MkId.hs 62.2 KB
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{-
(c) The University of Glasgow 2006
(c) The AQUA Project, Glasgow University, 1998

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This module contains definitions for the IdInfo for things that
have a standard form, namely:

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- data constructors
- record selectors
- method and superclass selectors
- primitive operations
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-}
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{-# LANGUAGE CPP #-}
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module MkId (
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        mkDictFunId, mkDictFunTy, mkDictSelId, mkDictSelRhs,
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        mkPrimOpId, mkFCallId,
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        wrapNewTypeBody, unwrapNewTypeBody,
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        wrapFamInstBody, unwrapFamInstScrut,
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        wrapTypeUnbranchedFamInstBody, unwrapTypeUnbranchedFamInstScrut,
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        DataConBoxer(..), mkDataConRep, mkDataConWorkId,
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        -- And some particular Ids; see below for why they are wired in
        wiredInIds, ghcPrimIds,
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        unsafeCoerceName, unsafeCoerceId, realWorldPrimId,
        voidPrimId, voidArgId,
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        nullAddrId, seqId, lazyId, lazyIdKey, runRWId,
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        coercionTokenId, magicDictId, coerceId,
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        proxyHashId, noinlineId, noinlineIdName,
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        -- Re-export error Ids
        module PrelRules
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    ) where

#include "HsVersions.h"

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import Rules
import TysPrim
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import TysWiredIn
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import PrelRules
import Type
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import FamInstEnv
import Coercion
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import TcType
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import MkCore
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import CoreUtils        ( exprType, mkCast )
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import CoreUnfold
import Literal
import TyCon
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import CoAxiom
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import Class
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import NameSet
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import Name
import PrimOp
import ForeignCall
import DataCon
import Id
import IdInfo
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import Demand
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import CoreSyn
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import Unique
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import UniqSupply
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import PrelNames
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import BasicTypes       hiding ( SuccessFlag(..) )
import Util
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import Pair
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import DynFlags
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import Outputable
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import FastString
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import ListSetOps
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import qualified GHC.LanguageExtensions as LangExt
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import Data.Maybe       ( maybeToList )
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{-
************************************************************************
*                                                                      *
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\subsection{Wired in Ids}
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*                                                                      *
************************************************************************
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Note [Wired-in Ids]
~~~~~~~~~~~~~~~~~~~
There are several reasons why an Id might appear in the wiredInIds:

(1) The ghcPrimIds are wired in because they can't be defined in
    Haskell at all, although the can be defined in Core.  They have
    compulsory unfoldings, so they are always inlined and they  have
    no definition site.  Their home module is GHC.Prim, so they
    also have a description in primops.txt.pp, where they are called
    'pseudoops'.

(2) The 'error' function, eRROR_ID, is wired in because we don't yet have
    a way to express in an interface file that the result type variable
    is 'open'; that is can be unified with an unboxed type

    [The interface file format now carry such information, but there's
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    no way yet of expressing at the definition site for these
    error-reporting functions that they have an 'open'
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    result type. -- sof 1/99]

(3) Other error functions (rUNTIME_ERROR_ID) are wired in (a) because
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    the desugarer generates code that mentions them directly, and
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    (b) for the same reason as eRROR_ID

(4) lazyId is wired in because the wired-in version overrides the
    strictness of the version defined in GHC.Base

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(5) noinlineId is wired in because when we serialize to interfaces
    we may insert noinline statements.

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In cases (2-4), the function has a definition in a library module, and
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can be called; but the wired-in version means that the details are
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never read from that module's interface file; instead, the full definition
is right here.
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-}
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wiredInIds :: [Id]
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wiredInIds
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  =  [lazyId, dollarId, oneShotId, runRWId, noinlineId]
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  ++ errorIds           -- Defined in MkCore
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  ++ ghcPrimIds
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-- These Ids are exported from GHC.Prim
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ghcPrimIds :: [Id]
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ghcPrimIds
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  = [   -- These can't be defined in Haskell, but they have
        -- perfectly reasonable unfoldings in Core
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    realWorldPrimId,
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    voidPrimId,
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    unsafeCoerceId,
    nullAddrId,
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    seqId,
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    magicDictId,
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    coerceId,
    proxyHashId
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    ]

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{-
************************************************************************
*                                                                      *
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\subsection{Data constructors}
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*                                                                      *
************************************************************************
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The wrapper for a constructor is an ordinary top-level binding that evaluates
any strict args, unboxes any args that are going to be flattened, and calls
the worker.

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We're going to build a constructor that looks like:

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        data (Data a, C b) =>  T a b = T1 !a !Int b
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        T1 = /\ a b ->
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             \d1::Data a, d2::C b ->
             \p q r -> case p of { p ->
                       case q of { q ->
                       Con T1 [a,b] [p,q,r]}}
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Notice that

* d2 is thrown away --- a context in a data decl is used to make sure
  one *could* construct dictionaries at the site the constructor
  is used, but the dictionary isn't actually used.

* We have to check that we can construct Data dictionaries for
  the types a and Int.  Once we've done that we can throw d1 away too.

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* We use (case p of q -> ...) to evaluate p, rather than "seq" because
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  all that matters is that the arguments are evaluated.  "seq" is
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  very careful to preserve evaluation order, which we don't need
  to be here.

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  You might think that we could simply give constructors some strictness
  info, like PrimOps, and let CoreToStg do the let-to-case transformation.
  But we don't do that because in the case of primops and functions strictness
  is a *property* not a *requirement*.  In the case of constructors we need to
  do something active to evaluate the argument.

  Making an explicit case expression allows the simplifier to eliminate
  it in the (common) case where the constructor arg is already evaluated.

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Note [Wrappers for data instance tycons]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In the case of data instances, the wrapper also applies the coercion turning
the representation type into the family instance type to cast the result of
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the wrapper.  For example, consider the declarations

  data family Map k :: * -> *
  data instance Map (a, b) v = MapPair (Map a (Pair b v))

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The tycon to which the datacon MapPair belongs gets a unique internal
name of the form :R123Map, and we call it the representation tycon.
In contrast, Map is the family tycon (accessible via
tyConFamInst_maybe). A coercion allows you to move between
representation and family type.  It is accessible from :R123Map via
tyConFamilyCoercion_maybe and has kind

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  Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
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The wrapper and worker of MapPair get the types
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        -- Wrapper
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  $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
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  $WMapPair a b v = MapPair a b v `cast` sym (Co123Map a b v)
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        -- Worker
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  MapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v
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This coercion is conditionally applied by wrapFamInstBody.
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It's a bit more complicated if the data instance is a GADT as well!
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   data instance T [a] where
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        T1 :: forall b. b -> T [Maybe b]
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Hence we translate to
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        -- Wrapper
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  $WT1 :: forall b. b -> T [Maybe b]
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  $WT1 b v = T1 (Maybe b) b (Maybe b) v
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                        `cast` sym (Co7T (Maybe b))
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        -- Worker
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  T1 :: forall c b. (c ~ Maybe b) => b -> :R7T c
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        -- Coercion from family type to representation type
  Co7T a :: T [a] ~ :R7T a

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Note [Newtype datacons]
~~~~~~~~~~~~~~~~~~~~~~~
The "data constructor" for a newtype should always be vanilla.  At one
point this wasn't true, because the newtype arising from
     class C a => D a
looked like
       newtype T:D a = D:D (C a)
so the data constructor for T:C had a single argument, namely the
predicate (C a).  But now we treat that as an ordinary argument, not
part of the theta-type, so all is well.

