[GHC] #13856: "Zero-argument" lambda expressions from pretty-print strangely

21 Jun 2017

      #13856: "Zero-argument" lambda expressions from pretty-print strangely
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  Haskell                            |
           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:
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 An amusing corner case of the language is that you can constructor lambdas
 with zero arguments using Template Haskell:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}

 import Language.Haskell.TH

 f :: Int
 f = $(lamE [] [| 42 |])
 }}}

 But if you try to compile that with `-ddump-splices` on, it'll look quite
 funny:

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs -ddump-splices
 GHCi, version 8.2.0.20170616: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )
 Bug.hs:6:7-22: Splicing expression lamE [] [| 42 |] ======> \ -> 42
 Ok, modules loaded: Main.
 }}}

 Oh dear, `\ -> 42` isn't a valid expression at all. The same thing happens
 with the Template Haskell pretty-printer:

 {{{
 λ> import Language.Haskell.TH
 λ> :set -XTemplateHaskell
 λ> putStrLn $(lamE [] [| 42 |] >>= stringE . pprint)
 <interactive>:4:12-48: Splicing expression
     lamE [] [| 42 |] >>= stringE . pprint ======> "\ -> 42"
 \ -> 42
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13856
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