Re: [GHC] #10572: Template Haskell does not implicitly quantify types

26 Jun 2015

      #10572: Template Haskell does not implicitly quantify types
-------------------------------------+-------------------------------------
        Reporter:  spinda            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Template Haskell  |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
Description changed by spinda:

Old description:
...
Consider the simple quasiquoter producing a type variable:
{{{
tv :: QuasiQuoter
tv = QuasiQuoter { quoteType = varT . mkName }
}}}
If a type variable is introduced elsewhere in a type signature, it is
implicitly quantified as normal and the TH name resolves properly:
{{{
id' :: a -> [tv|a|]
id' x = x
}}}
But if the type variable is not mentioned elsewhere in the type, an error
results:
{{{
const' :: a -> [tv|b|] -> a
const' x _ = x
}}}
{{{
Not in scope: type variable ‘b’
}}}
One would expect the type variable introduced by the TH splice to be
included in the implicit quantification of the type signature.
In fact, there is no way to introduce a new type variable in this
context, apart from using {{{Rank(2/N)Types}}} with an explicit
{{{forall}}}. But this prevents the type variable from being referred to
elsewhere in the type, eg:
{{{
const'' :: [tv|a|] -> b -> [tv|a|]
const'' x _ = x
}}}
New description:

 Consider this simple quasiquoter producing a type variable:

 {{{
 tv :: QuasiQuoter
 tv = QuasiQuoter { quoteType = varT . mkName }
 }}}

 If a type variable is introduced elsewhere in a type signature, it is
 implicitly quantified as normal and the TH name resolves properly:

 {{{
 id' :: a -> [tv|a|]
 id' x = x
 }}}

 But if the type variable is not mentioned elsewhere in the type, an error
 results:

 {{{
 const' :: a -> [tv|b|] -> a
 const' x _ = x
 }}}

 {{{
 Not in scope: type variable ‘b’
 }}}

 One would expect the type variable introduced by the TH splice to be
 included in the implicit quantification of the type signature.

 In fact, there is no way to introduce a new type variable in this context,
 apart from using {{{Rank(2/N)Types}}} with an explicit {{{forall}}}. But
 this prevents the type variable from being referred to elsewhere in the
 type, eg:

 {{{
 const'' :: [tv|a|] -> b -> [tv|a|]
 const'' x _ = x
 }}}

--

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10572#comment:1
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