding = (Pattern, Ident)

elimFP :: [LazyBinding] -> Pattern -> DsM ([LazyBinding], Pattern)
elimFP bs p@(LiteralPattern        _) = return (bs, p)
elimFP bs p@(NegativePattern     _ _) = return (bs, p)
elimFP bs p@(VariablePattern       _) = return (bs, p)
elimFP bs (ConstructorPattern   c ts) = second (ConstructorPattern c)
                                        <$> mapAccumM elimFP bs ts
elimFP bs (InfixPattern     t1 op t2) = do
  (bs1, t1') <- elimFP bs  t1
  (bs2, t2') <- elimFP bs1 t2
  return (bs2, InfixPattern t1' op t2')
elimFP bs (ParenPattern            t) = second ParenPattern <$> elimFP bs t
elimFP bs (TuplePattern       pos ts) = second (TuplePattern pos)
                                        <$> mapAccumM elimFP bs ts
elimFP bs (ListPattern        pos ts) = second (ListPattern pos)
                                        <$> mapAccumM elimFP bs ts
elimFP bs (AsPattern             v t) = second (AsPattern v) <$> elimFP bs t
elimFP bs (LazyPattern           r t) = second (LazyPattern r) <$> elimFP bs t
elimFP bs p@(FunctionPattern     _ _) = do
 v <- freshMonoTypeVar "_#funpatt" p
 return ((p, v) : bs, VariablePattern v)
elimFP bs p@(InfixFuncPattern  _ _ _) = do
 v <- freshMonoTypeVar "_#funpatt" p
 return ((p, v) : bs, VariablePattern v)
elimFP bs (RecordPattern        fs r) = second (flip RecordPattern r)
                                        <$> mapAccumM elimField bs fs
  where elimField b (Field p i t) = second (Field p i) <$> elimFP b t

genFPExpr :: Position -> [Ident] -> [LazyBinding] -> ([Decl], [Expression])
genFPExpr p vs bs
  | null bs   = ([]               , [])
  | null free = ([]               , cs)
  | otherwise = ([FreeDecl p free], cs)
  where
  mkLB (t, v) = let (t', es) = fp2Expr t
                in  (t' =:<= mkVar v) : es
  cs       = concatMap mkLB bs
  free     = nub $ filter (not . isAnonId) $ bv (map fst bs) \\ vs

fp2Expr :: Pattern -> (Expression, [Expression])
fp2Expr (LiteralPattern          l) = (Literal l, [])
fp2Expr (NegativePattern       _ l) = (Literal (negateLiteral l), [])
fp2Expr (VariablePattern         v) = (mkVar v, [])
fp2Expr (ConstructorPattern   c ts) =
  let (ts', ess) = unzip $ map fp2Expr ts
  in  (apply (Constructor c) ts', concat ess)
fp2Expr (InfixPattern     t1 op t2) =
  let (t1', es1) = fp2Expr t1
      (t2', es2) = fp2Expr t2
  in  (InfixApply t1' (InfixConstr op) t2', es1 ++ es2)
fp2Expr (ParenPattern            t) = first Paren (fp2Expr t)
fp2Expr (TuplePattern         r ts) =
  let (ts', ess) = unzip $ map fp2Expr ts
  in  (Tuple r ts', concat ess)
fp2Expr (ListPattern         rs ts) =
  let (ts', ess) = unzip $ map fp2Expr ts
  in  (List rs ts', concat ess)
fp2Expr (FunctionPattern      f ts) =
  let (ts', ess) = unzip $ map fp2Expr ts
  in  (apply (Variable f) ts', concat ess)
fp2Expr (InfixFuncPattern t1 op t2) =
  let (t1', es1) = fp2Expr t1
      (t2', es2) = fp2Expr t2
  in  (InfixApply t1' (InfixOp op) t2', es1 ++ es2)
fp2Expr (AsPattern             v t) =
  let (t', es) = fp2Expr t
  in  (mkVar v, (t' =:<= mkVar v):es)
fp2Expr t                           = internalError $
  "Desugar.fp2Expr: Unexpected constructor term: " ++ show t