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************************************************************************
*                                                                      *
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\subsection{Dictionary selectors}
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*                                                                      *
************************************************************************
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Selecting a field for a dictionary.  If there is just one field, then
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there's nothing to do.
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Dictionary selectors may get nested forall-types.  Thus:
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        class Foo a where
          op :: forall b. Ord b => a -> b -> b
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Then the top-level type for op is
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        op :: forall a. Foo a =>
              forall b. Ord b =>
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              a -> b -> b
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-}
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mkDictSelId :: Name          -- Name of one of the *value* selectors
                             -- (dictionary superclass or method)
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            -> Class -> Id
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mkDictSelId name clas
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  = mkGlobalId (ClassOpId clas) name sel_ty info
  where
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    tycon          = classTyCon clas
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    sel_names      = map idName (classAllSelIds clas)
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    new_tycon      = isNewTyCon tycon
    [data_con]     = tyConDataCons tycon
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    tyvars         = dataConUnivTyVarBinders data_con
    n_ty_args      = length tyvars
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    arg_tys        = dataConRepArgTys data_con  -- Includes the dictionary superclasses
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    val_index      = assoc "MkId.mkDictSelId" (sel_names `zip` [0..]) name

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    sel_ty = mkForAllTys tyvars $
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             mkFunTy (mkClassPred clas (mkTyVarTys (binderVars tyvars))) $
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             getNth arg_tys val_index

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    base_info = noCafIdInfo
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                `setArityInfo`          1
                `setStrictnessInfo`     strict_sig
                `setLevityInfoWithType` sel_ty
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    info | new_tycon
         = base_info `setInlinePragInfo` alwaysInlinePragma
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                     `setUnfoldingInfo`  mkInlineUnfoldingWithArity 1
                                           (mkDictSelRhs clas val_index)
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                   -- See Note [Single-method classes] in TcInstDcls
                   -- for why alwaysInlinePragma
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         | otherwise
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         = base_info `setRuleInfo` mkRuleInfo [rule]
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                   -- Add a magic BuiltinRule, but no unfolding
                   -- so that the rule is always available to fire.
                   -- See Note [ClassOp/DFun selection] in TcInstDcls
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    -- This is the built-in rule that goes
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    --      op (dfT d1 d2) --->  opT d1 d2
    rule = BuiltinRule { ru_name = fsLit "Class op " `appendFS`
                                     occNameFS (getOccName name)
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                       , ru_fn    = name
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                       , ru_nargs = n_ty_args + 1
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                       , ru_try   = dictSelRule val_index n_ty_args }
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        -- The strictness signature is of the form U(AAAVAAAA) -> T
        -- where the V depends on which item we are selecting
        -- It's worth giving one, so that absence info etc is generated
        -- even if the selector isn't inlined
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    strict_sig = mkClosedStrictSig [arg_dmd] topRes
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    arg_dmd | new_tycon = evalDmd
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            | otherwise = mkManyUsedDmd $
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                          mkProdDmd [ if name == sel_name then evalDmd else absDmd
                                    | sel_name <- sel_names ]

mkDictSelRhs :: Class
             -> Int         -- 0-indexed selector among (superclasses ++ methods)
             -> CoreExpr
mkDictSelRhs clas val_index
  = mkLams tyvars (Lam dict_id rhs_body)
  where
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    tycon          = classTyCon clas
    new_tycon      = isNewTyCon tycon
    [data_con]     = tyConDataCons tycon
    tyvars         = dataConUnivTyVars data_con
    arg_tys        = dataConRepArgTys data_con  -- Includes the dictionary superclasses
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    the_arg_id     = getNth arg_ids val_index
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    pred           = mkClassPred clas (mkTyVarTys tyvars)
    dict_id        = mkTemplateLocal 1 pred
    arg_ids        = mkTemplateLocalsNum 2 arg_tys
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    rhs_body | new_tycon = unwrapNewTypeBody tycon (mkTyVarTys tyvars) (Var dict_id)
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             | otherwise = Case (Var dict_id) dict_id (idType the_arg_id)
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                                [(DataAlt data_con, arg_ids, varToCoreExpr the_arg_id)]
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                                -- varToCoreExpr needed for equality superclass selectors
                                --   sel a b d = case x of { MkC _ (g:a~b) _ -> CO g }
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dictSelRule :: Int -> Arity -> RuleFun
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-- Tries to persuade the argument to look like a constructor
-- application, using exprIsConApp_maybe, and then selects
-- from it
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--       sel_i t1..tk (D t1..tk op1 ... opm) = opi
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--
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dictSelRule val_index n_ty_args _ id_unf _ args
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  | (dict_arg : _) <- drop n_ty_args args
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  , Just (_, _, con_args) <- exprIsConApp_maybe id_unf dict_arg
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  = Just (getNth con_args val_index)
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  | otherwise
  = Nothing
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{-
************************************************************************
*                                                                      *
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        Data constructors
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*                                                                      *
************************************************************************
-}
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mkDataConWorkId :: Name -> DataCon -> Id
mkDataConWorkId wkr_name data_con
  | isNewTyCon tycon
  = mkGlobalId (DataConWrapId data_con) wkr_name nt_wrap_ty nt_work_info
  | otherwise
  = mkGlobalId (DataConWorkId data_con) wkr_name alg_wkr_ty wkr_info

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  where
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    tycon = dataConTyCon data_con

