TcSplice.hs 63.5 KB
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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
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TcSplice: Template Haskell splices
-}
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{-# LANGUAGE CPP #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE InstanceSigs #-}
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{-# OPTIONS_GHC -fno-warn-orphans #-}
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module TcSplice(
     -- These functions are defined in stage1 and stage2
     -- The raise civilised errors in stage1
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     tcSpliceExpr, tcTypedBracket, tcUntypedBracket,
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--     runQuasiQuoteExpr, runQuasiQuotePat,
--     runQuasiQuoteDecl, runQuasiQuoteType,
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     runAnnotation,

#ifdef GHCI
     -- These ones are defined only in stage2, and are
     -- called only in stage2 (ie GHCI is on)
     runMetaE, runMetaP, runMetaT, runMetaD, runQuasi,
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     tcTopSpliceExpr, lookupThName_maybe,
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     defaultRunMeta, runMeta'
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#endif
      ) where
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#include "HsVersions.h"

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import HsSyn
import Annotations
import Name
import TcRnMonad
import TcType

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import Outputable
import TcExpr
import SrcLoc
import FastString
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import THNames
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import TcUnify
import TcEnv

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#ifdef GHCI
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import HscMain
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        -- These imports are the reason that TcSplice
        -- is very high up the module hierarchy
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import RnSplice( traceSplice, SpliceInfo(..) )
import RdrName
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import HscTypes
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import Convert
import RnExpr
import RnEnv
import RnTypes
import TcHsSyn
import TcSimplify
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import Type
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import Kind
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import NameSet
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import TcMType
import TcHsType
import TcIface
import TypeRep
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import FamInst
import FamInstEnv
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import InstEnv
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import NameEnv
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import PrelNames
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import OccName
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import Hooks
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import Var
import Module
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import LoadIface
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import Class
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import Inst
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import TyCon
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import CoAxiom
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import PatSyn ( patSynName )
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import ConLike
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import DataCon
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import TcEvidence( TcEvBinds(..) )
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import Id
import IdInfo
import DsExpr
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import DsMonad
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import Serialized
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import ErrUtils
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import Util
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import Unique
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import VarSet           ( isEmptyVarSet )
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import Data.Maybe
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import BasicTypes hiding( SuccessFlag(..) )
import Maybes( MaybeErr(..) )
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import DynFlags
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import Panic
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import Lexeme
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import qualified Language.Haskell.TH as TH
-- THSyntax gives access to internal functions and data types
import qualified Language.Haskell.TH.Syntax as TH
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-- Because GHC.Desugar might not be in the base library of the bootstrapping compiler
import GHC.Desugar      ( AnnotationWrapper(..) )
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import qualified Data.Map as Map
import Data.Dynamic  ( fromDynamic, toDyn )
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import Data.Typeable ( typeOf, Typeable )
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import Data.Data (Data)
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import GHC.Exts         ( unsafeCoerce# )
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#endif

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{-
************************************************************************
*                                                                      *
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\subsection{Main interface + stubs for the non-GHCI case
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*                                                                      *
************************************************************************
-}
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tcTypedBracket   :: HsBracket Name -> TcRhoType -> TcM (HsExpr TcId)
tcUntypedBracket :: HsBracket Name -> [PendingRnSplice] -> TcRhoType -> TcM (HsExpr TcId)
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tcSpliceExpr     :: HsSplice Name  -> TcRhoType -> TcM (HsExpr TcId)
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        -- None of these functions add constraints to the LIE

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-- runQuasiQuoteExpr :: HsQuasiQuote RdrName -> RnM (LHsExpr RdrName)
-- runQuasiQuotePat  :: HsQuasiQuote RdrName -> RnM (LPat RdrName)
-- runQuasiQuoteType :: HsQuasiQuote RdrName -> RnM (LHsType RdrName)
-- runQuasiQuoteDecl :: HsQuasiQuote RdrName -> RnM [LHsDecl RdrName]
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runAnnotation     :: CoreAnnTarget -> LHsExpr Name -> TcM Annotation
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{-
************************************************************************
*                                                                      *
\subsection{Quoting an expression}
*                                                                      *
************************************************************************
-}

-- See Note [How brackets and nested splices are handled]
-- tcTypedBracket :: HsBracket Name -> TcRhoType -> TcM (HsExpr TcId)
tcTypedBracket brack@(TExpBr expr) res_ty
  = addErrCtxt (quotationCtxtDoc brack) $
    do { cur_stage <- getStage
       ; ps_ref <- newMutVar []
       ; lie_var <- getConstraintVar   -- Any constraints arising from nested splices
                                       -- should get thrown into the constraint set
                                       -- from outside the bracket

       -- Typecheck expr to make sure it is valid,
       -- Throw away the typechecked expression but return its type.
       -- We'll typecheck it again when we splice it in somewhere
       ; (_tc_expr, expr_ty) <- setStage (Brack cur_stage (TcPending ps_ref lie_var)) $
                                tcInferRhoNC expr
                                -- NC for no context; tcBracket does that

