On Tue, Jan 10, 2006 at 04:44:33PM +0000, Ian Lynagh wrote:
>
> readChunks :: FirstMonad String
> readChunks = do xs <- get
> if null xs then return []
> else do let (ys, zs) = foo xs
> put zs
> rest <- readChunks
> return (ys ++ rest)
It looks like changing this let to a case fixes this example, but at the
time I'd experimented with that there must have been other issues
clouding the effect, such as the following.
Foo1 (attached) uses large amounts of memory whereas Foo2 (also
attached) runs in a little constant space. The difference is only
changing this:
else do chunk <- case foo xs of
(ys, zs) ->
do put zs
return ys
chunks <- readChunks
return (chunk ++ chunks)
to this:
else case foo xs of
(ys, zs) ->
do put zs
chunks <- readChunks
return (ys ++ chunks)
but I don't have a good feeling for why this should be the case given
I'd expect chunk to be forced, and thus the case evaluated, at the same
point in Foo1 as the case is evaluated in Foo2.
Is this just a case of GHC's optimiser's behaviour depending on subtle
source changes, or am I missing something?
Thanks
Ian
module Main (main) where
import Control.Monad (liftM)
import Control.Monad.State (State, runState, evalState, get, put)
main :: IO ()
main = do xs <- readFile "data"
ys <- readFile "data"
print (evalState readChunks xs == ys)
---
type FirstMonad = State String
readChunks :: FirstMonad String
readChunks = do xs <- get
if null xs then return []
else do chunk <- case foo xs of
(ys, zs) ->
do put zs
return ys
chunks <- readChunks
return (chunk ++ chunks)
---
type SecondMonad = State String
foo :: String -> (String, String)
foo = runState bar
bar :: SecondMonad String
bar = do inp <- get
case inp of
[] -> return []
x:xs -> do put xs
liftM (x:) bar
module Main (main) where
import Control.Monad (liftM)
import Control.Monad.State (State, runState, evalState, get, put)
main :: IO ()
main = do xs <- readFile "data"
ys <- readFile "data"
print (evalState readChunks xs == ys)
---
type FirstMonad = State String
readChunks :: FirstMonad String
readChunks = do xs <- get
if null xs then return []
else case foo xs of
(ys, zs) ->
do put zs
chunks <- readChunks
return (ys ++ chunks)
---
type SecondMonad = State String
foo :: String -> (String, String)
foo = runState bar
bar :: SecondMonad String
bar = do inp <- get
case inp of
[] -> return []
x:xs -> do put xs
liftM (x:) bar
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