[Haskell-cafe] questions on lazy pattern, StateT monad

Wolfgang Jeltsch wolfgang at jeltsch.net
Thu Nov 24 14:28:54 EST 2005

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,

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