[GHC] #10027: Importing constructor of associated data type fails

26 Jan 2015

      #10027: Importing constructor of associated data type fails
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              Reporter:  lspitzner   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  Unknown/Multiple
  constructor import associated      |   Type of failure:  None/Unknown
  data type                          |        Blocked By:
          Architecture:              |   Related Tickets:
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 I will base my description on the code you find when searching for
 "instance PersistEntity" on http://www.yesodweb.com/book/persistent. In
 summary, the code looks like

 {{{#!hs
 instance PersistEntity Person where
 ...
     data EntityField Person typ where
         PersonId   :: EntityField Person PersonId
         PersonName :: EntityField Person String
         PersonAge  :: EntityField Person Int
 ...
 }}}

 This code is generated from TH (see above). I have not tested if it
 matters that it is generated. A first step could be to test if manually
 replacing the TH part yields different results.

 The problem is: When i add this code in one module, do ''not'' create an
 export list for that module, and try to import the constructors explicitly
 (i.e. with a list) in a different module, it does not work.

 I read and tried to follow the description on
 https://wiki.haskell.org/GHC/Type_families, chapter 4.3.

 == Testcase 1 ==

 (not really expected to work; still intersting error message)

 {{{#!hs
 import MyModuleContainingInstancePerson ( PersonId )
 }}}

 ghc says

 {{{
       ‘PersonId’ is a data constructor of ‘EntityField’
     To import it use
       ‘import’ MyModuleContainingInstancePerson( EntityField(
 VersionVersion ) )
     or
       ‘import’ MyModuleContainingInstancePerson( EntityField(..) )
 }}}

 == Testcase 2 ==

 (no export list in {{{MyModuleContainingInstancePerson}}})

 {{{#!hs
 import MyModuleContainingInstancePerson ( EntityField(..) )
 }}}

 ghc says

 {{{
 Module ‘MyModuleContainingInstancePerson’ does not export
 ‘EntityField(..)’
 }}}

 == Testcase 3 ==

 But ''when i do provide'' an export list in
 {{{MyModuleContainingInstancePerson}}}, like
 {{{#!hs
 module MyModuleContainingInstancePerson ( Person(..), EntityField(..) )
 where ..
 }}}
 Testcase 2 suddenly works.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10027
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler