typing query

[Iavor S. Diatchki]

hi,

the read only "state" monad is usually called "environment", or "reader" 
monad.  since it is goind to be "read-only" there is no need to return a 
new state in the result (as it would presumably be the same as the input 
state).  so the type becomes:

newtype Env e a = E (e -> a)

for something simillar to what you have take a look at
the Hugs/GHC monad transformer libraries, or at 
www.cse.ogi.edu/~diatchki/MonadTransformers

bye
iavor


Amit Garg wrote:
> Hey all.
> 
> I am trying to declare a read-only state monad and a read-write state
> monad, so as to distinguish between methods on a data type that are
> read-only vs. read-write.
> 
> This is the best I could come up with:
> 
> newtype ST s a = ST ( s -> (s,a) ) -- read-only
> newtype SW s a = SW ( s -> (s,a) ) -- read-write
> 
> class ReadM m s a where
>     readM :: m s s
>     runM :: s -> m s a -> (a,s)
> 
> class WriteM m r s where
>     updateM :: (s -> s) -> m s ()
> 
> instance ReadM ST s a where
>     readM = ST (\s -> (s,s))
>     runM s (ST c) = c s -- Doesn't work
> 
> instance ReadM SW s a where
>     readM = SW (\s -> (s,s))
>     runM s (SW c) = c s -- Doesn't work
> 
> And later on ...
>     updateM s (SW c) = c s
> 
> Does that make sense? If not, how do I do it? If so, is there a
> simpler means of getting there? Thanks.
> 
> ;Amit
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-- 
==================================================
| Iavor S. Diatchki, Ph.D. student               |
| Department of Computer Science and Engineering |
| School of OGI at OHSU                          |
| http://www.cse.ogi.edu/~diatchki               |
==================================================