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))
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.
Cheers,
Fan
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