#5499: Tagging constructors with record/product phantom type
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Reporter: basvandijk | Owner:
Type: feature request | Status: new
Priority: normal | Component: libraries (other)
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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As mentioned in [http://www.haskell.org/pipermail/glasgow-haskell-
users/2011-September/020953.html this discussion] on the `glasgow-haskell-
users` mailinglist. I would like to propose making the following changes
to [http://www.haskell.org/ghc/docs/latest/html/libraries/ghc-prim-0.2.0.0
/GHC-Generics.html GHC.Generics]:
== Proposal ==
* Add a phantom type to the
[http://www.haskell.org/ghc/docs/latest/html/libraries/ghc-prim-0.2.0.0
/GHC-Generics.html#t:C C] type which specifies whether it's a record or a
normal product using the empty datatypes:
{{{
data Record
data Product
}}}
* Optionally add the type synonyms:
{{{
type R1 = M1 (C Record)
type P1 = M1 (C Product)
}}}
* Optionally remove the `conIsRecord` method of the `Constructor` type
class.
Of course GHC has to be modified too, so that the generated `C`s are
properly tagged.
== Motiviation ==
Having the information, whether a constructor is a record or not,
statically available instead of only dynamically, has the following
advantages:
* More efficient: programs can make a static instead of a dynamic choice.
(To be fair, the case branches on `conIsRecord` should in most cases be
optimized away)
* No unnecessary constraints for constructor instances:
See
[https://github.com/basvandijk/aeson/blob/5bdf30ed4b32f2cc169f6e4e58d1688bcb02c197/Data/Aeson/Types/Internal.hs#L925
the code] that triggered this proposal:
{{{
instance (Constructor c, GRecordToObject a, GToJSON a) => GToJSON (C1 c a)
where
gToJSON m1@(M1 x)
| conIsRecord m1 = Object $ gRecordToObject x
| otherwise = gToJSON x
}}}
Note the `GRecordToObject` constraint. This constraint is only really
necessary in case of a record. Down the line this extra constraint
requires
[https://github.com/basvandijk/aeson/blob/5bdf30ed4b32f2cc169f6e4e58d1688bcb02c197/Data/Aeson/Types/Internal.hs#L988
the following] ugly undefined instances:
{{{
instance GRecordToObject (a :+: b) where gRecordToObject = undefined
instance GRecordToObject U1 where gRecordToObject = undefined
instance GRecordToObject (K1 i c) where gRecordToObject = undefined
instance GRecordToObject (M1 i c f) where gRecordToObject = undefined
}}}
== Process ==
Since `GHC.Generics` is part of the `ghc-prim` package I don't think this
proposal has to go through the whole library submission process. But if it
must I will have no problem supervising it.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5499>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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