Hi,
this is true. The optimization only works with -O2. I'd like to have
more details about what's going on. How can I make sure, that this
optimization triggers?
Heinrich
On 18.02.2012 11:10, MigMit wrote:
Different kinds of optimization. I expect you'd have different results even if
you use one type, but different -O flags.
On 18 Feb 2012, at 13:28, Heinrich Hördegen wrote:
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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