       ; meta_ty <- tcTExpTy expr_ty
       ; co <- unifyType meta_ty res_ty
       ; ps' <- readMutVar ps_ref
       ; texpco <- tcLookupId unsafeTExpCoerceName
       ; return (mkHsWrapCo co (unLoc (mkHsApp (nlHsTyApp texpco [expr_ty])
                                               (noLoc (HsTcBracketOut brack ps'))))) }
tcTypedBracket other_brack _
  = pprPanic "tcTypedBracket" (ppr other_brack)

-- tcUntypedBracket :: HsBracket Name -> [PendingRnSplice] -> TcRhoType -> TcM (HsExpr TcId)
tcUntypedBracket brack ps res_ty
  = do { traceTc "tc_bracket untyped" (ppr brack $$ ppr ps)
       ; ps' <- mapM tcPendingSplice ps
       ; meta_ty <- tcBrackTy brack
       ; co <- unifyType meta_ty res_ty
       ; traceTc "tc_bracket done untyped" (ppr meta_ty)
       ; return (mkHsWrapCo co (HsTcBracketOut brack ps'))  }

---------------
tcBrackTy :: HsBracket Name -> TcM TcType
tcBrackTy (VarBr _ _) = tcMetaTy nameTyConName  -- Result type is Var (not Q-monadic)
tcBrackTy (ExpBr _)   = tcMetaTy expQTyConName  -- Result type is ExpQ (= Q Exp)
tcBrackTy (TypBr _)   = tcMetaTy typeQTyConName -- Result type is Type (= Q Typ)
tcBrackTy (DecBrG _)  = tcMetaTy decsQTyConName -- Result type is Q [Dec]
tcBrackTy (PatBr _)   = tcMetaTy patQTyConName  -- Result type is PatQ (= Q Pat)
tcBrackTy (DecBrL _)  = panic "tcBrackTy: Unexpected DecBrL"
tcBrackTy (TExpBr _)  = panic "tcUntypedBracket: Unexpected TExpBr"

---------------
tcPendingSplice :: PendingRnSplice -> TcM PendingTcSplice
tcPendingSplice (PendingRnSplice flavour splice_name expr)
  = do { res_ty <- tcMetaTy meta_ty_name
       ; expr' <- tcMonoExpr expr res_ty
       ; return (PendingTcSplice splice_name expr') }
  where
     meta_ty_name = case flavour of
                       UntypedExpSplice  -> expQTyConName
                       UntypedPatSplice  -> patQTyConName
                       UntypedTypeSplice -> typeQTyConName
                       UntypedDeclSplice -> decsQTyConName

---------------
-- Takes a type tau and returns the type Q (TExp tau)
tcTExpTy :: TcType -> TcM TcType
tcTExpTy tau
  = do { q    <- tcLookupTyCon qTyConName
       ; texp <- tcLookupTyCon tExpTyConName
       ; return (mkTyConApp q [mkTyConApp texp [tau]]) }

quotationCtxtDoc :: HsBracket Name -> SDoc
quotationCtxtDoc br_body
  = hang (ptext (sLit "In the Template Haskell quotation"))
         2 (ppr br_body)

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#ifndef GHCI
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tcSpliceExpr  e _      = failTH e "Template Haskell splice"
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-- runQuasiQuoteExpr q = failTH q "quasiquote"
-- runQuasiQuotePat  q = failTH q "pattern quasiquote"
-- runQuasiQuoteType q = failTH q "type quasiquote"
-- runQuasiQuoteDecl q = failTH q "declaration quasiquote"
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runAnnotation   _ q = failTH q "annotation"

#else
  -- The whole of the rest of the file is the else-branch (ie stage2 only)

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{-
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Note [How top-level splices are handled]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Top-level splices (those not inside a [| .. |] quotation bracket) are handled
very straightforwardly:

  1. tcTopSpliceExpr: typecheck the body e of the splice $(e)

  2. runMetaT: desugar, compile, run it, and convert result back to
     HsSyn RdrName (of the appropriate flavour, eg HsType RdrName,
     HsExpr RdrName etc)

  3. treat the result as if that's what you saw in the first place
     e.g for HsType, rename and kind-check
         for HsExpr, rename and type-check

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     (The last step is different for decls, because they can *only* be
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      top-level: we return the result of step 2.)

Note [How brackets and nested splices are handled]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Nested splices (those inside a [| .. |] quotation bracket),
are treated quite differently.

Remember, there are two forms of bracket
         typed   [|| e ||]
   and untyped   [|  e  |]

The life cycle of a typed bracket:
   * Starts as HsBracket

   * When renaming:
        * Set the ThStage to (Brack s RnPendingTyped)
        * Rename the body
        * Result is still a HsBracket

   * When typechecking:
        * Set the ThStage to (Brack s (TcPending ps_var lie_var))
        * Typecheck the body, and throw away the elaborated result
        * Nested splices (which must be typed) are typechecked, and
          the results accumulated in ps_var; their constraints
          accumulate in lie_var
        * Result is a HsTcBracketOut rn_brack pending_splices
          where rn_brack is the incoming renamed bracket

The life cycle of a un-typed bracket:
   * Starts as HsBracket

   * When renaming:
        * Set the ThStage to (Brack s (RnPendingUntyped ps_var))
        * Rename the body
        * Nested splices (which must be untyped) are renamed, and the
          results accumulated in ps_var
        * Result is still (HsRnBracketOut rn_body pending_splices)