-- -----------------------------------------------------------------------------
-- Desugaring of remaining patterns
-- -----------------------------------------------------------------------------
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-- The transformation of patterns is straight forward except for lazy
-- patterns. A lazy pattern '~t' is replaced by a fresh
-- variable 'v' and a new local declaration 't = v' in the
-- scope of the pattern. In addition, as-patterns 'v@t' where
-- 't' is a variable or an as-pattern are replaced by 't' in combination
-- with a local declaration for 'v'.

dsPattern :: Position -> [Decl] -> Pattern -> DsM ([Decl], Pattern)
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dsPattern _ ds v@(VariablePattern      _) = return (ds, v)
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dsPattern p ds (LiteralPattern         l) = do
  dl <- dsLiteral l
  case dl of
    Left  l'     -> return (ds, LiteralPattern l')
    Right (rs,ls) -> dsPattern p ds $ ListPattern rs $ map LiteralPattern ls
dsPattern p ds (NegativePattern      _ l) =
  dsPattern p ds (LiteralPattern (negateLiteral l))
dsPattern p ds (ConstructorPattern c [t]) = do
    tyEnv <- getValueEnv
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    (if isNewtypeConstr tyEnv c then id else second (constrPat c)) <$>
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          (dsPattern p ds t)
  where constrPat c' t' = ConstructorPattern c' [t']
dsPattern p ds (ConstructorPattern c ts) =
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  second (ConstructorPattern c) <$> mapAccumM (dsPattern p) ds ts
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dsPattern p ds (InfixPattern t1 op t2) =
  dsPattern p ds (ConstructorPattern op [t1,t2])
dsPattern p ds (ParenPattern      t) = dsPattern p ds t
dsPattern p ds (TuplePattern pos ts) =
  dsPattern p ds (ConstructorPattern (tupleConstr ts) ts)
  where tupleConstr ts' = addRef pos $
                         if null ts' then qUnitId else qTupleId (length ts')
dsPattern p ds (ListPattern pos ts) =
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  second (dsList pos cons nil) <$> mapAccumM (dsPattern p) ds ts
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  where nil  p' = ConstructorPattern (addRef p' qNilId) []
        cons p' t ts' = ConstructorPattern (addRef p' qConsId) [t,ts']
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dsPattern p ds (AsPattern   v t) = dsAs p v <$> dsPattern p ds t
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dsPattern p ds (LazyPattern r t) = dsLazy r p ds t
dsPattern p ds (FunctionPattern f ts) =
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  second (FunctionPattern f) <$> mapAccumM (dsPattern p) ds ts
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dsPattern p ds (InfixFuncPattern t1 f t2) =
  dsPattern p ds (FunctionPattern f [t1,t2])
dsPattern p ds (RecordPattern fs _)
  | null fs   = internalError "Desugar.dsPattern: empty record"
  | otherwise = do
    r   <- recordFromField (fieldLabel (head fs))
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    fs' <- (map fst . snd) <$> lookupRecord r
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    let ts = map (dsLabel (map field2Tuple fs)) fs'
    dsPattern p ds (ConstructorPattern r ts)
  where dsLabel fs' l = fromMaybe (VariablePattern anonId) (lookup l fs')

dsLiteral :: Literal -> DsM (Either Literal ([SrcRef], [Literal]))
dsLiteral c@(Char             _ _) = return $ Left c
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dsLiteral (Int                v i) = do
  tyEnv <- getValueEnv
  tcEnv <- getTyConsEnv
  return (Left (fixType tyEnv tcEnv))
  where fixType tyEnv' tcEnv'
          | typeOf tyEnv' tcEnv' v == floatType =
              Float (srcRefOf $ idPosition v) (fromIntegral i)
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          | otherwise = Int v i
dsLiteral f@(Float            _ _) = return $ Left f
dsLiteral (String (SrcRef [i]) cs) = return $ Right
  (consRefs i cs, zipWith (Char . SrcRef . (:[])) [i, i + 2 ..] cs)
  where consRefs r []     = [SrcRef [r]]
        consRefs r (_:xs) = let r' = r + 2
                            in  r' `seq` (SrcRef [r'] : consRefs r' xs)
dsLiteral (String is _) = internalError $
  "Desugar.dsLiteral: " ++ "wrong source ref for string "  ++ show is

