[Git][ghc/ghc][master] Comments only: document newtypes' DataConWrapId

[Marge Bot]

 Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
0345b1b0 by Richard Eisenberg at 2019-06-11T03:52:10Z
Comments only: document newtypes' DataConWrapId

- - - - -


1 changed file:

- compiler/basicTypes/MkId.hs


Changes:

=====================================
compiler/basicTypes/MkId.hs
=====================================
@@ -298,6 +298,24 @@ so the data constructor for T:C had a single argument, namely the
 predicate (C a).  But now we treat that as an ordinary argument, not
 part of the theta-type, so all is well.
 
+Note [Newtype workers]
+~~~~~~~~~~~~~~~~~~~~~~
+A newtype does not really have a worker. Instead, newtype constructors
+just unfold into a cast. But we need *something* for, say, MkAge to refer
+to. So, we do this:
+
+* The Id used as the newtype worker will have a compulsory unfolding to
+  a cast. See Note [Compulsory newtype unfolding]
+
+* This Id is labeled as a DataConWrapId. We don't want to use a DataConWorkId,
+  as those have special treatment in the back end.
+
+* There is no top-level binding, because the compulsory unfolding
+  means that it will be inlined (to a cast) at every call site.
+
+We probably should have a NewtypeWorkId, but these Ids disappear as soon as
+we desugar anyway, so it seems a step too far.
+
 Note [Compulsory newtype unfolding]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Newtype wrappers, just like workers, have compulsory unfoldings.
@@ -447,6 +465,8 @@ mkDataConWorkId :: Name -> DataCon -> Id
 mkDataConWorkId wkr_name data_con
   | isNewTyCon tycon
   = mkGlobalId (DataConWrapId data_con) wkr_name wkr_ty nt_work_info
+      -- See Note [Newtype workers]
+
   | otherwise
   = mkGlobalId (DataConWorkId data_con) wkr_name wkr_ty alg_wkr_info
 



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/commit/0345b1b0f62c8fac72d07a7b848d14b9893e9ac9

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/commit/0345b1b0f62c8fac72d07a7b848d14b9893e9ac9
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