[Haskell-cafe] State separation/combination pattern question

Udo Stenzel u.stenzel at web.de
Fri Dec 22 08:31:29 EST 2006

Reto Kramer wrote:
> What I'm really looking for is not so much the chaining of StateT  
> compositions, but rather the isolation of StateA from StateB while  
> they both flow from the search loop into the respective library calls  
> (foo, bar) transparently to the application programmer.

How about this?

-- these two should be defined in two separate library modules, of course
trueFoo :: MonadState StateA m => m ()
trueBar :: MonadState StateB m => m ()

data AppStateRec = AppStateRec { a :: StateA, b :: StateB }

type Eval a = StateT AppStateRec Identity a

exec :: Eval ()
exec = do foo
    -- you might want to define combinators for the following pattern,
    -- but for just two functions this is good enough
    foo = do AppStateRec a b <- get
             a' <- runStateT trueFoo a
	     put $ AppStateRec a' b
    bar = do AppStateRec a b <- get
             b' <- runStateT trueBar b
	     put $ AppStateRec a b'

"In the software business there are many enterprises for which it is not
clear that science can help them; that science should try is not clear
	-- E. W. Dijkstra
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