dsList :: [SrcRef] -> (SrcRef -> b -> b -> b) -> (SrcRef -> b) -> [b] -> b
dsList pos cons nil xs = snd (foldr cons' nil' xs)
  where rNil : rCs = reverse pos
        nil'                 = (rCs , nil rNil)
        cons' t (rC:rCs',ts) = (rCs', cons rC t ts)
        cons' _ ([],_) = error "Desugar.dsList.cons': empty list"

dsAs :: Position -> Ident -> ([Decl], Pattern) -> ([Decl], Pattern)
dsAs p v (ds, t) = case t of
  VariablePattern v' -> (varDecl p v (mkVar v') : ds, t)
  AsPattern     v' _ -> (varDecl p v (mkVar v') : ds, t)
  _                  -> (ds, AsPattern v t)

dsLazy :: SrcRef -> Position -> [Decl] -> Pattern -> DsM ([Decl], Pattern)
dsLazy pos p ds t = case t of
  VariablePattern   _ -> return (ds, t)
  ParenPattern     t' -> dsLazy pos p ds t'
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  AsPattern      v t' -> dsAs p v <$> dsLazy pos p ds t'
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  LazyPattern pos' t' -> dsLazy pos' p ds t'
  _                   -> do
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   v' <- addPositionIdent (AST pos) <$> freshMonoTypeVar "_#lazy" t
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   return (patDecl p { astRef = pos } t (mkVar v') : ds, VariablePattern v')

negateLiteral :: Literal -> Literal
negateLiteral (Int    v i) = Int   v  (-i)
negateLiteral (Float p' f) = Float p' (-f)
negateLiteral _            = internalError "Desugar.negateLiteral"

-- A list of boolean guards is expanded into a nested if-then-else
-- expression, whereas a constraint guard is replaced by a case
-- expression. Note that if the guard type is 'Success' only a
-- single guard is allowed for each equation (This change was
-- introduced in version 0.8 of the Curry report.). We check for the
-- type 'Bool' of the guard because the guard's type defaults to
-- 'Success' if it is not restricted by the guard expression.

dsRhs :: Position -> (Expression -> Expression) -> Rhs -> DsM Rhs
dsRhs p f rhs = do
  e' <- expandRhs prelFailed f rhs >>= dsExpr p
  return (SimpleRhs p e' [])

expandRhs :: Expression -> (Expression -> Expression) -> Rhs -> DsM Expression
expandRhs _  f (SimpleRhs _ e ds) = return $ Let ds (f e)
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expandRhs e0 f (GuardedRhs es ds) = (Let ds . f) <$> expandGuards e0 es
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expandGuards :: Expression -> [CondExpr] -> DsM Expression
expandGuards e0 es = do
  tyEnv <- getValueEnv
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  tcEnv <- getTyConsEnv
  return $ if booleanGuards tyEnv tcEnv es
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              then foldr mkIfThenElse e0 es
              else mkCond es
  where mkIfThenElse (CondExpr p g e) = IfThenElse (srcRefOf p) g e
        mkCond       [CondExpr _ g e] = apply prelCond [g, e]
        mkCond _ = error "Desugar.expandGuards.mkCond: non-unary list"

