+\end{code}
+
+
+%************************************************************************
+%* *
+ Error expressions
+%* *
+%************************************************************************
+
+\begin{code}
+mkRuntimeErrorApp
+ :: Id -- Should be of type (forall a. Addr# -> a)
+ -- where Addr# points to a UTF8 encoded string
+ -> Type -- The type to instantiate 'a'
+ -> String -- The string to print
+ -> CoreExpr
+
+mkRuntimeErrorApp err_id res_ty err_msg
+ = mkApps (Var err_id) [Type res_ty, err_string]
+ where
+ err_string = Lit (mkMachString err_msg)
+
+mkImpossibleExpr :: Type -> CoreExpr
+mkImpossibleExpr res_ty
+ = mkRuntimeErrorApp rUNTIME_ERROR_ID res_ty "Impossible case alternative"
+\end{code}
+
+%************************************************************************
+%* *
+ Error Ids
+%* *
+%************************************************************************
+
+GHC randomly injects these into the code.
+
+@patError@ is just a version of @error@ for pattern-matching
+failures. It knows various ``codes'' which expand to longer
+strings---this saves space!
+
+@absentErr@ is a thing we put in for ``absent'' arguments. They jolly
+well shouldn't be yanked on, but if one is, then you will get a
+friendly message from @absentErr@ (rather than a totally random
+crash).
+
+@parError@ is a special version of @error@ which the compiler does
+not know to be a bottoming Id. It is used in the @_par_@ and @_seq_@
+templates, but we don't ever expect to generate code for it.
+
+\begin{code}
+errorIds :: [Id]
+errorIds
+ = [ eRROR_ID, -- This one isn't used anywhere else in the compiler
+ -- But we still need it in wiredInIds so that when GHC
+ -- compiles a program that mentions 'error' we don't
+ -- import its type from the interface file; we just get
+ -- the Id defined here. Which has an 'open-tyvar' type.
+
+ rUNTIME_ERROR_ID,
+ iRREFUT_PAT_ERROR_ID,
+ nON_EXHAUSTIVE_GUARDS_ERROR_ID,
+ nO_METHOD_BINDING_ERROR_ID,
+ pAT_ERROR_ID,
+ rEC_CON_ERROR_ID,
+ rEC_SEL_ERROR_ID,
+ aBSENT_ERROR_ID ]
+
+recSelErrorName, runtimeErrorName, absentErrorName :: Name
+irrefutPatErrorName, recConErrorName, patErrorName :: Name
+nonExhaustiveGuardsErrorName, noMethodBindingErrorName :: Name
+
+recSelErrorName = err_nm "recSelError" recSelErrorIdKey rEC_SEL_ERROR_ID
+absentErrorName = err_nm "absentError" absentErrorIdKey aBSENT_ERROR_ID
+runtimeErrorName = err_nm "runtimeError" runtimeErrorIdKey rUNTIME_ERROR_ID
+irrefutPatErrorName = err_nm "irrefutPatError" irrefutPatErrorIdKey iRREFUT_PAT_ERROR_ID
+recConErrorName = err_nm "recConError" recConErrorIdKey rEC_CON_ERROR_ID
+patErrorName = err_nm "patError" patErrorIdKey pAT_ERROR_ID
+
+noMethodBindingErrorName = err_nm "noMethodBindingError"
+ noMethodBindingErrorIdKey nO_METHOD_BINDING_ERROR_ID
+nonExhaustiveGuardsErrorName = err_nm "nonExhaustiveGuardsError"
+ nonExhaustiveGuardsErrorIdKey nON_EXHAUSTIVE_GUARDS_ERROR_ID
+
+err_nm :: String -> Unique -> Id -> Name
+err_nm str uniq id = mkWiredInIdName cONTROL_EXCEPTION_BASE (fsLit str) uniq id
+
+rEC_SEL_ERROR_ID, rUNTIME_ERROR_ID, iRREFUT_PAT_ERROR_ID, rEC_CON_ERROR_ID :: Id
+pAT_ERROR_ID, nO_METHOD_BINDING_ERROR_ID, nON_EXHAUSTIVE_GUARDS_ERROR_ID :: Id
+rEC_SEL_ERROR_ID = mkRuntimeErrorId recSelErrorName
+rUNTIME_ERROR_ID = mkRuntimeErrorId runtimeErrorName
+iRREFUT_PAT_ERROR_ID = mkRuntimeErrorId irrefutPatErrorName
+rEC_CON_ERROR_ID = mkRuntimeErrorId recConErrorName
+pAT_ERROR_ID = mkRuntimeErrorId patErrorName
+nO_METHOD_BINDING_ERROR_ID = mkRuntimeErrorId noMethodBindingErrorName
+nON_EXHAUSTIVE_GUARDS_ERROR_ID = mkRuntimeErrorId nonExhaustiveGuardsErrorName
+
+aBSENT_ERROR_ID :: Id
+-- Not bottoming; no unfolding! See Note [Absent error Id] in WwLib
+aBSENT_ERROR_ID = mkVanillaGlobal absentErrorName runtimeErrorTy
+
+mkRuntimeErrorId :: Name -> Id
+mkRuntimeErrorId name = pc_bottoming_Id name runtimeErrorTy
+
+runtimeErrorTy :: Type
+-- The runtime error Ids take a UTF8-encoded string as argument
+runtimeErrorTy = mkSigmaTy [openAlphaTyVar] [] (mkFunTy addrPrimTy openAlphaTy)
+\end{code}
+
+\begin{code}
+errorName :: Name
+errorName = mkWiredInIdName gHC_ERR (fsLit "error") errorIdKey eRROR_ID
+
+eRROR_ID :: Id
+eRROR_ID = pc_bottoming_Id errorName errorTy
+
+errorTy :: Type
+errorTy = mkSigmaTy [openAlphaTyVar] [] (mkFunTys [mkListTy charTy] openAlphaTy)
+ -- Notice the openAlphaTyVar. It says that "error" can be applied
+ -- to unboxed as well as boxed types. This is OK because it never
+ -- returns, so the return type is irrelevant.
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Utilities}
+%* *
+%************************************************************************
+
+\begin{code}
+pc_bottoming_Id :: Name -> Type -> Id
+-- Function of arity 1, which diverges after being given one argument
+pc_bottoming_Id name ty
+ = mkVanillaGlobalWithInfo name ty bottoming_info
+ where
+ bottoming_info = vanillaIdInfo `setStrictnessInfo` Just strict_sig
+ `setArityInfo` 1
+ -- Make arity and strictness agree
+
+ -- Do *not* mark them as NoCafRefs, because they can indeed have
+ -- CAF refs. For example, pAT_ERROR_ID calls GHC.Err.untangle,
+ -- which has some CAFs
+ -- In due course we may arrange that these error-y things are
+ -- regarded by the GC as permanently live, in which case we
+ -- can give them NoCaf info. As it is, any function that calls
+ -- any pc_bottoming_Id will itself have CafRefs, which bloats
+ -- SRTs.
+
+ strict_sig = mkStrictSig (mkTopDmdType [evalDmd] BotRes)
+ -- These "bottom" out, no matter what their arguments
+\end{code}
+
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