Why does GHC infer type of `IO b` for an application of `hSetBuffering`?
문제
I am trying to use hSetBuffering
in a Haskell program using GHC 6.10.
When I try this very simple program:
module Ctlc
where
import IO
main :: ()
main = do hSetBuffering stdout NoBuffering
return ()
I get a baffling error message:
ctlc.hs:8:10:
Couldn't match expected type `()' against inferred type `IO b'
In a stmt of a 'do' expression: hSetBuffering stdout NoBuffering
In the expression:
do hSetBuffering stdout NoBuffering
return ()
In the definition of `main':
main = do hSetBuffering stdout NoBuffering
return ()
I don't get why GHC is inferring a type of IO b
, since ghci claims
Prelude Data.List IO> :t hSetBuffering
hSetBuffering :: Handle -> BufferMode -> IO ()
ANSWER: I stupidly put the wrong type on main
. Thanks ja for sharp eyes.
해결책
You've declared main to be of type (), not IO ().
다른 팁
That function has to have some IO type, because the implementation is going to do a system call.
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