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.