[GHC] #16232: Add setField to HasField

[GHC]

#16232: Add setField to HasField
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        Reporter:  adamgundry        |                Owner:  adamgundry
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler (Type    |              Version:  8.7
  checker)                           |
      Resolution:                    |             Keywords:  ORF
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by adamgundry):

 * owner:  (none) => adamgundry


Comment:

 > I suggest taking the direct path, and generate an update function for
 each field.

 Sorry, to be clear, do you mean the approach of generating renamed syntax
 at definition sites?


 > Its more work than Matthew says because we may need to evaluate the
 argument (for strict constructors) or unbox it (if the argument is
 UNPACKed).

 Exactly, perhaps my example didn't do a good job of conveying the awkward
 points here. Let me see if I understand correctly what generating Core
 directly needs to do in the general case:
 * When we pattern match in Core, we match on the *worker* (binding its
 existential tyvars, theta and the arguments as given by `dataConInstSig`).
 * We can use `dataConBoxer` to build let-bindings for the arguments to the
 wrapper out of the bound variables from matching on the worker.
 * We need to construct an application of the *wrapper* to appropriate
 boxed arguments, replacing the one that corresponds to the field being
 updated with the new value. I don't understand very clearly how to do
 this. In particular, I'm not sure how to figure out the order in which to
 instantiate type variables and dictionaries in the wrapper's type.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16232#comment:3>
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