        ----------- Workers for data types --------------
    alg_wkr_ty = dataConRepType data_con
    wkr_arity = dataConRepArity data_con
    wkr_info  = noCafIdInfo
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                `setArityInfo`          wkr_arity
                `setStrictnessInfo`     wkr_sig
                `setUnfoldingInfo`      evaldUnfolding  -- Record that it's evaluated,
                                                        -- even if arity = 0
                `setLevityInfoWithType` alg_wkr_ty
                  -- NB: unboxed tuples have workers, so we can't use
                  -- setNeverLevPoly
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    wkr_sig = mkClosedStrictSig (replicate wkr_arity topDmd) (dataConCPR data_con)
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        --      Note [Data-con worker strictness]
        -- Notice that we do *not* say the worker is strict
        -- even if the data constructor is declared strict
        --      e.g.    data T = MkT !(Int,Int)
        -- Why?  Because the *wrapper* is strict (and its unfolding has case
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        -- expressions that do the evals) but the *worker* itself is not.
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        -- If we pretend it is strict then when we see
        --      case x of y -> $wMkT y
        -- the simplifier thinks that y is "sure to be evaluated" (because
        --  $wMkT is strict) and drops the case.  No, $wMkT is not strict.
        --
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        -- When the simplifier sees a pattern
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        --      case e of MkT x -> ...
        -- it uses the dataConRepStrictness of MkT to mark x as evaluated;
        -- but that's fine... dataConRepStrictness comes from the data con
        -- not from the worker Id.

        ----------- Workers for newtypes --------------
    (nt_tvs, _, nt_arg_tys, _) = dataConSig data_con
    res_ty_args  = mkTyVarTys nt_tvs
    nt_wrap_ty   = dataConUserType data_con
    nt_work_info = noCafIdInfo          -- The NoCaf-ness is set by noCafIdInfo
                  `setArityInfo` 1      -- Arity 1
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                  `setInlinePragInfo`     alwaysInlinePragma
                  `setUnfoldingInfo`      newtype_unf
                  `setLevityInfoWithType` nt_wrap_ty
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    id_arg1      = mkTemplateLocal 1 (head nt_arg_tys)
    newtype_unf  = ASSERT2( isVanillaDataCon data_con &&
                            isSingleton nt_arg_tys, ppr data_con  )
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                              -- Note [Newtype datacons]
                   mkCompulsoryUnfolding $
                   mkLams nt_tvs $ Lam id_arg1 $
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                   wrapNewTypeBody tycon res_ty_args (Var id_arg1)
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dataConCPR :: DataCon -> DmdResult
dataConCPR con
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  | isDataTyCon tycon     -- Real data types only; that is,
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                          -- not unboxed tuples or newtypes
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  , null (dataConExTyVars con)  -- No existentials
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  , wkr_arity > 0
  , wkr_arity <= mAX_CPR_SIZE
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  = if is_prod then vanillaCprProdRes (dataConRepArity con)
               else cprSumRes (dataConTag con)
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  | otherwise
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  = topRes
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  where
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    is_prod   = isProductTyCon tycon
    tycon     = dataConTyCon con
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    wkr_arity = dataConRepArity con

    mAX_CPR_SIZE :: Arity
    mAX_CPR_SIZE = 10
    -- We do not treat very big tuples as CPR-ish:
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    --      a) for a start we get into trouble because there aren't
    --         "enough" unboxed tuple types (a tiresome restriction,
    --         but hard to fix),
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    --      b) more importantly, big unboxed tuples get returned mainly
    --         on the stack, and are often then allocated in the heap
    --         by the caller.  So doing CPR for them may in fact make
    --         things worse.
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{-
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-------------------------------------------------
--         Data constructor representation
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--
-- This is where we decide how to wrap/unwrap the
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-- constructor fields
--
--------------------------------------------------
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-}
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type Unboxer = Var -> UniqSM ([Var], CoreExpr -> CoreExpr)
  -- Unbox: bind rep vars by decomposing src var

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data Boxer = UnitBox | Boxer (TCvSubst -> UniqSM ([Var], CoreExpr))
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  -- Box:   build src arg using these rep vars

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-- | Data Constructor Boxer
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newtype DataConBoxer = DCB ([Type] -> [Var] -> UniqSM ([Var], [CoreBind]))
                       -- Bind these src-level vars, returning the
                       -- rep-level vars to bind in the pattern

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{-
Note [Inline partially-applied constructor wrappers]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We allow the wrapper to inline when partially applied to avoid
boxing values unnecessarily. For example, consider

   data Foo a = Foo !Int a

   instance Traversable Foo where
     traverse f (Foo i a) = Foo i <$> f a

This desugars to

   traverse f foo = case foo of
        Foo i# a -> let i = I# i#
                    in map ($WFoo i) (f a)

