diff options
| -rw-r--r-- | compiler/stranal/WwLib.lhs | 20 |
1 files changed, 20 insertions, 0 deletions
diff --git a/compiler/stranal/WwLib.lhs b/compiler/stranal/WwLib.lhs index fc94c9b921..4acf2557db 100644 --- a/compiler/stranal/WwLib.lhs +++ b/compiler/stranal/WwLib.lhs @@ -540,6 +540,7 @@ mkWWcpr body_ty res Just con_tag | Just stuff <- deepSplitCprType_maybe con_tag body_ty -> mkWWcpr_help stuff | otherwise + -- See Note [non-algebraic or open body type warning] -> WARN( True, text "mkWWcpr: non-algebraic or open body type" <+> ppr body_ty ) return (id, id, body_ty) @@ -590,6 +591,25 @@ mkUnpackCase scrut co uniq boxing_con unpk_args body bndr = mk_ww_local uniq (exprType casted_scrut) \end{code} +Note [non-algebraic or open body type warning] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +There are a few cases where the W/W transformation is told that something +returns a constructor, but the type at hand doesn't really match this. One +real-world example involves unsafeCoerce: + foo = IO a + foo = unsafeCoere c_exit + foreign import ccall "c_exit" c_exit :: IO () +Here CPR will tell you that `foo` returns a () constructor for sure, but trying +to create a worker/wrapper for type `a` obviously fails. +(This was a real example until ee8e792 in libraries/base.) + +It does not seem feasilbe to avoid all such cases already in the analyser (and +after all, the analysis is not really wrong), so we simply do nothing here in +mkWWcpr. But we still want to emit warning with -DDEBUG, to hopefully catch +other cases where something went avoidably wrong. + + Note [Profiling and unpacking] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If the original function looked like |