Lennart Augustsson wrote:
On Sep 7, 2006, at 08:50 , Brian Hulley wrote:
Deokhwan Kim wrote:
What is the practical meaning of monad laws?
Afaiu the monad laws are needed so the compiler can do various
optimizations, especially in regard to the "do" notation. Consider:
g c = do
if c
then p
else return ()
q
which can further be optimized to:
g c = if c then (p >>= (\_ -> q)) else q
Brian,
Are you really sure Haskell compilers do that optimization?
I would regard a compiler that does optimizations that are justified
by laws that the compiler cannot check as broken.
I think at least GHC does, if I understand the -ddump-simpl output below
properly:
-- in Monad.hs
module Main where
import Control.Monad
test :: Bool -> IO ()
test c = do
if c
then putStr "True"
else return ()
putStrLn "Finish"
main = test False
ghc --make -O2 -ddump-simpl monad
gives:
==================== Tidy Core ====================
Main.s :: GHC.Base.String
[GlobalId]
[Str: DmdType]
Main.s = GHC.Base.unpackCString# "Finish"
Main.main :: GHC.IOBase.IO ()
[GlobalId]
[Arity 1
Str: DmdType L]
Main.main = \ (eta_a26L :: GHC.Prim.State# GHC.Prim.RealWorld) ->
case (# GHC.Prim.State# GHC.Prim.RealWorld, () #) GHC.IO.hPutStr
GHC.Handle.stdout Main.s eta_a26L
of wild_a26O { (# new_s_a26M, a85_a26N #) ->
System.IO.lvl1 new_s_a26M
}
Main.s1 :: GHC.Base.String
[GlobalId]
[Str: DmdType]
Main.s1 = GHC.Base.unpackCString# "True"
Main.test :: GHC.Base.Bool -> GHC.IOBase.IO ()
[GlobalId]
[Arity 2
Str: DmdType SL]
Main.test = \ (c_a19p :: GHC.Base.Bool)
(eta_s27s :: GHC.Prim.State# GHC.Prim.RealWorld) ->
case (# GHC.Prim.State# GHC.Prim.RealWorld, () #) c_a19p of wild_B1 {
GHC.Base.False -> Main.main eta_s27s;
GHC.Base.True ->
case (# GHC.Prim.State# GHC.Prim.RealWorld, () #) GHC.IO.hPutStr
GHC.Handle.stdout Main.s1 eta_s27s
of wild1_a27o { (# new_s_a27m, a85_a27n #) ->
Main.main new_s_a27m
}
}
So when the condition in Main.test is False, the compiler immediately
executes Main.main eta_s27s which does the putStr "Finish" directly, so the
return () has been optimized out.
Whether this is because ghc has used the monad laws, or has applied some
different optimization due to it's knowledge of the built-in IO monad etc I
don't know.
Even if the compiler is not itself making use of the laws, other parts of
standard library code might be, and their correctness would therefore also
depend on something which the compiler can't verify.
It seems a pity that having gone to the trouble of ensuring a monad obeys
the laws, the compiler can't make use of them. What then *is* the point of
the monad laws?
Regards, Brian.
--
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.
http://www.metamilk.com
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