In case of +, the reason might be that it's cheap, but the function add
could do something else than + (It was just a small example).
Ok, thank you for your useful comments. I will read about cse.
Heinrich
On 18.02.2012 13:42, Victor Gorokgov wrote:
+ on Int is extremely cheap. It is always faster to add again rather
than store the value.
But Integer is a different story. Addition time on this type can grow
to several minutes.
18.02.2012 13:28, Heinrich Hördegen пишет:
Dear all,
I have a question about evaluation with respect to types and
currying. Consider this programm:
import Debug.Trace
-- add :: Integer -> Integer -> Integer
add :: Int -> Int -> Int
add x y = x + y
f a b c = trace "b" (add x c) where x = trace "a" (add a b)
main :: IO ()
main = do
print (f 1 2 3)
print (f 1 2 4)
Compiled with ghc-7.0.3:
$ ghc --make Main.hs -o main -O2
The function add has to types. When we use type Int -> Int -> Int,
the programm produces "b a 6 b a 7" as output which shows that the x
from the where clause in f is evaluated twice. However, when we use
type Integer -> Integer -> Integer, this will give "b a 6 b 7" which
shows that x is evaluated only once. This was rather unexpected to me.
Why does the number of evaluation steps depend on a type? Can anybody
explain this or give a hint?
Thank you very much,
Heinrich
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