[GHC] #7851: Give better diagnostic when arguments are omitted to a function call in do-notation

20 Apr 2013

      #7851: Give better diagnostic when arguments are omitted to a function call in do-
notation
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Reporter:  JohnWiegley                        |          Owner:                   
    Type:  feature request                    |         Status:  new              
Priority:  normal                             |      Component:  Compiler (Parser)
 Version:  7.4.2                              |       Keywords:                   
      Os:  MacOS X                            |   Architecture:  x86_64 (amd64)   
 Failure:  Incorrect warning at compile-time  |      Blockedby:                   
Blocking:                                     |        Related:                   
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 When using any Monad other than (->) e, it is almost always an error for a
 function call to yield another function, rather than a monadic value.  For
 example:

     import Control.Monad.Trans.State

     main = flip runStateT 10 $ do
         print
         print "Hello"

 The error you get from this code is (with GHC 7.4.2):

     Couldn't match expected type `StateT b0 m0 a0'
                     with actual type `a1 -> IO ()'

 While this is fully correct, I think the compiler could do much better.
 The fact that the naked call to "print" doesn't return an IO value, but
 rather a function type, could be immediately detected as an error,
 allowing GHC to say something like:

     Detected function type (a -> IO ()) returned when IO () was expected,
 perhaps missing an argument in call to "print"?

 My example with StateT, IO and print is rather trivial, but when you start
 getting into nested monad transformers, in polymorphic functions where the
 transformer and underlying monad types are parameterized, the errors from
 GHC become downright mysterious.  Only through experience have I become
 able to read them as saying "You've forgotten an argument in your call to
 f".

 Since I can't think of a case (outside the function monad), where
 statements within a do-notation block should yield a function type rather
 than a monadic value appropriate to that Monad, perhaps we could do better
 here in guiding the user to the real source of the problem.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7851
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