   * When typechecking a HsRnBracketOut
        * Typecheck the pending_splices individually
        * Ignore the body of the bracket; just check that the context
          expects a bracket of that type (e.g. a [p| pat |] bracket should
          be in a context needing a (Q Pat)
        * Result is a HsTcBracketOut rn_brack pending_splices
          where rn_brack is the incoming renamed bracket


In both cases, desugaring happens like this:
  * HsTcBracketOut is desugared by DsMeta.dsBracket.  It
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      a) Extends the ds_meta environment with the PendingSplices
         attached to the bracket

      b) Converts the quoted (HsExpr Name) to a CoreExpr that, when
         run, will produce a suitable TH expression/type/decl.  This
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         is why we leave the *renamed* expression attached to the bracket:
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         the quoted expression should not be decorated with all the goop
         added by the type checker

  * Each splice carries a unique Name, called a "splice point", thus
    ${n}(e).  The name is initialised to an (Unqual "splice") when the
    splice is created; the renamer gives it a unique.

  * When DsMeta (used to desugar the body of the bracket) comes across
    a splice, it looks up the splice's Name, n, in the ds_meta envt,
    to find an (HsExpr Id) that should be substituted for the splice;
    it just desugars it to get a CoreExpr (DsMeta.repSplice).

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Example:
    Source:       f = [| Just $(g 3) |]
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      The [| |] part is a HsBracket

    Typechecked:  f = [| Just ${s7}(g 3) |]{s7 = g Int 3}
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      The [| |] part is a HsBracketOut, containing *renamed*
        (not typechecked) expression
      The "s7" is the "splice point"; the (g Int 3) part
        is a typechecked expression
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    Desugared:    f = do { s7 <- g Int 3
                         ; return (ConE "Data.Maybe.Just" s7) }
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Note [Template Haskell state diagram]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here are the ThStages, s, their corresponding level numbers
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(the result of (thLevel s)), and their state transitions.
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The top level of the program is stage Comp:
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     Start here
         |
         V
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      -----------     $      ------------   $
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      |  Comp   | ---------> |  Splice  | -----|
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      |   1     |            |    0     | <----|
      -----------            ------------
        ^     |                ^      |
      $ |     | [||]         $ |      | [||]
        |     v                |      v
   --------------          ----------------
   | Brack Comp |          | Brack Splice |
   |     2      |          |      1       |
   --------------          ----------------

* Normal top-level declarations start in state Comp
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       (which has level 1).
  Annotations start in state Splice, since they are
       treated very like a splice (only without a '$')

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* Code compiled in state Splice (and only such code)
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  will be *run at compile time*, with the result replacing
  the splice

* The original paper used level -1 instead of 0, etc.

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* The original paper did not allow a splice within a
  splice, but there is no reason not to. This is the
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  $ transition in the top right.

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Note [Template Haskell levels]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Imported things are impLevel (= 0)
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* However things at level 0 are not *necessarily* imported.
      eg  $( \b -> ... )   here b is bound at level 0

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* In GHCi, variables bound by a previous command are treated
  as impLevel, because we have bytecode for them.

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* Variables are bound at the "current level"
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* The current level starts off at outerLevel (= 1)
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* The level is decremented by splicing $(..)
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               incremented by brackets [| |]
               incremented by name-quoting 'f
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When a variable is used, we compare
        bind:  binding level, and
        use:   current level at usage site
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  Generally
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        bind > use      Always error (bound later than used)
                        [| \x -> $(f x) |]
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        bind = use      Always OK (bound same stage as used)
                        [| \x -> $(f [| x |]) |]

        bind < use      Inside brackets, it depends
                        Inside splice, OK
                        Inside neither, OK
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  For (bind < use) inside brackets, there are three cases:
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    - Imported things   OK      f = [| map |]
    - Top-level things  OK      g = [| f |]
    - Non-top-level     Only if there is a liftable instance
                                h = \(x:Int) -> [| x |]
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  To track top-level-ness we use the ThBindEnv in TcLclEnv
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  For example:
           f = ...
           g1 = $(map ...)         is OK
           g2 = $(f ...)           is not OK; because we havn't compiled f yet
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-}
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{-
************************************************************************
*                                                                      *
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\subsection{Splicing an expression}
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*                                                                      *
************************************************************************
-}
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tcSpliceExpr splice@(HsTypedSplice name expr) res_ty
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  = addErrCtxt (spliceCtxtDoc splice) $
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    setSrcSpan (getLoc expr)    $ do
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    { stage <- getStage
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    ; case stage of
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        Splice {}            -> tcTopSplice expr res_ty
        Comp                 -> tcTopSplice expr res_ty
        Brack pop_stage pend -> tcNestedSplice pop_stage pend name expr res_ty }
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tcSpliceExpr splice _
  = pprPanic "tcSpliceExpr" (ppr splice)
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tcNestedSplice :: ThStage -> PendingStuff -> Name
                -> LHsExpr Name -> TcRhoType -> TcM (HsExpr Id)
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    -- See Note [How brackets and nested splices are handled]
    -- A splice inside brackets
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tcNestedSplice pop_stage (TcPending ps_var lie_var) splice_name expr res_ty
  = do { meta_exp_ty <- tcTExpTy res_ty
       ; expr' <- setStage pop_stage $
                  setConstraintVar lie_var $
                  tcMonoExpr expr meta_exp_ty
       ; untypeq <- tcLookupId unTypeQName
       ; let expr'' = mkHsApp (nlHsTyApp untypeq [res_ty]) expr'
       ; ps <- readMutVar ps_var
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       ; writeMutVar ps_var (PendingTcSplice splice_name expr'' : ps)
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       -- The returned expression is ignored; it's in the pending splices
       ; return (panic "tcSpliceExpr") }

tcNestedSplice _ _ splice_name _ _
  = pprPanic "tcNestedSplice: rename stage found" (ppr splice_name)
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tcTopSplice :: LHsExpr Name -> TcRhoType -> TcM (HsExpr Id)
tcTopSplice expr res_ty
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  = do { -- Typecheck the expression,
         -- making sure it has type Q (T res_ty)
         meta_exp_ty <- tcTExpTy res_ty
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       ; zonked_q_expr <- tcTopSpliceExpr True $
                          tcMonoExpr expr meta_exp_ty
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         -- Run the expression
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       ; expr2 <- runMetaE zonked_q_expr
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       ; traceSplice (SpliceInfo { spliceDescription = "expression"
                                 , spliceIsDecl      = False
                                 , spliceSource      = Just expr
                                 , spliceGenerated   = ppr expr2 })
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         -- Rename and typecheck the spliced-in expression,
         -- making sure it has type res_ty
         -- These steps should never fail; this is a *typed* splice
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       ; addErrCtxt (spliceResultDoc expr) $ do
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       { (exp3, _fvs) <- rnLExpr expr2
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       ; exp4 <- tcMonoExpr exp3 res_ty
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       ; return (unLoc exp4) } }
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{-
************************************************************************
*                                                                      *
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\subsection{Error messages}
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*                                                                      *
************************************************************************
-}
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spliceCtxtDoc :: HsSplice Name -> SDoc
spliceCtxtDoc splice
  = hang (ptext (sLit "In the Template Haskell splice"))
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         2 (pprSplice splice)
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spliceResultDoc :: LHsExpr Name -> SDoc
spliceResultDoc expr
  = sep [ ptext (sLit "In the result of the splice:")
        , nest 2 (char '$' <> pprParendExpr expr)
        , ptext (sLit "To see what the splice expanded to, use -ddump-splices")]
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-------------------
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tcTopSpliceExpr :: Bool -> TcM (LHsExpr Id) -> TcM (LHsExpr Id)
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-- Note [How top-level splices are handled]
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-- Type check an expression that is the body of a top-level splice
--   (the caller will compile and run it)
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-- Note that set the level to Splice, regardless of the original level,
-- before typechecking the expression.  For example:
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--      f x = $( ...$(g 3) ... )
-- The recursive call to tcMonoExpr will simply expand the
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-- inner escape before dealing with the outer one

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tcTopSpliceExpr isTypedSplice tc_action
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  = checkNoErrs $  -- checkNoErrs: must not try to run the thing
                   -- if the type checker fails!
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    unsetGOptM Opt_DeferTypeErrors $
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                   -- Don't defer type errors.  Not only are we
                   -- going to run this code, but we do an unsafe
                   -- coerce, so we get a seg-fault if, say we
                   -- splice a type into a place where an expression
                   -- is expected (Trac #7276)
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    setStage (Splice isTypedSplice) $
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    do {    -- Typecheck the expression
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         (expr', lie) <- captureConstraints tc_action
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        -- Solve the constraints
        ; const_binds <- simplifyTop lie

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          -- Zonk it and tie the knot of dictionary bindings
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       ; zonkTopLExpr (mkHsDictLet (EvBinds const_binds) expr') }
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{-
************************************************************************
*                                                                      *
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        Annotations
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*                                                                      *
************************************************************************
-}
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runAnnotation target expr = do
    -- Find the classes we want instances for in order to call toAnnotationWrapper
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    loc <- getSrcSpanM
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    data_class <- tcLookupClass dataClassName
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    to_annotation_wrapper_id <- tcLookupId toAnnotationWrapperName
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    -- Check the instances we require live in another module (we want to execute it..)
    -- and check identifiers live in other modules using TH stage checks. tcSimplifyStagedExpr
    -- also resolves the LIE constraints to detect e.g. instance ambiguity
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    zonked_wrapped_expr' <- tcTopSpliceExpr False $
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           do { (expr', expr_ty) <- tcInferRhoNC expr
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                -- We manually wrap the typechecked expression in a call to toAnnotationWrapper
                -- By instantiating the call >here< it gets registered in the
                -- LIE consulted by tcTopSpliceExpr
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                -- and hence ensures the appropriate dictionary is bound by const_binds
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              ; wrapper <- instCall AnnOrigin [expr_ty] [mkClassPred data_class [expr_ty]]
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              ; let specialised_to_annotation_wrapper_expr
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                      = L loc (HsWrap wrapper (HsVar to_annotation_wrapper_id))
              ; return (L loc (HsApp specialised_to_annotation_wrapper_expr expr')) }
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    -- Run the appropriately wrapped expression to get the value of
    -- the annotation and its dictionaries. The return value is of
    -- type AnnotationWrapper by construction, so this conversion is
    -- safe
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    serialized <- runMetaAW zonked_wrapped_expr'
    return Annotation {
               ann_target = target,
               ann_value = serialized
           }

convertAnnotationWrapper :: AnnotationWrapper -> Either MsgDoc Serialized
convertAnnotationWrapper  annotation_wrapper = Right $
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        case annotation_wrapper of
            AnnotationWrapper value | let serialized = toSerialized serializeWithData value ->
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                -- Got the value and dictionaries: build the serialized value and
                -- call it a day. We ensure that we seq the entire serialized value
                -- in order that any errors in the user-written code for the
                -- annotation are exposed at this point.  This is also why we are
                -- doing all this stuff inside the context of runMeta: it has the
                -- facilities to deal with user error in a meta-level expression
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                seqSerialized serialized `seq` serialized