If the wrapper `$WFoo` is not inlined, we get a fruitless reboxing of `i`.
But if we inline the wrapper, we get

   map (\a. case i of I# i# a -> Foo i# a) (f a)

and now case-of-known-constructor eliminates the redundant allocation.
-}

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mkDataConRep :: DynFlags
             -> FamInstEnvs
             -> Name
             -> Maybe [HsImplBang]
                -- See Note [Bangs on imported data constructors]
             -> DataCon
             -> UniqSM DataConRep
mkDataConRep dflags fam_envs wrap_name mb_bangs data_con
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  | not wrapper_reqd
  = return NoDataConRep
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  | otherwise
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  = do { wrap_args <- mapM newLocal wrap_arg_tys
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       ; wrap_body <- mk_rep_app (wrap_args `zip` dropList eq_spec unboxers)
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                                 initial_wrap_app

       ; let wrap_id = mkGlobalId (DataConWrapId data_con) wrap_name wrap_ty wrap_info
             wrap_info = noCafIdInfo
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                         `setArityInfo`         wrap_arity
                             -- It's important to specify the arity, so that partial
                             -- applications are treated as values
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                         `setInlinePragInfo`    wrap_prag
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                         `setUnfoldingInfo`     wrap_unf
                         `setStrictnessInfo`    wrap_sig
                             -- We need to get the CAF info right here because TidyPgm
                             -- does not tidy the IdInfo of implicit bindings (like the wrapper)
                             -- so it not make sure that the CAF info is sane
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                         `setNeverLevPoly`      wrap_ty
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             wrap_sig = mkClosedStrictSig wrap_arg_dmds (dataConCPR data_con)
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             wrap_arg_dmds = map mk_dmd arg_ibangs
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             mk_dmd str | isBanged str = evalDmd
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                        | otherwise           = topDmd
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             wrap_prag = alwaysInlinePragma `setInlinePragmaActivation`
                         ActiveAfter NoSourceText 2
                         -- See Note [Activation for data constructor wrappers]

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             -- The wrapper will usually be inlined (see wrap_unf), so its
             -- strictness and CPR info is usually irrelevant. But this is
             -- not always the case; GHC may choose not to inline it. In
             -- particular, the wrapper constructor is not inlined inside
             -- an INLINE rhs or when it is not applied to any arguments.
             -- See Note [Inline partially-applied constructor wrappers]
             -- Passing Nothing here allows the wrapper to inline when
             -- unsaturated.
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             wrap_unf = mkInlineUnfolding wrap_rhs
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             wrap_tvs = (univ_tvs `minusList` map eqSpecTyVar eq_spec) ++ ex_tvs
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             wrap_rhs = mkLams wrap_tvs $
                        mkLams wrap_args $
                        wrapFamInstBody tycon res_ty_args $
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                        wrap_body

       ; return (DCR { dcr_wrap_id = wrap_id
                     , dcr_boxer   = mk_boxer boxers
                     , dcr_arg_tys = rep_tys
                     , dcr_stricts = rep_strs
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                     , dcr_bangs   = arg_ibangs }) }
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  where
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    (univ_tvs, ex_tvs, eq_spec, theta, orig_arg_tys, _orig_res_ty)
      = dataConFullSig data_con
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    res_ty_args  = substTyVars (mkTvSubstPrs (map eqSpecPair eq_spec)) univ_tvs
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    tycon        = dataConTyCon data_con       -- The representation TyCon (not family)
    wrap_ty      = dataConUserType data_con
    ev_tys       = eqSpecPreds eq_spec ++ theta
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    all_arg_tys  = ev_tys ++ orig_arg_tys
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    ev_ibangs    = map (const HsLazy) ev_tys
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    orig_bangs   = dataConSrcBangs data_con
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    wrap_arg_tys = theta ++ orig_arg_tys
    wrap_arity   = length wrap_arg_tys
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             -- The wrap_args are the arguments *other than* the eq_spec
             -- Because we are going to apply the eq_spec args manually in the
             -- wrapper
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    arg_ibangs =
      case mb_bangs of
        Nothing    -> zipWith (dataConSrcToImplBang dflags fam_envs)
                              orig_arg_tys orig_bangs
        Just bangs -> bangs

    (rep_tys_w_strs, wrappers)
      = unzip (zipWith dataConArgRep all_arg_tys (ev_ibangs ++ arg_ibangs))

