[GHC] #16049: Add a warning flag that warns when a datatype could be a newtype

[GHC]

#16049: Add a warning flag that warns when a datatype could be a newtype
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        Reporter:  chessai           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:  flags
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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Description changed by chessai:

Old description:

> More often than not, `newtype` is desirable over `data` if a type has a
> single field. I propose a warning flag `-Wdata-could-be-newtype` that
> would warn users if a data type they've defined has a single field (and
> thus could be a `newtype`). It would need to be explicitly enabled (and
> of course enabled by `-Weverything`). An example (relevant Trac ticket:
> #15995):
>
> {{{#!hs
> -- Control.Concurrent.QSem:
>
> data QSem = QSem !(MVar (Int, [MVar ()], [MVar ()]))
>
> }}}
>
> There's a bang pattern on its single field, but this could just be
> supplied at every pattern-matching site of `QSem`.

New description:

 More often than not, `newtype` is desirable over `data` if a type has a
 single field. I propose a warning flag `-Wdata-could-be-newtype` that
 would warn users if a data type they've defined has a single field (and
 thus could be a `newtype`). It would need to be explicitly enabled (and of
 course enabled by `-Weverything`). An example (relevant Trac ticket:
 #15995):

 {{{#!hs
 -- Control.Concurrent.QSem:

 data QSem = QSem !(MVar (Int, [MVar ()], [MVar ()]))

 }}}

 could be a newtype

 {{{#!hs
 -- Control.Concurrent.QSem:

 newtype QSem = QSem (MVar (Int, [MVar ()], [MVar ()]))
 }}}

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16049#comment:1>
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