addConstraints :: [Expression] -> Expression -> Expression
addConstraints cs e
  | null cs   = e
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  | otherwise = apply prelCond [foldr1 (&>) cs, e]
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booleanGuards :: ValueEnv -> TCEnv -> [CondExpr] -> Bool
booleanGuards _     _     []                    = False
booleanGuards tyEnv tcEnv (CondExpr _ g _ : es) =
  not (null es) || typeOf tyEnv tcEnv g == boolType
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dsExpr :: Position -> Expression -> DsM Expression
dsExpr p (Literal l) =
  dsLiteral l >>=
  either (return . Literal) (\ (pos, ls) -> dsExpr p $ List pos $ map Literal ls)
dsExpr _ var@(Variable v)
  | isAnonId (unqualify v) = return prelUnknown
  | otherwise              = return var
dsExpr _ c@(Constructor _) = return c
dsExpr p (Paren         e) = dsExpr p e
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dsExpr p (Typed      e ty) = Typed <$> dsExpr p e <*> dsTypeExpr ty
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dsExpr p (Tuple    pos es) = apply (Constructor $ tupleConstr es)
                             <$> mapM (dsExpr p) es
  where tupleConstr es1 = addRef pos
                        $ if null es1 then qUnitId else qTupleId (length es1)
dsExpr p (List     pos es) = dsList pos cons nil <$> mapM (dsExpr p) es
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  where nil p'  = Constructor (addRef p' qNilId)
        cons p' = Apply . Apply (Constructor $ addRef p' qConsId)
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dsExpr p (ListCompr    r e []    ) = dsExpr p (List [r,r] [e])
dsExpr p (ListCompr    r e (q:qs)) = dsQual p q (ListCompr r e qs)
dsExpr p (EnumFrom              e) = Apply prelEnumFrom <$> dsExpr p e
dsExpr p (EnumFromThen      e1 e2) = apply prelEnumFromThen
                                     <$> mapM (dsExpr p) [e1, e2]
dsExpr p (EnumFromTo        e1 e2) = apply prelEnumFromTo
                                     <$> mapM (dsExpr p) [e1, e2]
dsExpr p (EnumFromThenTo e1 e2 e3) = apply prelEnumFromThenTo
                                     <$> mapM (dsExpr p) [e1, e2, e3]
dsExpr p (UnaryMinus         op e) = do
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  ty <- getTypeOf e
  e' <- dsExpr p e
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  negativeLitsEnabled <- checkNegativeLitsExtension
  return $ case e' of
    Literal l | negativeLitsEnabled -> Literal $ negateLiteral l
    _                               -> Apply (unaryMinus op ty) e'
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  where
  unaryMinus op1 ty'
    | op1 ==  minusId = if ty' == floatType then prelNegateFloat else prelNegate
    | op1 == fminusId = prelNegateFloat
    | otherwise       = internalError "Desugar.unaryMinus"
dsExpr p (Apply (Constructor c) e) = do
  tyEnv <- getValueEnv
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  (if isNewtypeConstr tyEnv c then id else (Apply (Constructor c))) <$>
    dsExpr p e
dsExpr p (Apply e1 e2) = Apply <$> dsExpr p e1 <*> dsExpr p e2
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dsExpr p (InfixApply e1 op e2) = do
  op' <- dsExpr p (infixOp op)
  e1' <- dsExpr p e1
  e2' <- dsExpr p e2
  return $ apply op' [e1', e2']
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dsExpr p (LeftSection  e op) = Apply <$> dsExpr p (infixOp op) <*> dsExpr p e
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dsExpr p (RightSection op e) = do
  op' <- dsExpr p (infixOp op)
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  e'  <- dsExpr p e
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  return $ apply prelFlip [op', e']
dsExpr p expr@(Lambda r ts e) = do
  ty <- getTypeOf expr
  f  <- freshIdent "_#lambda" (length ts) (polyType ty)
  dsExpr p $ Let [funDecl (AST r) f ts e] $ mkVar f
dsExpr p (Let ds e) = do
  ds' <- dsDeclGroup ds
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  e'  <- dsExpr p e
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  return (if null ds' then e' else Let ds' e')
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dsExpr p (Do sts e) = dsExpr p (foldr desugarStmt e sts)
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  where desugarStmt (StmtExpr r e1) e' = apply (prelBind_ r) [e1,e']
        desugarStmt (StmtBind r t e1) e' = apply (prelBind r) [e1,Lambda r [t] e']
        desugarStmt (StmtDecl ds) e' = Let ds e'
dsExpr p (IfThenElse r e1 e2 e3) = do
  e1' <- dsExpr p e1
  e2' <- dsExpr p e2
  e3' <- dsExpr p e3
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  return $ Case r Rigid e1' [caseAlt p truePat e2', caseAlt p falsePat e3']
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dsExpr p (Case r ct e alts)
  | null alts = return prelFailed
  | otherwise = do
    m  <- getModuleIdent
    e' <- dsExpr p e
    v  <- freshMonoTypeVar "_#case" e
    alts'  <- mapM dsAltLhs alts
    alts'' <- mapM (expandAlt v ct) (init (tails alts')) >>= mapM dsAltRhs
    return (mkCase m v e' alts'')
  where
  mkCase m1 v e1 alts1
    | v `elem` qfv m1 alts1 = Let [varDecl p v e1] (Case r ct (mkVar v) alts1)
    | otherwise             = Case r ct e1 alts1
dsExpr p (RecordConstr fs)
  | null fs   = internalError "Desugar.dsExpr: empty record construction"
  | otherwise = do
    r <- recordFromField (fieldLabel (head fs))
    dsRecordConstr p r (map field2Tuple fs)
dsExpr p (RecordSelection e l) = do
  m <- getModuleIdent
  r <- recordFromField l
  dsExpr p (Apply (Variable (qualRecSelectorId m r l)) e)
dsExpr p (RecordUpdate fs rexpr)
  | null fs   = internalError "Desugar.dsExpr: empty record update"
  | otherwise = do
    r <- recordFromField (fieldLabel (head fs))
    dsRecordUpdate p r rexpr (map field2Tuple fs)