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    (unboxers, boxers) = unzip wrappers
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    (rep_tys, rep_strs) = unzip (concat rep_tys_w_strs)

    wrapper_reqd = not (isNewTyCon tycon)  -- Newtypes have only a worker
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                && (any isBanged (ev_ibangs ++ arg_ibangs)
                      -- Some forcing/unboxing (includes eq_spec)
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                    || isFamInstTyCon tycon  -- Cast result
                    || (not $ null eq_spec)) -- GADT
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    initial_wrap_app = Var (dataConWorkId data_con)
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                       `mkTyApps`  res_ty_args
                       `mkVarApps` ex_tvs
                       `mkCoApps`  map (mkReflCo Nominal . eqSpecType) eq_spec
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    mk_boxer :: [Boxer] -> DataConBoxer
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    mk_boxer boxers = DCB (\ ty_args src_vars ->
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                      do { let (ex_vars, term_vars) = splitAtList ex_tvs src_vars
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                               subst1 = zipTvSubst univ_tvs ty_args
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                               subst2 = extendTvSubstList subst1 ex_tvs
                                                          (mkTyVarTys ex_vars)
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                         ; (rep_ids, binds) <- go subst2 boxers term_vars
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                         ; return (ex_vars ++ rep_ids, binds) } )

    go _ [] src_vars = ASSERT2( null src_vars, ppr data_con ) return ([], [])
    go subst (UnitBox : boxers) (src_var : src_vars)
      = do { (rep_ids2, binds) <- go subst boxers src_vars
           ; return (src_var : rep_ids2, binds) }
    go subst (Boxer boxer : boxers) (src_var : src_vars)
      = do { (rep_ids1, arg)  <- boxer subst
           ; (rep_ids2, binds) <- go subst boxers src_vars
           ; return (rep_ids1 ++ rep_ids2, NonRec src_var arg : binds) }
    go _ (_:_) [] = pprPanic "mk_boxer" (ppr data_con)

    mk_rep_app :: [(Id,Unboxer)] -> CoreExpr -> UniqSM CoreExpr
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    mk_rep_app [] con_app
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      = return con_app
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    mk_rep_app ((wrap_arg, unboxer) : prs) con_app
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      = do { (rep_ids, unbox_fn) <- unboxer wrap_arg
           ; expr <- mk_rep_app prs (mkVarApps con_app rep_ids)
           ; return (unbox_fn expr) }

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{- Note [Activation for data constructor wrappers]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Activation on a data constructor wrapper allows it to inline in
Phase 2 and later (1, 0).  But not in the InitialPhase.  That gives
rewrite rules a chance to fire (in the InitialPhase) if they mention
a data constructor on the left
   RULE "foo"  f (K a b) = ...
Since the LHS of rules are simplified with InitialPhase, we won't
inline the wrapper on the LHS either.

People have asked for this before, but now that even the InitialPhase
does some inlining, it has become important.


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Note [Bangs on imported data constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We pass Maybe [HsImplBang] to mkDataConRep to make use of HsImplBangs
from imported modules.

- Nothing <=> use HsSrcBangs
- Just bangs <=> use HsImplBangs

For imported types we can't work it all out from the HsSrcBangs,
because we want to be very sure to follow what the original module
(where the data type was declared) decided, and that depends on what
flags were enabled when it was compiled. So we record the decisions in
the interface file.

The HsImplBangs passed are in 1-1 correspondence with the
dataConOrigArgTys of the DataCon.

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Note [Data con wrappers and unlifted types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   data T = MkT !Int#

We certainly do not want to make a wrapper
   $WMkT x = case x of y { DEFAULT -> MkT y }

For a start, it's still to generate a no-op.  But worse, since wrappers
are currently injected at TidyCore, we don't even optimise it away!
So the stupid case expression stays there.  This actually happened for
the Integer data type (see Trac #1600 comment:66)!
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-}

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-------------------------
newLocal :: Type -> UniqSM Var
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newLocal ty = do { uniq <- getUniqueM
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                 ; return (mkSysLocalOrCoVar (fsLit "dt") uniq ty) }
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-- | Unpack/Strictness decisions from source module
dataConSrcToImplBang
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   :: DynFlags
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   -> FamInstEnvs
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   -> Type
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   -> HsSrcBang
   -> HsImplBang
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dataConSrcToImplBang dflags fam_envs arg_ty
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                     (HsSrcBang ann unpk NoSrcStrict)
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  | xopt LangExt.StrictData dflags -- StrictData => strict field
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  = dataConSrcToImplBang dflags fam_envs arg_ty
                  (HsSrcBang ann unpk SrcStrict)
  | otherwise -- no StrictData => lazy field
  = HsLazy
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dataConSrcToImplBang _ _ _ (HsSrcBang _ _ SrcLazy)
  = HsLazy
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dataConSrcToImplBang dflags fam_envs arg_ty
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                     (HsSrcBang _ unpk_prag SrcStrict)
  | isUnliftedType arg_ty
  = HsLazy  -- For !Int#, say, use HsLazy
            -- See Note [Data con wrappers and unlifted types]

