[GHC] #12360: Extend support for binding implicit parameters

2 Jul 2016

      #12360: Extend support for binding implicit parameters
-------------------------------------+-------------------------------------
           Reporter:  MichaelBurge   |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Implicit parameters can only be bound using simple let and where
 declarations. In this example, one would expect ?t to be bound to the
 result of getCurrentTime, however the program is rejected with a syntax
 error:

 {{{
 {-# LANGUAGE ImplicitParams #-}

 import Data.Time.Clock

 printTime :: (?t :: UTCTime) => IO ()
 printTime = putStrLn $ show ?t

 main = do
   ?t <- getCurrentTime
   printTime
 }}}

 Instead, one must first bind to a regular variable and then bind the
 implicit parameter to it:

 {{{
 {-# LANGUAGE ImplicitParams #-}

 import Data.Time.Clock

 printTime :: (?t :: UTCTime) => IO ()
 printTime = putStrLn $ show ?t

 main = do
   t <- getCurrentTime
   let ?t = t
   printTime
 }}}

 In general, it seems like any pattern binding involving implicit
 parameters could be rewritten as a pattern binding to ordinary variables,
 followed by a binding of the implicit parameters to the ordinary
 variables. So you could bind implicit parameters buried in variables,
 tuples, record fields, etc.:

 Sugared:
 {{{
 {-# LANGUAGE ImplicitParams #-}

 data Example = Example { a :: Int, b :: Int, c :: Int }

 f :: (?a :: Int, ?b :: Int, ?c :: Int) => Int
 f = ?a + ?b + ?c

 main = do
   let x = Example 1 2 3
   let (Example { a = ?a, b = ?b, c = ?c }) = x
   let y = f
   putStrLn $ show y
 }}}

 Desugared:
 {{{
 {-# LANGUAGE ImplicitParams #-}

 data Example = Example { a :: Int, b :: Int, c :: Int }

 f :: (?a :: Int, ?b :: Int, ?c :: Int) => Int
 f = ?a + ?b + ?c

 main = do
   let x = Example 1 2 3
   let (Example { a = a, b = b, c = c }) = x
   let ?a = a
       ?b = b
       ?c = c
   let y = f
   putStrLn $ show y
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12360
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler