Effective Haskell: Chapter 10 (pages 379-383) - code does not compile

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1

Under “Building a Basic Metrics System with IORefs”, have created a sample file with the code snippets from page 379-383 as follows:

module Metrics where

import qualified Data.Map.Strict as Map
import Data.IORef
import Data.Maybe (fromMaybe)
import Data.Time.Clock
( diffUTCTime
, getCurrentTime
, nominalDiffTimeToSeconds
)

data AppMetrics = AppMetrics
{ successCount :: Int
, failureCount :: Int
, callDuration :: Map.Map String Int
} deriving Show

metrics :: IO (IORef AppMetrics)
metrics = newIORef AppMetrics
{ successCount = 0
, failureCount = 0
, callDuration = Map.empty
}

printMetrics :: IO ()
printMetrics = metrics >>= readIORef >>= print

incrementSuccess :: IO ()
incrementSuccess =
metrics >>= flip modifyIORef incrementSuccess
where
incrementSuccess m =
m { successCount = 1 + successCount m}

successfullyPrintHello :: IO ()
successfullyPrintHello = do
print “Hello”
incrementSuccess

printHelloAndMetrics = do
successfullyPrintHello
printMetrics

newtype Metrics = Metrics { appMetricsStore :: IORef AppMetrics}

newMetrics :: IO Metrics
newMetrics =
let
emptyAppMetrics = AppMetrics
{ successCount = 0
, failureCount = 0
, callDuration = Map.empty
}
in Metrics <$> newIORef emptyAppMetrics

tickSuccess :: Metrics → IO ()
tickSuccess (Metrics metricsRef) = modifyIORef metricsRef $ \m →
m { successCount = 1 + successCount m}

tickFailure :: Metrics → IO ()
tickFailure (Metrics metricsRef) = modifyIORef metricsRef $ \m →
m { failureCount = 1 + failureCount m}

timeFunction :: Metrics → String → IO a → IO a
timeFunction (Metrics metrics) actionName action = do
startTime ← getCurrentTime
result ← action
endTime ← getCurrentTime

modifyIORef metrics $ \oldMetrics -> 
    let
        oldDurationValue = 
            fromMaybe 0 $ Map.lookup actionName (callDuration oldMetrics)

        runDuration = 
            floor . nominalDiffTimeToSeconds $ 
               diffUTCTime endTime startTime
        
        newDurationValue = oldDurationValue + runDuration

    in oldMetrics {
        callDuration = 
            Map.insert actionName newDurationValue $
              callDuration oldMetrics}

    pure result

Code does not compile, throws this error:

The function ‘oldMetrics
{callDuration = Map.insert actionName newDurationValue
$ callDuration oldMetrics}’
is applied to two value arguments,
but its type ‘AppMetrics’ has none

Not able to figure out the problem.

2

Hi, sorry about the trouble here. The problem is that you need to un-indent the final pure result. Right now it’s indented to be part of the function being passed to modifyIORef, but it should be part of the outer do block. I think the page break in example makes it really hard to see the right level of indentation in this example. In the final version we’re trying to avoid dangling code lines like this as much as possible, so hopefully you won’t keep running into this kind of problem.

3

That did the trick. Amen to fixup on dangling code lines. Loving the book. Thx for speedy reply!

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