Am Donnerstag, 24. November 2005 19:24 schrieb Fan Wu:
Hi Wolfgang,
I don't know the history so maybe this is a new implementation of State transformer. The Peek and poke functions are defined below (copied from StateT.hs):
instance Monad m => StateM (StateT s m) s where peek = S (\s -> return (s,s)) poke s = S (\s1 -> return (s1,s))
This is obviously a newer implementation. peek and poke are obviously what get and put were in the old one.
The question is why can't the mplus be written as simple as what Andrew suggested:
mplus m1 m2 = S (\s -> let m1' = runState s m1 m2' = runState s m2 in ~(a, s') <- mplus m1' m2' return (a, s'))
this is easier to understand. I don't see what's the purpose of the peek, lift, poke, return in the new implementattion.
[There is a "do" missing before the lazy pattern, isn't it?] Maybe the point is that Andrew's implementation needs access to the internal structure of a state transformer while the solution with peek, poke etc. doesn't need this kind of access. So the non-Andrew solution can be considered more elegant because it doesn't depend on the implementation of state transformers, can be put into a different module and shows clearly that mplus is not yet another primitive StateT operation but can be defined in terms of primitives.
Cheers, Fan
Best wishes, Wolfgang