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  | not (gopt Opt_OmitInterfacePragmas dflags) -- Don't unpack if -fomit-iface-pragmas
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          -- Don't unpack if we aren't optimising; rather arbitrarily,
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          -- we use -fomit-iface-pragmas as the indication
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  , let mb_co   = topNormaliseType_maybe fam_envs arg_ty
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                     -- Unwrap type families and newtypes
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        arg_ty' = case mb_co of { Just (_,ty) -> ty; Nothing -> arg_ty }
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  , isUnpackableType dflags fam_envs arg_ty'
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  , (rep_tys, _) <- dataConArgUnpack arg_ty'
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  , case unpk_prag of
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      NoSrcUnpack ->
        gopt Opt_UnboxStrictFields dflags
            || (gopt Opt_UnboxSmallStrictFields dflags
                && length rep_tys <= 1) -- See Note [Unpack one-wide fields]
      srcUnpack -> isSrcUnpacked srcUnpack
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  = case mb_co of
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      Nothing     -> HsUnpack Nothing
      Just (co,_) -> HsUnpack (Just co)
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  | otherwise -- Record the strict-but-no-unpack decision
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  = HsStrict
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-- | Wrappers/Workers and representation following Unpack/Strictness
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-- decisions
dataConArgRep
  :: Type
  -> HsImplBang
  -> ([(Type,StrictnessMark)] -- Rep types
     ,(Unboxer,Boxer))

dataConArgRep arg_ty HsLazy
  = ([(arg_ty, NotMarkedStrict)], (unitUnboxer, unitBoxer))
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dataConArgRep arg_ty HsStrict
  = ([(arg_ty, MarkedStrict)], (seqUnboxer, unitBoxer))

dataConArgRep arg_ty (HsUnpack Nothing)
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  | (rep_tys, wrappers) <- dataConArgUnpack arg_ty
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  = (rep_tys, wrappers)
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dataConArgRep _ (HsUnpack (Just co))
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  | let co_rep_ty = pSnd (coercionKind co)
  , (rep_tys, wrappers) <- dataConArgUnpack co_rep_ty
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  = (rep_tys, wrapCo co co_rep_ty wrappers)
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-------------------------
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wrapCo :: Coercion -> Type -> (Unboxer, Boxer) -> (Unboxer, Boxer)
wrapCo co rep_ty (unbox_rep, box_rep)  -- co :: arg_ty ~ rep_ty
  = (unboxer, boxer)
  where
    unboxer arg_id = do { rep_id <- newLocal rep_ty
                        ; (rep_ids, rep_fn) <- unbox_rep rep_id
                        ; let co_bind = NonRec rep_id (Var arg_id `Cast` co)
                        ; return (rep_ids, Let co_bind . rep_fn) }
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    boxer = Boxer $ \ subst ->
            do { (rep_ids, rep_expr)
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                    <- case box_rep of
                         UnitBox -> do { rep_id <- newLocal (TcType.substTy subst rep_ty)
                                       ; return ([rep_id], Var rep_id) }
                         Boxer boxer -> boxer subst
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               ; let sco = substCoUnchecked subst co
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               ; return (rep_ids, rep_expr `Cast` mkSymCo sco) }

------------------------
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seqUnboxer :: Unboxer
seqUnboxer v = return ([v], \e -> Case (Var v) v (exprType e) [(DEFAULT, [], e)])

unitUnboxer :: Unboxer
unitUnboxer v = return ([v], \e -> e)