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{-
************************************************************************
*                                                                      *
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\subsection{Running an expression}
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*                                                                      *
************************************************************************
-}
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runQuasi :: TH.Q a -> TcM a
runQuasi act = TH.runQ act

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runQResult :: (a -> String) -> (SrcSpan -> a -> b) -> SrcSpan -> TH.Q a -> TcM b
runQResult show_th f expr_span hval
  = do { th_result <- TH.runQ hval
       ; traceTc "Got TH result:" (text (show_th th_result))
       ; return (f expr_span th_result) }
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-----------------
runMeta :: (MetaHook TcM -> LHsExpr Id -> TcM hs_syn)
        -> LHsExpr Id
        -> TcM hs_syn
runMeta unwrap e
  = do { h <- getHooked runMetaHook defaultRunMeta
       ; unwrap h e }

defaultRunMeta :: MetaHook TcM
defaultRunMeta (MetaE r)
  = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsExpr)
defaultRunMeta (MetaP r)
  = fmap r . runMeta' True ppr (runQResult TH.pprint convertToPat)
defaultRunMeta (MetaT r)
  = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsType)
defaultRunMeta (MetaD r)
  = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsDecls)
defaultRunMeta (MetaAW r)
  = fmap r . runMeta' False (const empty) (const (return . convertAnnotationWrapper))
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    -- We turn off showing the code in meta-level exceptions because doing so exposes
    -- the toAnnotationWrapper function that we slap around the users code

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----------------
runMetaAW :: LHsExpr Id         -- Of type AnnotationWrapper
          -> TcM Serialized
runMetaAW = runMeta metaRequestAW
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runMetaE :: LHsExpr Id          -- Of type (Q Exp)
         -> TcM (LHsExpr RdrName)
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runMetaE = runMeta metaRequestE
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runMetaP :: LHsExpr Id          -- Of type (Q Pat)
         -> TcM (LPat RdrName)
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runMetaP = runMeta metaRequestP
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runMetaT :: LHsExpr Id          -- Of type (Q Type)
         -> TcM (LHsType RdrName)
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runMetaT = runMeta metaRequestT
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runMetaD :: LHsExpr Id          -- Of type Q [Dec]
         -> TcM [LHsDecl RdrName]
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runMetaD = runMeta metaRequestD
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---------------
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runMeta' :: Bool                 -- Whether code should be printed in the exception message
         -> (hs_syn -> SDoc)                                    -- how to print the code
         -> (SrcSpan -> x -> TcM (Either MsgDoc hs_syn))        -- How to run x
         -> LHsExpr Id           -- Of type x; typically x = Q TH.Exp, or something like that
         -> TcM hs_syn           -- Of type t
runMeta' show_code ppr_hs run_and_convert expr
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  = do  { traceTc "About to run" (ppr expr)
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        ; recordThSpliceUse -- seems to be the best place to do this,
                            -- we catch all kinds of splices and annotations.
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        -- Check that we've had no errors of any sort so far.
        -- For example, if we found an error in an earlier defn f, but
        -- recovered giving it type f :: forall a.a, it'd be very dodgy
        -- to carry ont.  Mind you, the staging restrictions mean we won't
        -- actually run f, but it still seems wrong. And, more concretely,
        -- see Trac #5358 for an example that fell over when trying to
        -- reify a function with a "?" kind in it.  (These don't occur
        -- in type-correct programs.
        ; failIfErrsM

        -- Desugar
        ; ds_expr <- initDsTc (dsLExpr expr)
        -- Compile and link it; might fail if linking fails
        ; hsc_env <- getTopEnv
        ; src_span <- getSrcSpanM
        ; traceTc "About to run (desugared)" (ppr ds_expr)
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        ; either_hval <- tryM $ liftIO $
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                         HscMain.hscCompileCoreExpr hsc_env src_span ds_expr
        ; case either_hval of {
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            Left exn   -> fail_with_exn "compile and link" exn ;
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            Right hval -> do

        {       -- Coerce it to Q t, and run it

                -- Running might fail if it throws an exception of any kind (hence tryAllM)
                -- including, say, a pattern-match exception in the code we are running
                --
                -- We also do the TH -> HS syntax conversion inside the same
                -- exception-cacthing thing so that if there are any lurking
                -- exceptions in the data structure returned by hval, we'll
                -- encounter them inside the try
                --
                -- See Note [Exceptions in TH]
          let expr_span = getLoc expr
        ; either_tval <- tryAllM $
                         setSrcSpan expr_span $ -- Set the span so that qLocation can
                                                -- see where this splice is
             do { mb_result <- run_and_convert expr_span (unsafeCoerce# hval)
                ; case mb_result of
                    Left err     -> failWithTc err
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                    Right result -> do { traceTc "Got HsSyn result:" (ppr_hs result)
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                                       ; return $! result } }
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        ; case either_tval of
            Right v -> return v
            Left se -> case fromException se of
                         Just IOEnvFailure -> failM -- Error already in Tc monad
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                         _ -> fail_with_exn "run" se -- Exception
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        }}}
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  where
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    -- see Note [Concealed TH exceptions]
    fail_with_exn phase exn = do
        exn_msg <- liftIO $ Panic.safeShowException exn
        let msg = vcat [text "Exception when trying to" <+> text phase <+> text "compile-time code:",
                        nest 2 (text exn_msg),
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                        if show_code then text "Code:" <+> ppr expr else empty]
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        failWithTc msg
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{-
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Note [Exceptions in TH]
~~~~~~~~~~~~~~~~~~~~~~~
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Suppose we have something like this
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        $( f 4 )
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where
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        f :: Int -> Q [Dec]
        f n | n>3       = fail "Too many declarations"
            | otherwise = ...
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The 'fail' is a user-generated failure, and should be displayed as a
perfectly ordinary compiler error message, not a panic or anything
like that.  Here's how it's processed:

  * 'fail' is the monad fail.  The monad instance for Q in TH.Syntax
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    effectively transforms (fail s) to
        qReport True s >> fail
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    where 'qReport' comes from the Quasi class and fail from its monad
    superclass.

  * The TcM monad is an instance of Quasi (see TcSplice), and it implements
    (qReport True s) by using addErr to add an error message to the bag of errors.
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    The 'fail' in TcM raises an IOEnvFailure exception
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 * 'qReport' forces the message to ensure any exception hidden in unevaluated
   thunk doesn't get into the bag of errors. Otherwise the following splice
   will triger panic (Trac #8987):
        $(fail undefined)
   See also Note [Concealed TH exceptions]

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  * So, when running a splice, we catch all exceptions; then for
        - an IOEnvFailure exception, we assume the error is already
                in the error-bag (above)
        - other errors, we add an error to the bag
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    and then fail

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Note [Concealed TH exceptions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When displaying the error message contained in an exception originated from TH
code, we need to make sure that the error message itself does not contain an
exception.  For example, when executing the following splice:

    $( error ("foo " ++ error "bar") )

the message for the outer exception is a thunk which will throw the inner
exception when evaluated.

For this reason, we display the message of a TH exception using the
'safeShowException' function, which recursively catches any exception thrown
when showing an error message.

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To call runQ in the Tc monad, we need to make TcM an instance of Quasi:
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-}
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instance TH.Quasi (IOEnv (Env TcGblEnv TcLclEnv)) where
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  qNewName s = do { u <- newUnique
                  ; let i = getKey u
                  ; return (TH.mkNameU s i) }
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  -- 'msg' is forced to ensure exceptions don't escape,
  -- see Note [Exceptions in TH]
  qReport True msg  = seqList msg $ addErr  (text msg)
  qReport False msg = seqList msg $ addWarn (text msg)
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  qLocation = do { m <- getModule
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                 ; l <- getSrcSpanM
                 ; r <- case l of
                        UnhelpfulSpan _ -> pprPanic "qLocation: Unhelpful location"
                                                    (ppr l)
                        RealSrcSpan s -> return s
                 ; return (TH.Loc { TH.loc_filename = unpackFS (srcSpanFile r)
                                  , TH.loc_module   = moduleNameString (moduleName m)
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                                  , TH.loc_package  = packageKeyString (modulePackageKey m)
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                                  , TH.loc_start = (srcSpanStartLine r, srcSpanStartCol r)
                                  , TH.loc_end = (srcSpanEndLine   r, srcSpanEndCol   r) }) }

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  qLookupName       = lookupName
  qReify            = reify
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  qReifyFixity nm   = lookupThName nm >>= reifyFixity
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  qReifyInstances   = reifyInstances
  qReifyRoles       = reifyRoles
  qReifyAnnotations = reifyAnnotations
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  qReifyModule      = reifyModule
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        -- For qRecover, discard error messages if
        -- the recovery action is chosen.  Otherwise
        -- we'll only fail higher up.  c.f. tryTcLIE_
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  qRecover recover main = do { (msgs, mb_res) <- tryTcErrs main
