Re: [GHC] #14076: cons operator for list give weird output

2 Aug 2017

      #14076: cons operator for list give weird output
-------------------------------------+-------------------------------------
        Reporter:  AshHash           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.2.1
      Resolution:                    |             Keywords:  cons(:)
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 This is expected, someone could define a `Num` instance for lists in a
 separate module

 {{{#!hs
 instance Num a => Num [a] where
   fromInteger :: Integer -> [a]
   fromInteger i = [fromInteger]

   (+) :: [a] -> [a] -> [a]
   (+) = liftA2 (+)

   -- ..
 }}}

 at which point your expression reduces to `[1,2]`

 {{{
...
...
:t 1 : 2
 1 : 2 :: Num a => [a]
1 : 2
 [1,2]
 }}}
-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14076#comment:3
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler