import qualified Control.Exception as Exception( userErrors )
\end{code}
+Note [Template Haskell levels]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+* Imported things are impLevel (= 0)
+
+* In GHCi, variables bound by a previous command are treated
+ as impLevel, because we have bytecode for them.
+
+* Variables are bound at the "current level"
+
+* The current level starts off at topLevel (= 1)
+
+* The level is decremented by splicing $(..)
+ incremented by brackets [| |]
+ incremented by name-quoting 'f
+
+When a variable is used, we compare
+ bind: binding level, and
+ use: current level at usage site
+
+ Generally
+ bind > use Always error (bound later than used)
+ [| \x -> $(f x) |]
+
+ bind = use Always OK (bound same stage as used)
+ [| \x -> $(f [| x |]) |]
+
+ bind < use Inside brackets, it depends
+ Inside splice, OK
+ Inside neither, OK
+
+ For (bind < use) inside brackets, there are three cases:
+ - 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 |]
+
+See Note [What is a top-level Id?]
+
+Note [Quoting names]
+~~~~~~~~~~~~~~~~~~~~
+A quoted name 'n is a bit like a quoted expression [| n |], except that we
+have no cross-stage lifting (c.f. TcExpr.thBrackId). So, after incrementing
+the use-level to account for the brackets, the cases are:
+
+ bind > use Error
+ bind = use OK
+ bind < use
+ Imported things OK
+ Top-level things OK
+ Non-top-level Error
+
+See Note [What is a top-level Id?] in TcEnv. Examples:
+
+ f 'map -- OK; also for top-level defns of this module
+
+ \x. f 'x -- Not ok (whereas \x. f [| x |] might have been ok, by
+ -- cross-stage lifting
+
+ \y. [| \x. $(f 'y) |] -- Not ok (same reason)
+
+ [| \x. $(f 'x) |] -- OK
+
+
+Note [What is a top-level Id?]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In the level-control criteria above, we need to know what a "top level Id" is.
+There are three kinds:
+ * Imported from another module (GlobalId, ExternalName)
+ * Bound at the top level of this module (ExternalName)
+ * In GHCi, bound by a previous stmt (GlobalId)
+It's strange that there is no one criterion tht picks out all three, but that's
+how it is right now. (The obvious thing is to give an ExternalName to GHCi Ids
+bound in an earlier Stmt, but what module would you choose? See
+Note [Interactively-bound Ids in GHCi] in TcRnDriver.)
+
+The predicate we use is TcEnv.thTopLevelId.
+
%************************************************************************
%* *
getLIEVar `thenM` \ lie_var ->
setStage (Brack next_level pending_splices lie_var) (
- getLIE (tc_bracket brack)
+ getLIE (tc_bracket next_level brack)
) `thenM` \ (meta_ty, lie) ->
tcSimplifyBracket lie `thenM_`
returnM (noLoc (HsBracketOut brack pendings))
}
-tc_bracket :: HsBracket Name -> TcM TcType
-tc_bracket (VarBr name) -- Note [Quoting names]
+tc_bracket :: ThLevel -> HsBracket Name -> TcM TcType
+tc_bracket use_lvl (VarBr name) -- Note [Quoting names]
= do { thing <- tcLookup name
; case thing of
AGlobal _ -> return ()
- ATcId { tct_level = bind_lvl }
- | isExternalName name -- C.f isExternalName case of
- -> keepAliveTc name -- TcExpr.thBrackId
+ ATcId { tct_level = bind_lvl, tct_id = id }
+ | thTopLevelId id -- C.f thTopLevelId case of
+ -> keepAliveTc id -- TcExpr.thBrackId
| otherwise
- -> do { use_stage <- getStage
- ; checkTc (thLevel use_stage == bind_lvl)
+ -> do { checkTc (use_lvl == bind_lvl)
(quotedNameStageErr name) }
other -> pprPanic "th_bracket" (ppr name)
; tcMetaTy nameTyConName -- Result type is Var (not Q-monadic)
}
-tc_bracket (ExpBr expr)
+tc_bracket use_lvl (ExpBr expr)
= do { any_ty <- newFlexiTyVarTy liftedTypeKind
; tcMonoExpr expr any_ty
; tcMetaTy expQTyConName }
-- Result type is Expr (= Q Exp)
-tc_bracket (TypBr typ)
+tc_bracket use_lvl (TypBr typ)
= do { tcHsSigType ExprSigCtxt typ
; tcMetaTy typeQTyConName }
-- Result type is Type (= Q Typ)
-tc_bracket (DecBr decls)
+tc_bracket use_lvl (DecBr decls)
= do { tcTopSrcDecls emptyModDetails decls
-- Typecheck the declarations, dicarding the result
-- We'll get all that stuff later, when we splice it in
-- Result type is Q [Dec]
}
-tc_bracket (PatBr _)
+tc_bracket use_lvl (PatBr _)
= failWithTc (ptext SLIT("Tempate Haskell pattern brackets are not supported yet"))
quotedNameStageErr v
, ptext SLIT("must be used at the same stage at which is is bound")]
\end{code}
-Note [Quoting names]
-~~~~~~~~~~~~~~~~~~~~
-A quoted name is a bit like a quoted expression, except that we have no
-cross-stage lifting (c.f. TcExpr.thBrackId). Examples:
-
- f 'map -- OK; also for top-level defns of this module
-
- \x. f 'x -- Not ok (whereas \x. f [| x |] might have been ok, by
- -- cross-stage lifting
-
- \y. [| \x. $(f 'y) |] -- Not ok (same reason)
-
- [| \x. $(f 'x) |] -- OK
%************************************************************************
%* *
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