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                             ; case mb_res of
                                 Just val -> do { addMessages msgs      -- There might be warnings
                                                ; return val }
                                 Nothing  -> recover                    -- Discard all msgs
                          }
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  qRunIO io = liftIO io
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  qAddDependentFile fp = do
    ref <- fmap tcg_dependent_files getGblEnv
    dep_files <- readTcRef ref
    writeTcRef ref (fp:dep_files)
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  qAddTopDecls thds = do
      l <- getSrcSpanM
      let either_hval = convertToHsDecls l thds
      ds <- case either_hval of
              Left exn -> pprPanic "qAddTopDecls: can't convert top-level declarations" exn
              Right ds -> return ds
      mapM_ (checkTopDecl . unLoc) ds
      th_topdecls_var <- fmap tcg_th_topdecls getGblEnv
      updTcRef th_topdecls_var (\topds -> ds ++ topds)
    where
      checkTopDecl :: HsDecl RdrName -> TcM ()
      checkTopDecl (ValD binds)
        = mapM_ bindName (collectHsBindBinders binds)
      checkTopDecl (SigD _)
        = return ()
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      checkTopDecl (AnnD _)
        = return ()
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      checkTopDecl (ForD (ForeignImport (L _ name) _ _ _))
        = bindName name
      checkTopDecl _
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        = addErr $ text "Only function, value, annotation, and foreign import declarations may be added with addTopDecl"
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      bindName :: RdrName -> TcM ()
      bindName (Exact n)
        = do { th_topnames_var <- fmap tcg_th_topnames getGblEnv
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             ; updTcRef th_topnames_var (\ns -> extendNameSet ns n)
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             }

      bindName name =
          addErr $
          hang (ptext (sLit "The binder") <+> quotes (ppr name) <+> ptext (sLit "is not a NameU."))
             2 (text "Probable cause: you used mkName instead of newName to generate a binding.")
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  qAddModFinalizer fin = do
      th_modfinalizers_var <- fmap tcg_th_modfinalizers getGblEnv
      updTcRef th_modfinalizers_var (\fins -> fin:fins)
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  qGetQ :: forall a. Typeable a => IOEnv (Env TcGblEnv TcLclEnv) (Maybe a)
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  qGetQ = do
      th_state_var <- fmap tcg_th_state getGblEnv
      th_state <- readTcRef th_state_var
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      -- See #10596 for why we use a scoped type variable here.
      -- ToDo: convert @undefined :: a@ to @proxy :: Proxy a@ when
      -- we drop support for GHC 7.6.
      return (Map.lookup (typeOf (undefined :: a)) th_state >>= fromDynamic)
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  qPutQ x = do
      th_state_var <- fmap tcg_th_state getGblEnv
      updTcRef th_state_var (\m -> Map.insert (typeOf x) (toDyn x) m)
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{-
************************************************************************
*                                                                      *
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            Instance Testing
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*                                                                      *
************************************************************************
-}
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reifyInstances :: TH.Name -> [TH.Type] -> TcM [TH.Dec]
reifyInstances th_nm th_tys
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   = addErrCtxt (ptext (sLit "In the argument of reifyInstances:")
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                 <+> ppr_th th_nm <+> sep (map ppr_th th_tys)) $
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     do { loc <- getSrcSpanM
        ; rdr_ty <- cvt loc (mkThAppTs (TH.ConT th_nm) th_tys)
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          -- #9262 says to bring vars into scope, like in HsForAllTy case
          -- of rnHsTyKi
        ; let (kvs, tvs) = extractHsTyRdrTyVars rdr_ty
              tv_bndrs   = userHsTyVarBndrs loc tvs
              hs_tvbs    = mkHsQTvs tv_bndrs
          -- Rename  to HsType Name
        ; ((rn_tvbs, rn_ty), _fvs)
            <- bindHsTyVars doc Nothing kvs hs_tvbs $ \ rn_tvbs ->
               do { (rn_ty, fvs) <- rnLHsType doc rdr_ty
                  ; return ((rn_tvbs, rn_ty), fvs) }
        ; (ty, _kind) <- tcHsTyVarBndrs rn_tvbs $ \ _tvs ->
                         tcLHsType rn_ty
        ; ty <- zonkTcTypeToType emptyZonkEnv ty
                -- Substitute out the meta type variables
                -- In particular, the type might have kind
                -- variables inside it (Trac #7477)
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        ; traceTc "reifyInstances" (ppr ty $$ ppr (typeKind ty))
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        ; case splitTyConApp_maybe ty of   -- This expands any type synonyms
            Just (tc, tys)                 -- See Trac #7910
               | Just cls <- tyConClass_maybe tc
               -> do { inst_envs <- tcGetInstEnvs
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                     ; let (matches, unifies, _) = lookupInstEnv False inst_envs cls tys
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                     ; traceTc "reifyInstances1" (ppr matches)
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                     ; reifyClassInstances cls (map fst matches ++ unifies) }
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               | isOpenFamilyTyCon tc
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               -> do { inst_envs <- tcGetFamInstEnvs
                     ; let matches = lookupFamInstEnv inst_envs tc tys
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                     ; traceTc "reifyInstances2" (ppr matches)
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                     ; reifyFamilyInstances tc (map fim_instance matches) }
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            _  -> bale_out (hang (ptext (sLit "reifyInstances:") <+> quotes (ppr ty))
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                               2 (ptext (sLit "is not a class constraint or type family application"))) }
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  where
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    doc = ClassInstanceCtx
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    bale_out msg = failWithTc msg

    cvt :: SrcSpan -> TH.Type -> TcM (LHsType RdrName)
    cvt loc th_ty = case convertToHsType loc th_ty of
                      Left msg -> failWithTc msg
                      Right ty -> return ty
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{-
************************************************************************
*                                                                      *
