[GHC] #1200: ghci scripts ending in printf lines fail with Exception: Prelude.undefined

[GHC]

#1200: ghci scripts ending in printf lines fail with Exception: Prelude.undefined
------------------------+---------------------------------------------------
    Reporter:  dons     |       Owner:         
        Type:  bug      |      Status:  new    
    Priority:  normal   |   Milestone:         
   Component:  GHCi     |     Version:  6.6    
    Severity:  normal   |    Keywords:         
  Difficulty:  Unknown  |    Testcase:         
Architecture:  Unknown  |          Os:  Unknown
------------------------+---------------------------------------------------
There appears to be some differences in runhaskell/ghci and ghc when it
 comes to printf.

 Consider this program:
 {{{
 import Text.Printf
 import System.Environment

 main = do
     [who] <- getArgs
     printf "hello, %s\n" who
 }}}

 when compiled:

 {{{
 $ ghc A.hs
 $ ./A world
 hello, world
 }}}

 When run in GHci:

 {{{
 $ ghci A.hs
 Prelude Main> :set args world
 Prelude Main> main
 hello, world
 *** Exception: Prelude.undefined
 }}}

 Hmm! And in runhaskell:

 {{{
 $ runhaskell A.hs world
 hello, world
 *** Exception: Prelude.undefined
 }}}

 An ugly 'return ()' seems to help:

 {{{
 import Text.Printf
 import System.Environment

 main = do
     [who] <- getArgs
     printf "hello, %s\n" who
     return ()
 }}}

 which produces:

 {{{
 $ runhaskell A.hs world
 hello, world
 }}}

 As does an explicit annotation:

 {{{
 $ cat A.hs
 import Text.Printf
 import System.Environment

 main = do
     [who] <- getArgs
     printf "hello, %s\n" who :: IO ()
 }}}

 So some defaulting is coming into play?

 {{{
 $ ghci
 Prelude> :l A.hs
 *Main> :t main
 main :: IO t

 *Main> :set args world
 *Main> main :: IO ()
 hello, world

 *Main> main :: IO String
 hello, world
 "*** Exception: Prelude.undefined
 }}}

 Is GHCi/runhaskell giving an overly generous type to 'main'? I note the
 following
 is also valid "runhaskell" programs:

 {{{
 $ cat A.hs
 main = return "hello, world"

 $ runhaskell A.hs
 "hello, world"
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1200>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler