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
bar
foo
foo
bar
where
-- 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'
-Udo
--
"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
either."
-- E. W. Dijkstra
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