newtype workers

[Simon Peyton Jones]

|  I'm working on unlifted newtypes.
|  
|  In MkId.mkDataConWorkId, I see
|  
|  > mkDataConWorkId wkr_name data_con
|  >   | isNewTyCon tycon
|  >   = mkGlobalId (DataConWrapId data_con) wkr_name wkr_ty nt_work_info
|  >   | otherwise
|  >   = mkGlobalId (DataConWorkId data_con) wkr_name wkr_ty alg_wkr_info
|  
|  Why is a newtype worker called a wrapper (DataConWrapId)? Note that some
|  newtype constructors have separate wrappers (which is relatively new).
|  This doesn't appear to be hurting anything right now, but it's odd, and
|  there's no commentary on this.

That is indeed odd, I agree.

Some points

* Newtypes can have wrappers: see Note [Data con wrappers and GADT syntax]
  in MkId.hs

* The "worker" for a newtype data constructor is just a function
  with a compulsory unfolding, of form  (\x. x |> co), where co
  is the newtype coercion.


* I agree that it's not right to describe the "worker" for a newtype
  data constructor as a "DataConWrapId"; after all, it may have
  a wrapper too (see previous bullet).

* Bu not is it rigt to call it a regular DataConWorkId. Those are 
  *data* constructors, head normal forms, very very special.

The kosher thing would be to create a NewtypeWorkId.  But that's seems a terrible fiddle, because these things are inlined compulsorily so they never exist for long.

Maybe it should just be a vanilla Id.  While there are a few places where
we do test for data-con-wrap-ids -- eg in isImplicitId -- but remember that these things are inlined so aggressively that we will only see them very briefly.

Or leave it as-is, with the above commentary as a Note -- that might be the easiest thing!   Might you make a little patch?

Simon 

|  -----Original Message-----
|  From: Richard Eisenberg <rae at cs.brynmawr.edu>
|  Sent: 25 May 2019 11:50
|  To: Simon Peyton Jones <simonpj at microsoft.com>
|  Subject: newtype workers
|  
|  Hi Simon,
|  
|  I'm working on unlifted newtypes.
|  
|  In MkId.mkDataConWorkId, I see
|  
|  > mkDataConWorkId wkr_name data_con
|  >   | isNewTyCon tycon
|  >   = mkGlobalId (DataConWrapId data_con) wkr_name wkr_ty nt_work_info
|  >   | otherwise
|  >   = mkGlobalId (DataConWorkId data_con) wkr_name wkr_ty alg_wkr_info
|  
|  Why is a newtype worker called a wrapper (DataConWrapId)? Note that some
|  newtype constructors have separate wrappers (which is relatively new).
|  This doesn't appear to be hurting anything right now, but it's odd, and
|  there's no commentary on this.
|  
|  Thanks,
|  Richard