#1127: Make mtl lazier
-------------------------------+--------------------------------------------
Reporter: igloo | Owner:
Type: proposal | Status: closed
Priority: normal | Milestone: Not GHC
Component: libraries (other) | Version: 6.6
Severity: normal | Resolution: fixed
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
-------------------------------+--------------------------------------------
Changes (by igloo):
* resolution: => fixed
* status: new => closed
Comment:
While this started off as just making StateT lazy, it has ultimately ended
up as a much larger rejig of the mtl library. The patch description is:
{{{
Sat Mar 3 15:22:21 GMT 2007 Ian Lynagh <[EMAIL PROTECTED]>
* Rejig mtl; trac proposal #1127
Moved the MonadFoo classes to Control.Monad.Foo.Class.
Put the instances consistently in the module which defines the type
they
give an instance for.
Made the existing transformers lazy to match the existing monads, and
put
them in Control.Monad.Foo.Lazy. Also added Control.Monad.Foo.Strict
with
strict monads and transformers. Control.Monad.Foo still exports what
it
used to.
Created a MonadRWS class.
Made the MonadWriter w (ErrorT e m) instance strict to match
everything
else.
}}}
Neil Mitchell advocated State and State' rather than State.Lazy and
State.Strict but there seemed to be a concensus for the latter (also used
by some modules in the base package).
Don Stewart asked for an example showing the different behaviours, to
which I replied:
{{{
The attached without the ~ gives:
$ time ./p 1000000
Stack space overflow: current size 8388608 bytes.
Use `+RTS -Ksize' to increase it.
./p 1000000 1.96s user 0.05s system 96% cpu 2.095 total
whereas with the ~ you can pump up the argument as high as you like
without any runtime or stack use worries:
$ time ./p 1000000
'a'
./p 1000000 0.00s user 0.00s system 93% cpu 0.004 total
$ time ./p 1000000000
'a'
./p 1000000000 0.00s user 0.00s system 101% cpu 0.004 total
For a non-contrived example see the compression package in hackage.
}}}
with this program attached:
{{{
import Control.Monad.Identity
import Control.Monad.State
import System.Environment
type M = StateT Char Identity
main = do [x] <- getArgs
print $ head $ runIdentity $ evalStateT (f $ read x) 'a'
f :: Int -> M [Char]
f 0 = return []
f (n+1) = do st <- get
rest <- f n
return (st:rest)
}}}
Should you wish to see the whole thread, it begins with
http://www.haskell.org/pipermail/libraries/2007-January/006805.html
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1127>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
