Re: [GHC] #11351: Scoped type variables in pattern synonyms

6 Jan 2016

      #11351: Scoped type variables in pattern synonyms
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

Old description:
...
Unsure of pattern synonym scoping rules.
I want to be able to refer to a type in the signature (assuming `symbol`
from #11349):
{{{#!hs
symbol :: forall s. KnownSymbol s => String
symbol = symbolVal @s Proxy
-- Not in scope: type variable ‘s’
-- Not in scope: type variable ‘s’
pattern Symbol :: forall s. KnownSymbol s => String
pattern Symbol <- ((== symbol @s) -> True) where
         Symbol = symbol @s
}}}
Without `TypeApplications`:
{{{#!hs
-- • Could not deduce (KnownSymbol n0)
--     arising from a use of ‘symbolVal’
--   from the context: KnownSymbol s
--     bound by the type signature for pattern synonym ‘Symbol’:
--                String
pattern Symbol :: forall s. KnownSymbol s => String
pattern Symbol <- ((== symbolVal (Proxy :: Proxy s)) -> True)
}}}
but it (GHCi, version 8.1.20160102) says the type variable `s` is not in
scope. The desired outcome would be something like (this touches on
ticket #11350):
{{{#!hs
...
...
...
Symbol @"blessed duck"
"blessed duck"
isDuck :: String -> Bool
isDuck (Symbol @"duck") = True
isDuck _                = False
}}}
With a nudge and a wink to #9671.
New description:

 Unsure of pattern synonym scoping rules.
 I want to be able to refer to a type in the signature (assuming `symbol`
 from #11349):

 {{{#!hs
 symbol :: forall s. KnownSymbol s => String
 symbol = symbolVal @s Proxy

 -- Not in scope: type variable ‘s’
 -- Not in scope: type variable ‘s’
 pattern Symbol :: forall s. KnownSymbol s => String
 pattern Symbol <- ((== symbol @s) -> True) where
          Symbol = symbol @s
 }}}

 Without `TypeApplications`:

 {{{#!hs
 -- • Could not deduce (KnownSymbol n0)
 --     arising from a use of ‘symbolVal’
 --   from the context: KnownSymbol s
 --     bound by the type signature for pattern synonym ‘Symbol’:
 --                String
 pattern Symbol :: forall s. KnownSymbol s => String
 pattern Symbol <- ((== symbolVal (Proxy :: Proxy s)) -> True)
 }}}

 but it (GHCi, version 8.1.20160102) says the type variable `s` is not in
 scope. Is this intentional?

--

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11351#comment:1
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