Remove the type signature for b and you will see the
loss of sharing.
It mostly hurts people like John Hughes that don't
have the energy to put in type signatures. ;)
On the subject of type signatures, I don't want to
make them mandatory, but I think they should be strongly
encouraged. I don't buy the argument that they make
refactoring programs that much harder. It's still
very easy to do, the type checker will tell you exactly
where. :)
-- Lennart
Cale Gibbard wrote:
On 28/01/06, Taral <[EMAIL PROTECTED]> wrote:
On 1/28/06, Cale Gibbard <[EMAIL PROTECTED]> wrote:
Do you have an example of such a program handy?
b = (x, x) where { x :: Num a => a; x = fromInteger 1 }
fromInteger is called twice.
--- mr.hs ---
{-# OPTIONS_GHC -fno-monomorphism-restriction #-}
import Debug.Trace
b :: Num a => (a,a)
b = (x,x)
where x :: Num a => a
x = (trace "x" . fromInteger) 1
main = print b
------------
[EMAIL PROTECTED] ghci -fno-monomorphism-restriction mr.hs
Loading package base-1.0 ... linking ... done.
Compiling Main ( mr.hs, interpreted )
Ok, modules loaded: Main.
*Main> :t b
b :: (Num a) => (a, a)
*Main> b
(x
1,1)
------------
[EMAIL PROTECTED] ghc -fno-monomorphism-restriction -o mr mr.hs && ./mr
x
(1,1)
-----------
If x isn't being shared, then Debug.Trace at least seems incapable of
resolving that fact.
Let's try profiling:
--- mr.hs, revised -----
{-# OPTIONS_GHC -fno-monomorphism-restriction #-}
b :: Num a => (a,a)
b = (x,x)
where x :: Num a => a
x = {-# SCC "x" #-} fromInteger 1
main = print b
--------------
[EMAIL PROTECTED] ghc -fno-monomorphism-restriction -prof -auto-all -o
mr mr.hs && ./mr +RTS -p
(1,1)
[EMAIL PROTECTED] cat mr.prof
Sat Jan 28 18:21 2006 Time and Allocation Profiling Report (Final)
mr +RTS -p -RTS
total time = 0.00 secs (0 ticks @ 20 ms)
total alloc = 17,972 bytes (excludes profiling overheads)
COST CENTRE MODULE %time %alloc
CAF GHC.Handle 0.0 48.2
CAF System.IO 0.0 1.4
CAF Main 0.0 50.3
individual inherited
COST CENTRE MODULE
no. entries %time %alloc %time %alloc
MAIN MAIN
1 0 0.0 0.0 0.0 100.0
CAF Main
150 6 0.0 50.3 0.0 50.5
b Main
157 1 0.0 0.1 0.0 0.2
x Main
158 1 0.0 0.1 0.0 0.1
main Main
156 1 0.0 0.0 0.0 0.0
CAF System.IO
105 1 0.0 1.4 0.0 1.4
CAF GHC.Handle
103 3 0.0 48.2 0.0 48.2
-------
One entry to x. So where is this lack of sharing I keep hearing about?
Even if I repeat these tests with -fno-cse, the results are the same.
- Cale
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