[Haskell-cafe] I don't understand how ST works

[Nicu Ionita]

Ok, the error was: I was using Control.Monad.ST.Lazy. Importing 
Control.Monad.ST compiles immediately without problem. (Is this because 
I'm using unboxed mutable vectors?)

Now, that's a little bit odd.

It's clear that the strict and lazy forms of ST are different types. But 
unfortunately they are named the same! So actually any error message 
from the compiler drives you crazy, because it's refering to another type.

Probably the reason to name the types with the same name is for easy 
interchangeability. But as we see, the types are not (always) 
interchangeable.

Anyway, now it compiles.

Thanks,
Nicu

Am 08.06.2012 23:15, schrieb Nicu Ionita:
> Hi,
>
> I created a gist with a minimal (still 111 lines) module:
>
> https://gist.github.com/2898128
>
> I still get the errors:
>
> WhatsWrong.hs:53:5:
>     Couldn't match type `s' with `PrimState (ST s)'
>       `s' is a rigid type variable bound by
>           a type expected by the context: ST s [Move] at 
> WhatsWrong.hs:48:21
>     In a stmt of a 'do' block: listMoves ml
>     In the second argument of `($)', namely
>       `do { v <- U.new maxMovesPerPos;
>             let ml = ...;
>             listMoves ml }'
>     In the expression:
>       runST
>       $ do { v <- U.new maxMovesPerPos;
>              let ml = ...;
>              listMoves ml }
>
> WhatsWrong.hs:65:44:
>     Couldn't match type `s' with `PrimState (ST s)'
>       `s' is a rigid type variable bound by
>           the type signature for nextPhaseOnlyCapts :: GenPhase s
>           at WhatsWrong.hs:64:1
>     Expected type: U.MVector (PrimState (ST s)) Move
>       Actual type: U.MVector s Move
>     In the return type of a call of `mlVec'
>     In the third argument of `genCapts', namely `(mlVec ml)'
>
> Thanks,
> Nicu
>
> Am 08.06.2012 02:47, schrieb Silvio Frischknecht:
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>>
>>> Now comes my question: in the impure values there is always that
>>> "s". I was thinking that the whole structure should have s as a
>>> parameter:
>> Yes
>>
>>>> data MList s = MList { mlVec :: MVector s Move, mlNextPh :: MList
>>>> ->
>>> ST s (Maybe (MList s)) }
>> you probably meant:
>>
>>      data MList s = MList { ... , mlNextPh :: Mlist s ->  ... }
>>
>> Now I'm not sure about your exact problem since the following compiles
>> for me.
>>
>>      import Data.Vector
>>      import Data.Vector.Mutable
>>      import Control.Monad.ST
>>
>>      type Move = ()
>>      data MList s = MList {
>>          mvVec :: MVector s Move,
>>          mlNextPh :: MList s ->  ST s (Maybe (MList s)) }
>>
>>      splitMove :: MList s ->  ST s (Maybe (Move, MList s))
>>      splitMove ml = do
>>          m<- unsafeRead (mvVec ml) 0
>>          undefined
>>
>> Something you always have to watch out for when dealing with ST is not
>> to return something that depends on s in the last statement (the one
>> you use runST on). In other words, if you want to return a vector you
>> have to freeze it, so it's not mutable anymore.
>>
>> If you still can't figure it out paste some complete example that
>> doesn't work.
>>
>> silvio
>> -----BEGIN PGP SIGNATURE-----
>> Version: GnuPG v1.4.11 (GNU/Linux)
>> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/
>>
>> iQIcBAEBAgAGBQJP0Uu5AAoJEDLsP+zrbatWKFoP+wYdmAwO3aKPIibOydDwPlcu
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>> Q7aAmMed/DBlY/Anh2xY
>> =X9CL
>> -----END PGP SIGNATURE-----
>
>
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