Why Integral makes a function much slower than Int?
I implemented a simple function to calculate the length of a list.
However, I noticed that the speed of the function changes significantly
if I make I minor change to the type signature but keep the
implementation the same otherwise. I was wondering why this happens.
Contents of mylength.hs:
module MyLength where
-- Here is the first implementation:
myLength :: Integral n => [a] -> n
myLength xs = run xs 0
run  n = n
run (_:ys) n = run ys (n+1)
-- Then, the other implementation just fixes the return type without
-- modifying the actual implementation:
myLength' :: [a] -> Int
myLength' = myLength
When I compile these implementations with:
ghc -dynamic -O2 mylength.hs
Then, launch ghci:
And in ghci:
(myLength [1..10000000]) :: Int
(2.30 secs, 1,612,737,944 bytes)
(myLength' [1..10000000]) :: Int
(0.38 secs, 720,077,536 bytes)
So, the first implementation is much worse, although I've fixed the
return type to Int too.
What's going on? Does this mean one shouldn't use Integral but Int
instead most of the time?