On 12-02-14 03:01 PM, JP Moresmau wrote:
I'm confused: I'm using GHC 7.0.2 and Cabal 1.10.1.0 with
cabal-install 0.10.2. I use -Wall in my Cabal file. If I build a
Haskell file with unused do binds, via the GHC API I get no warning,
which is normal, since the doc states: The warnings that are not
enabled by -Wall are ..., -fwarn-unused-do-bind .... But if I build my
project through cabal it gives me the warning! Why is compiling with
Cabal giving that extra warning that GHC on its own with the same
flags doesn't give? Using --verbose on cabal build does not give any
clue, no suspicious extra flag is passed on.
The plot thickens as I perform some experiments.
Experiment #1:
main :: IO ()
main = do { x <- getLine; putStrLn "thank you" }
ghc -Wall =>
Warning: Defined but not used: `x'
ghc -fwarn-unused-do-bind =>
(no warning)
Apparently, warn-unused-do-bind does not mean that x is unused.
Experiment #2:
main :: IO ()
main = do { getLine; putStrLn "thank you" }
ghc -fwarn-unused-do-bind =>
Warning: A do-notation statement discarded a result of type String.
Suppress this warning by saying "_ <- getLine",
or by using the flag -fno-warn-unused-do-bind
ghc -Wall =>
Warning: A do-notation statement discarded a result of type String.
Suppress this warning by saying "_ <- getLine",
or by using the flag -fno-warn-unused-do-bind
ghc -Wall -fno-warn-unused-do-bind =>
(no warning)
Apparently, -Wall turns on -fwarn-unused-do-bind, despite the user guide.
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