Re: [GHC] #13876: Check 'pure' method of 'Applicative (WrappedMonad m)'

27 Jun 2017

      #13876: Check 'pure' method of 'Applicative (WrappedMonad m)'
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 To explain: this is not about defining `pure` / `return` differently but
 rather defining `pure @V3` in terms of `pure @(WrappedMonad V3)` to make
 use of the methods of `Monad` to define the methods of `Applicative`

 {{{#!hs
   pure @V3 @a
 = coerce (pure @(WrappedMonad V3) @a)
 = coerce (WrapMonad . pure @V3 @a)
 = pure @V3 @a
 }}}

 With my suggestion this works

 {{{#!hs
   pure @V3 @a
 = coerce (pure @(WrappedMonad V3) @a)
 = coerce (WrapMonad . return @V3 @a)
 = return @V3 @a
 }}}

 If MRP goes through then I would need a `newtype` wrapped with a `Pointed`
 & `Monad` constraint to define `Applicative`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13876#comment:4
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