GHC inlines past lambda in do-notation

Hi,

a friend of mine discovered this weird behaviour:

The program, when run on the attached file, takes quite some time.

import System.IO
import Control.Monad
import qualified Data.Map as M

main :: IO ()
main = do
    n <- read `fmap` getLine
    q <- read `fmap` getLine
    
    mts <- replicateM n $ do
        [a,b] <- words `fmap` getLine
        return (a,b)
    
    let mtdb2 = M.fromList mts
    
    --M.null mtdb2 `seq` return ()

    replicateM_ q $ do
        a <- getLine
        print $ M.lookup a mtdb2
    return ()

But if I uncomment the use of mtdb2 outside the replicate, it runs fast. My conclusion is that GHC inlines mtdb2 into the body of the loop, repeating the construction of the map over and over.

If forM [1..q] $ \_ -> do is used instead, this does not happen – probably because GHC does not inline past that lambda.

(I didn’t attempt to understand the core yet)

Trac metadata

Trac field	Value
Version	7.8.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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