unitBoxer :: Boxer
unitBoxer = UnitBox

-------------------------
dataConArgUnpack
   :: Type
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   ->  ( [(Type, StrictnessMark)]   -- Rep types
       , (Unboxer, Boxer) )
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dataConArgUnpack arg_ty
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  | Just (tc, tc_args) <- splitTyConApp_maybe arg_ty
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  , Just con <- tyConSingleAlgDataCon_maybe tc
      -- NB: check for an *algebraic* data type
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      -- A recursive newtype might mean that
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      -- 'arg_ty' is a newtype
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  , let rep_tys = dataConInstArgTys con tc_args
  = ASSERT( isVanillaDataCon con )
    ( rep_tys `zip` dataConRepStrictness con
    ,( \ arg_id ->
       do { rep_ids <- mapM newLocal rep_tys
          ; let unbox_fn body
                  = Case (Var arg_id) arg_id (exprType body)
                         [(DataAlt con, rep_ids, body)]
          ; return (rep_ids, unbox_fn) }
     , Boxer $ \ subst ->
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       do { rep_ids <- mapM (newLocal . TcType.substTyUnchecked subst) rep_tys
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          ; return (rep_ids, Var (dataConWorkId con)
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                             `mkTyApps` (substTysUnchecked subst tc_args)
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                             `mkVarApps` rep_ids ) } ) )
  | otherwise
  = pprPanic "dataConArgUnpack" (ppr arg_ty)
    -- An interface file specified Unpacked, but we couldn't unpack it

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isUnpackableType :: DynFlags -> FamInstEnvs -> Type -> Bool
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-- True if we can unpack the UNPACK the argument type
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-- See Note [Recursive unboxing]
-- We look "deeply" inside rather than relying on the DataCons
-- we encounter on the way, because otherwise we might well
-- end up relying on ourselves!
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isUnpackableType dflags fam_envs ty
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  | Just (tc, _) <- splitTyConApp_maybe ty
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  , Just con <- tyConSingleAlgDataCon_maybe tc
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  , isVanillaDataCon con
  = ok_con_args (unitNameSet (getName tc)) con
  | otherwise
  = False
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  where
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    ok_arg tcs (ty, bang) = not (attempt_unpack bang) || ok_ty tcs norm_ty
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        where
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          norm_ty = topNormaliseType fam_envs ty
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    ok_ty tcs ty
      | Just (tc, _) <- splitTyConApp_maybe ty
      , let tc_name = getName tc
      =  not (tc_name `elemNameSet` tcs)
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      && case tyConSingleAlgDataCon_maybe tc of
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            Just con | isVanillaDataCon con
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                    -> ok_con_args (tcs `extendNameSet` getName tc) con
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            _ -> True
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      | otherwise
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      = True

    ok_con_args tcs con
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       = all (ok_arg tcs) (dataConOrigArgTys con `zip` dataConSrcBangs con)
         -- NB: dataConSrcBangs gives the *user* request;
         -- We'd get a black hole if we used dataConImplBangs
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    attempt_unpack (HsSrcBang _ SrcUnpack NoSrcStrict)
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      = xopt LangExt.StrictData dflags
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    attempt_unpack (HsSrcBang _ SrcUnpack SrcStrict)
      = True
    attempt_unpack (HsSrcBang _  NoSrcUnpack SrcStrict)
      = True  -- Be conservative
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    attempt_unpack (HsSrcBang _  NoSrcUnpack NoSrcStrict)
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      = xopt LangExt.StrictData dflags -- Be conservative
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    attempt_unpack _ = False
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{-
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Note [Unpack one-wide fields]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The flag UnboxSmallStrictFields ensures that any field that can
(safely) be unboxed to a word-sized unboxed field, should be so unboxed.
For example:

    data A = A Int#
    newtype B = B A
    data C = C !B
    data D = D !C
    data E = E !()
    data F = F !D
    data G = G !F !F

All of these should have an Int# as their representation, except
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G which should have two Int#s.
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However
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    data T = T !(S Int)
    data S = S !a

Here we can represent T with an Int#.

Note [Recursive unboxing]
~~~~~~~~~~~~~~~~~~~~~~~~~
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Consider
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  data R = MkR {-# UNPACK #-} !S Int
  data S = MkS {-# UNPACK #-} !Int
The representation arguments of MkR are the *representation* arguments
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of S (plus Int); the rep args of MkS are Int#.  This is all fine.

But be careful not to try to unbox this!
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        data T = MkT {-# UNPACK #-} !T Int
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Because then we'd get an infinite number of arguments.

Here is a more complicated case:
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        data S = MkS {-# UNPACK #-} !T Int
        data T = MkT {-# UNPACK #-} !S Int
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Each of S and T must decide independently whether to unpack
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and they had better not both say yes. So they must both say no.

Also behave conservatively when there is no UNPACK pragma
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        data T = MkS !T Int
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with -funbox-strict-fields or -funbox-small-strict-fields
we need to behave as if there was an UNPACK pragma there.
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But it's the *argument* type that matters. This is fine:
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        data S = MkS S !Int
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because Int is non-recursive.