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                        Reification
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*                                                                      *
************************************************************************
-}
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lookupName :: Bool      -- True  <=> type namespace
                        -- False <=> value namespace
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           -> String -> TcM (Maybe TH.Name)
lookupName is_type_name s
  = do { lcl_env <- getLocalRdrEnv
       ; case lookupLocalRdrEnv lcl_env rdr_name of
           Just n  -> return (Just (reifyName n))
           Nothing -> do { mb_nm <- lookupGlobalOccRn_maybe rdr_name
                         ; return (fmap reifyName mb_nm) } }
  where
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    th_name = TH.mkName s       -- Parses M.x into a base of 'x' and a module of 'M'
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    occ_fs :: FastString
    occ_fs = mkFastString (TH.nameBase th_name)

    occ :: OccName
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    occ | is_type_name
        = if isLexCon occ_fs then mkTcOccFS    occ_fs
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                             else mkTyVarOccFS occ_fs
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        | otherwise
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        = if isLexCon occ_fs then mkDataOccFS occ_fs
                             else mkVarOccFS  occ_fs

    rdr_name = case TH.nameModule th_name of
                 Nothing  -> mkRdrUnqual occ
                 Just mod -> mkRdrQual (mkModuleName mod) occ

getThing :: TH.Name -> TcM TcTyThing
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getThing th_name
  = do  { name <- lookupThName th_name
        ; traceIf (text "reify" <+> text (show th_name) <+> brackets (ppr_ns th_name) <+> ppr name)
        ; tcLookupTh name }
        -- ToDo: this tcLookup could fail, which would give a
        --       rather unhelpful error message
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  where
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    ppr_ns (TH.Name _ (TH.NameG TH.DataName  _pkg _mod)) = text "data"
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    ppr_ns (TH.Name _ (TH.NameG TH.TcClsName _pkg _mod)) = text "tc"
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    ppr_ns (TH.Name _ (TH.NameG TH.VarName   _pkg _mod)) = text "var"
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    ppr_ns _ = panic "reify/ppr_ns"
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reify :: TH.Name -> TcM TH.Info
reify th_name
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  = do  { traceTc "reify 1" (text (TH.showName th_name))
        ; thing <- getThing th_name
        ; traceTc "reify 2" (ppr thing)
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        ; reifyThing thing }
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lookupThName :: TH.Name -> TcM Name
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lookupThName th_name = do
    mb_name <- lookupThName_maybe th_name
    case mb_name of
        Nothing   -> failWithTc (notInScope th_name)
        Just name -> return name

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lookupThName_maybe :: TH.Name -> TcM (Maybe Name)
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lookupThName_maybe th_name
  =  do { names <- mapMaybeM lookup (thRdrNameGuesses th_name)
          -- Pick the first that works
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          -- E.g. reify (mkName "A") will pick the class A in preference to the data constructor A
        ; return (listToMaybe names) }
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  where
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    lookup rdr_name
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        = do {  -- Repeat much of lookupOccRn, becase we want
                -- to report errors in a TH-relevant way
             ; rdr_env <- getLocalRdrEnv
             ; case lookupLocalRdrEnv rdr_env rdr_name of
                 Just name -> return (Just name)
                 Nothing   -> lookupGlobalOccRn_maybe rdr_name }
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tcLookupTh :: Name -> TcM TcTyThing
-- This is a specialised version of TcEnv.tcLookup; specialised mainly in that
-- it gives a reify-related error message on failure, whereas in the normal
-- tcLookup, failure is a bug.
tcLookupTh name
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  = do  { (gbl_env, lcl_env) <- getEnvs
        ; case lookupNameEnv (tcl_env lcl_env) name of {
                Just thing -> return thing;
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                Nothing    ->

          case lookupNameEnv (tcg_type_env gbl_env) name of {
                Just thing -> return (AGlobal thing);
                Nothing    ->

          if nameIsLocalOrFrom (tcg_mod gbl_env) name
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          then  -- It's defined in this module
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                failWithTc (notInEnv name)
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          else
     do { mb_thing <- tcLookupImported_maybe name
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        ; case mb_thing of
            Succeeded thing -> return (AGlobal thing)
            Failed msg      -> failWithTc msg
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    }}}}
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notInScope :: TH.Name -> SDoc
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notInScope th_name = quotes (text (TH.pprint th_name)) <+>
                     ptext (sLit "is not in scope at a reify")
        -- Ugh! Rather an indirect way to display the name
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notInEnv :: Name -> SDoc
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notInEnv name = quotes (ppr name) <+>
                     ptext (sLit "is not in the type environment at a reify")
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------------------------------
reifyRoles :: TH.Name -> TcM [TH.Role]
reifyRoles th_name
  = do { thing <- getThing th_name
       ; case thing of
           AGlobal (ATyCon tc) -> return (map reify_role (tyConRoles tc))
           _ -> failWithTc (ptext (sLit "No roles associated with") <+> (ppr thing))
       }
  where
    reify_role Nominal          = TH.NominalR
    reify_role Representational = TH.RepresentationalR
    reify_role Phantom          = TH.PhantomR

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------------------------------
reifyThing :: TcTyThing -> TcM TH.Info
-- The only reason this is monadic is for error reporting,
-- which in turn is mainly for the case when TH can't express
-- some random GHC extension

reifyThing (AGlobal (AnId id))
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  = do  { ty <- reifyType (idType id)
        ; let v = reifyName id
        ; case idDetails id of
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            ClassOpId cls -> return (TH.ClassOpI v ty (reifyName cls))
            _             -> return (TH.VarI     v ty Nothing)
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    }