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dsTypeExpr :: TypeExpr -> DsM TypeExpr
dsTypeExpr ty = do
  tcEnv <- getTyConsEnv
  let expType = expandType tcEnv (toType [] ty)
  return $ fromType expType

expandType :: TCEnv -> Type -> Type
expandType tcEnv (TypeConstructor tc tys) = case qualLookupTC tc tcEnv of
  [DataType     tc' _  _] -> TypeConstructor tc' tys'
  [RenamingType tc' _  _] -> TypeConstructor tc' tys'
  [AliasType    _   _ ty] -> expandAliasType tys' ty
  _ -> internalError $ "Desugar.expandType " ++ show tc
  where tys' = map (expandType tcEnv) tys
expandType _     tv@(TypeVariable      _) = tv
expandType _     tc@(TypeConstrained _ _) = tc
expandType tcEnv (TypeArrow      ty1 ty2) =
  TypeArrow (expandType tcEnv ty1) (expandType tcEnv ty2)
expandType _     ts@(TypeSkolem        _) = ts
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expandType tcEnv (TypeRecord          fs) =
  TypeRecord (map (\ (l, ty) -> (l, expandType tcEnv ty)) fs)
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-- If an alternative in a case expression has boolean guards and all of
-- these guards return 'False', the enclosing case expression does
-- not fail but continues to match the remaining alternatives against the
-- selector expression. In order to implement this semantics, which is
-- compatible with Haskell, we expand an alternative with boolean guards
-- such that it evaluates a case expression with the remaining cases that
-- are compatible with the matched pattern when the guards fail.

