1 files changed, 58 insertions, 0 deletions
diff --git a/libraries/ghc-prim/GHC/Prim/Panic.hs b/libraries/ghc-prim/GHC/Prim/Panic.hs
index 21ebc56a88..0aa07813c1 100644
--- a/libraries/ghc-prim/GHC/Prim/Panic.hs
+++ b/libraries/ghc-prim/GHC/Prim/Panic.hs
@@ -9,6 +9,7 @@
 module GHC.Prim.Panic
    ( absentSumFieldError
    , panicError
+   , absentError
    )
 where
 
@@ -17,10 +18,56 @@ import GHC.Magic
 
 default () -- Double and Integer aren't available yet
 
+{-
+Note [Compiler error functions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Most error functions (such as pattern match failure) are defined
+in base:Control.Exception.Base.  But absentError# and absentSumFieldError#
+are defined here in the ghc-prim package for two reasons:
+
+* GHC itself generates calls to these functions as a result of
+  strictness analysis, over which the programmer has no control. So
+  it is hard to ensure that no such calls exist in the modules
+  compiled "before" Control.Base.Exception.  (E.g. when compiling
+  with -fdicts-strict.)
+
+* A consequence of defining them in ghc-prim is that the libraries
+  defining exceptions have not yet been built, so we can't make them
+  into proper Haskell exceptions.
+
+  However, if these functions are ever called, it's a /compiler/ error,
+  not a user error, so it seems acceptable that they cannot be caught.
+
+One might wonder why absentError doesn't just call panic#.
+For absent error we want to combine two parts, one static, one call site
+dependent into one error message. While for absentSumFieldError it's a
+static string.
+
+The easiest way to combine the two parts for absentError is to use a
+format string with `barf` in the RTS passing the *dynamic* part of the
+error as argument. There is no need to do any of this for
+absentSumFieldError as it's a static string there.
+
+The alternatives would be to:
+* Drop the call site specific information from absentError.
+  The call site specific information is at times very helpful for debugging
+  so I don't consider this an option.
+* Remove the common prefix. We would then need to include the prefix
+  in the call site specific string we pass to absentError. Increasing
+  code size for no good reason.
+
+Both of which seem worse than having an stg_absentError function specific to
+absentError to me.
+-}
+
 -- `stg_panic#` never returns but it can't just return `State# RealWorld` so we
 -- indicate that it returns `(# #)` too to make the compiler happy.
+-- See Note [Compiler error functions]
 foreign import prim "stg_paniczh" panic# :: Addr# -> State# RealWorld -> (# State# RealWorld, (# #) #)
 
+-- See Note [Compiler error functions]
+foreign import prim "stg_absentErrorzh" stg_absentError# :: Addr# -> State# RealWorld -> (# State# RealWorld, (# #) #)
+
 -- | Display the CString whose address is given as an argument and exit.
 panicError :: Addr# -> a
 panicError errmsg =
@@ -43,3 +90,14 @@ absentSumFieldError = panicError "entered absent sum field!"#
 -- introduced in Stg.Unarise, long after inlining has stopped, but it seems
 -- more direct simply to give it a NOINLINE pragma
 {-# NOINLINE absentSumFieldError #-}
+
+-- | Displays "Oops!  Entered absent arg" ++ errormsg and exits the program.
+{-# NOINLINE absentError #-}
+absentError :: Addr# -> a
+absentError errmsg =
+  runRW# (\s ->
+    case stg_absentError# errmsg s of
+      (# _, _ #) -> -- This bottom is unreachable but we can't
+                    -- use an empty case lest the pattern match
+                    -- checker squawks.
+                    let x = x in x)