[Haskell-cafe] [template haskell] Sharing names between constructors and types

Tom Smeding x at tomsmeding.com
Fri May 20 06:11:15 UTC 2022

A `NameG`, the constructor of `Name` that your name probably has, has a
`NameSpace` field that indicates in what namespace the name lives. See
the documentation:

If you have the name of a type (with namespace `TcClsName`) and want to
use it as a data constructor (which normally has namespace `DataName`),
you probably need to change the namespace.

- Tom

On 20/05/2022 00:11, Patrick L Redmond via Haskell-Cafe wrote:
> I worked around the issue. Apologies for the noise.
> I'm still learning, but I think the reason for this error is:
>    * The name passed to `voidType` contains module information, and
> possibly other information.
>    * By reusing the name in the output of `voidType` (as a type) we are
> incorrectly overriding the existing name (of a constructor), even
> though they're ostensibly different namespaces.
> A workaround is to perform `mkName $ nameBase n` inside `voidType`.
> On Thu, May 19, 2022 at 1:15 PM Patrick L Redmond <plredmond at ucsc.edu> wrote:
>> Hello Haskell Cafe,
>> When can a type not share the name of a constructor?
>> Defining a type and a constructor with the same name, manually, as
>> follows, seems to be acceptable:
>>      data F = F | T
>>      data T
>> However, if the type is generated by Template Haskell, it is no longer
>> acceptable, as follows:
>>      voidType :: Name -> DecsQ
>>      voidType n = do
>>          d <- dataD (return []) n [] Nothing [] []
>>          return [d]
>> In a separate module:
>>      data F = F | T
>>      $(voidType 'T)
>> This will result in a "Multiple declarations of ‘T’" error. Is this expected?
>> Thank you,
>> Patrick
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