dsAltLhs :: Alt -> DsM Alt
dsAltLhs (Alt p t rhs) = do
  (ds', t') <- dsPattern p [] t
  return $ Alt p t' (addDecls ds' rhs)

dsAltRhs :: Alt -> DsM Alt
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dsAltRhs (Alt p t rhs) = Alt p t <$> dsRhs p id rhs
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expandAlt :: Ident -> CaseType -> [Alt] -> DsM Alt
expandAlt _ _  []                   = error "Desugar.expandAlt: empty list"
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expandAlt v ct (Alt p t rhs : alts) = caseAlt p t <$> expandRhs e0 id rhs
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  where
  e0 | ct == Flex = prelFailed
     | otherwise  = Case (srcRefOf p) ct (mkVar v)
                         (filter (isCompatible t . altPattern) alts)
  altPattern (Alt _ t1 _) = t1

isCompatible :: Pattern -> Pattern -> Bool
isCompatible (VariablePattern _) _                   = True
isCompatible _                   (VariablePattern _) = True
isCompatible (AsPattern    _ t1) t2                  = isCompatible t1 t2
isCompatible t1                  (AsPattern    _ t2) = isCompatible t1 t2
isCompatible (ConstructorPattern c1 ts1) (ConstructorPattern c2 ts2)
  = and ((c1 == c2) : zipWith isCompatible ts1 ts2)
isCompatible (LiteralPattern         l1) (LiteralPattern         l2)
  = canon l1 == canon l2
  where canon (Int _ i) = Int anonId i
        canon l         = l
isCompatible _                    _                  = False

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-- -----------------------------------------------------------------------------
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-- Desugaring of Records
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-- -----------------------------------------------------------------------------
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recordFromField :: Ident -> DsM QualIdent
recordFromField lbl = do
  tyEnv <- getValueEnv
  case lookupValue lbl tyEnv of
    [Label _ r _] -> return r
    _             -> internalError $
      "Desugar.recordFromField: unknown label: " ++ show lbl

lookupRecord :: QualIdent -> DsM (Int, [(Ident, Type)])
lookupRecord r = do
  tcEnv <- getTyConsEnv
  case qualLookupTC r tcEnv of
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    [AliasType _ n (TypeRecord fs)] -> return (n, fs)
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    _                                 ->
      internalError $ "Desugar.lookupRecord: no record: " ++ show r

dsRecordDecl :: Decl -> DsM [Decl]
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dsRecordDecl (TypeDecl p r vs (RecordType fss)) = do
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  m     <- getModuleIdent
  let qr = qualifyWith m r
  (n, fs') <- lookupRecord qr
  let tys   = concatMap (\ (ls, ty) -> replicate (length ls) ty) fss
      --tys' = map (elimRecordTypes tyEnv) tys
      rdecl = DataDecl p r vs [ConstrDecl p [] r tys]
      rty'  = TypeConstructor qr (map TypeVariable [0 .. n - 1])
      rcts' = ForAllExist 0 n (foldr TypeArrow rty' (map snd fs'))
  rfuncs <- mapM (genRecordFuncs p qr rty' (map fst fs')) fs'
  modifyValueEnv
      (bindGlobalInfo (flip DataConstructor (length tys)) m r rcts')
  return $ rdecl : concat rfuncs
dsRecordDecl d = return [d]

genRecordFuncs :: Position -> QualIdent -> Type -> [Ident] -> (Ident, Type)
               -> DsM [Decl]
genRecordFuncs p r rty ls (l, ty) = do
  m <- getModuleIdent
  let (selId, selFunc) = genSelectFunc p r ls l
      (updId, updFunc) = genUpdateFunc p r ls l
      selType = polyType (TypeArrow rty ty)
      updType = polyType (TypeArrow rty $ TypeArrow ty rty)
  modifyValueEnv (bindFun m selId 1 selType . bindFun m updId 2 updType)
  return [selFunc, updFunc]

genSelectFunc :: Position -> QualIdent -> [Ident] -> Ident -> (Ident, Decl)
genSelectFunc p r ls l = (selId, funDecl p selId [cpatt] (mkVar l))
  where
  selId  = recSelectorId r l
  cpatt  = ConstructorPattern r (map VariablePattern ls)

genUpdateFunc :: Position -> QualIdent -> [Ident] -> Ident -> (Ident, Decl)
genUpdateFunc p r ls l = (updId, funDecl p updId [cpatt1, cpatt2] cexpr)
  where
  updId  = recUpdateId r l
  vs     = [ VariablePattern (if v == l then anonId else v) | v <- ls]
  cpatt1 = ConstructorPattern r vs
  cpatt2 = VariablePattern l
  cexpr  = apply (Constructor r) (map mkVar ls)

