CODEWITHSUNDEEP
haskelllazy-evaluation
Chul-Woong Yang
Chul-Woong Yang

Reputation: 1243

seq to force evaluation

To run in constant space, mean2 uses sumlen2 which seqs summation and count. But again my belief and textbook, mean2 still runs with space leak. What's the mistake?

Any helps will be appreciated deeply.

-- Thinking Functionally with Haskell by R. Bird
-- Chapter 7.2 Controlling space

sumlen1 = foldl' g (0,0)
  where g (s,n) x = (s+x,n+1)

sumlen2 = foldl' g (0,0)
  where g (s,n) x = s `seq` n `seq` (s+x,n+1)

-- has space leak
mean1 [] = 0
mean1 xs = s / fromIntegral n
  where (s,n) = sumlen1 xs        

-- should run in constant space    
mean2 [] = 0
mean2 xs = s / fromIntegral n
  where (s,n) = sumlen2 xs        

λ> mean1 [1..1000000]
500000.5
(1.99 secs, 520,962,648 bytes)
λ> mean2 [1..1000000]
500000.5
(1.36 secs, 516,288,128 bytes)

Upvotes: 2

Views: 96

Answers (1)

Chul-Woong Yang
Chul-Woong Yang

Reputation: 1243

As @Clinton commented out, the performance metrics from ghci is the total bytes allocation. sumlen2 runs with constant space.

Upvotes: 1

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