dsRecordConstr :: Position -> QualIdent -> [(Ident, Expression)]
               -> DsM Expression
dsRecordConstr p r fs = do
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  fs' <- (map fst . snd) <$> lookupRecord r
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  let cts = map (\ l -> fromMaybe (internalError "Desugar.dsRecordConstr")
                            (lookup l fs)) fs'
  dsExpr p (apply (Constructor r) cts)

dsRecordUpdate :: Position -> QualIdent -> Expression
               -> [(Ident, Expression)] -> DsM Expression
dsRecordUpdate p r rexpr fs = do
  m <- getModuleIdent
  dsExpr p (foldl (genRecordUpdate m r) rexpr fs)
  where
  genRecordUpdate m1 r1 rexpr1 (l,e) =
   apply (Variable $ qualRecUpdateId m1 r1 l) [rexpr1, e]

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-- -----------------------------------------------------------------------------
-- Desugaring of List Comprehension
-- -----------------------------------------------------------------------------

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-- In general, a list comprehension of the form
-- '[e | t <- l, qs]'
-- is transformed into an expression 'foldr f [] l' where 'f'
-- is a new function defined as
--
--     f x xs =
--       case x of
--           t -> [e | qs] ++ xs
--           _ -> xs
--
-- Note that this translation evaluates the elements of 'l' rigidly,
-- whereas the translation given in the Curry report is flexible.
-- However, it does not seem very useful to have the comprehension
-- generate instances of 't' which do not contribute to the list.

-- Actually, we generate slightly better code in a few special cases.
-- When 't' is a plain variable, the 'case' expression degenerates
-- into a let-binding and the auxiliary function thus becomes an alias
-- for '(++)'. Instead of 'foldr (++)' we use the
-- equivalent prelude function 'concatMap'. In addition, if the
-- remaining list comprehension in the body of the auxiliary function has
-- no qualifiers -- i.e., if it is equivalent to '[e]' -- we
-- avoid the construction of the singleton list by calling '(:)'
-- instead of '(++)' and 'map' in place of 'concatMap', respectively.

dsQual :: Position -> Statement -> Expression -> DsM Expression
dsQual p (StmtExpr   r b) e = dsExpr p (IfThenElse r b e (List [r] []))
dsQual p (StmtDecl    ds) e = dsExpr p (Let ds e)
dsQual p (StmtBind r t l) e
  | isVarPattern t = dsExpr p (qualExpr t e l)
  | otherwise      = do
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    v   <- addRefId r <$> freshMonoTypeVar "_#var" t
    l'  <- addRefId r <$> freshMonoTypeVar "_#var" e
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    dsExpr p (apply (prelFoldr r) [foldFunct v l' e, List [r] [], l])
  where
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  qualExpr v (ListCompr _ e1 []) l1 = apply (prelMap       r)
                                      [Lambda r [v] e1, l1]
  qualExpr v e1                  l1 = apply (prelConcatMap r)
                                      [Lambda r [v] e1, l1]
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  foldFunct v l1 e1
    = Lambda r (map VariablePattern [v,l1])
       (Case r Rigid (mkVar v)
          [ caseAlt p t (append e1 (mkVar l1))
          , caseAlt p (VariablePattern v) (mkVar l1)])

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  append (ListCompr _ e1 []) l1 = apply prelCons       [e1, l1]
  append e1                  l1 = apply (prelAppend r) [e1, l1]
  prelCons                      = Constructor $ addRef r $ qConsId
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-- ---------------------------------------------------------------------------
-- Prelude entities
-- ---------------------------------------------------------------------------

prelBind :: SrcRef -> Expression
prelBind = prel ">>="

prelBind_ :: SrcRef -> Expression
prelBind_ = prel ">>"

prelFlip :: Expression
prelFlip = Variable $ preludeIdent "flip"

prelEnumFrom :: Expression
prelEnumFrom = Variable $ preludeIdent "enumFrom"

prelEnumFromTo :: Expression
prelEnumFromTo = Variable $ preludeIdent "enumFromTo"

prelEnumFromThen :: Expression
prelEnumFromThen = Variable $ preludeIdent "enumFromThen"

prelEnumFromThenTo :: Expression
prelEnumFromThenTo = Variable $ preludeIdent "enumFromThenTo"

prelFailed :: Expression
prelFailed = Variable $ preludeIdent "failed"

prelUnknown :: Expression
prelUnknown = Variable $ preludeIdent "unknown"

prelMap :: SrcRef -> Expression
prelMap r = Variable $ addRef r $ preludeIdent "map"

prelFoldr :: SrcRef -> Expression
prelFoldr = prel "foldr"

prelAppend :: SrcRef -> Expression
prelAppend = prel "++"

prelConcatMap :: SrcRef -> Expression
prelConcatMap = prel "concatMap"

prelNegate :: Expression
prelNegate = Variable $ preludeIdent "negate"

prelNegateFloat :: Expression
prelNegateFloat = Variable $ preludeIdent "negateFloat"

prelCond :: Expression
prelCond = Variable $ preludeIdent "cond"

(=:<=) :: Expression -> Expression -> Expression
e1 =:<= e2 = apply prelFPEq [e1, e2]

prelFPEq :: Expression
prelFPEq = Variable $ preludeIdent "=:<="

(=:=) :: Expression -> Expression -> Expression
e1 =:= e2 = apply prelSEq [e1, e2]

prelSEq :: Expression
prelSEq = Variable $ preludeIdent "=:="

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(&>) :: Expression -> Expression -> Expression
e1 &> e2 = apply prelCond [e1, e2]
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prel :: String -> SrcRef -> Expression
prel s r = Variable $ addRef r $ preludeIdent s

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truePat :: Pattern
truePat = ConstructorPattern qTrueId []
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falsePat :: Pattern
falsePat = ConstructorPattern qFalseId []
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preludeIdent :: String -> QualIdent
preludeIdent = qualifyWith preludeMIdent . mkIdent

-- ---------------------------------------------------------------------------
-- Auxiliary definitions
-- ---------------------------------------------------------------------------

isNewtypeConstr :: ValueEnv -> QualIdent -> Bool
isNewtypeConstr tyEnv c = case qualLookupValue c tyEnv of
  [NewtypeConstructor _ _] -> True
  [DataConstructor  _ _ _] -> False
  x -> internalError $ "Transformations.Desugar.isNewtypeConstr: "
                        ++ show c ++ " is " ++ show x

isVarPattern :: Pattern -> Bool
isVarPattern (VariablePattern _) = True
isVarPattern (ParenPattern    t) = isVarPattern t
isVarPattern (AsPattern     _ t) = isVarPattern t
isVarPattern (LazyPattern   _ _) = True
isVarPattern _                   = False

funDecl :: Position -> Ident -> [Pattern] -> Expression -> Decl
funDecl p f ts e = FunctionDecl p f
  [Equation p (FunLhs f ts) (SimpleRhs p e [])]

patDecl :: Position -> Pattern -> Expression -> Decl
patDecl p t e = PatternDecl p t (SimpleRhs p e [])

varDecl :: Position -> Ident -> Expression -> Decl
varDecl p = patDecl p . VariablePattern

addDecls :: [Decl] -> Rhs -> Rhs
addDecls ds (SimpleRhs p e ds') = SimpleRhs p e (ds ++ ds')
addDecls ds (GuardedRhs es ds') = GuardedRhs es (ds ++ ds')

caseAlt :: Position -> Pattern -> Expression -> Alt
caseAlt p t e = Alt p t (SimpleRhs p e [])

apply :: Expression -> [Expression] -> Expression
apply = foldl Apply

mkVar :: Ident -> Expression
mkVar = Variable . qualify