Re: [Haskell-beginners] Control.Monad.State: State CTOR unneeded to create a State?

1 Nov 2008

      On 10/30/08 21:20, Antoine Latter wrote:
...
On Thu, Oct 30, 2008 at 10:07 PM, Larry Evans 
 wrote:
...
Page:
http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona...
...
suggests, with the content:
Constructors
  State         runState :: (s -> (a, s))
that State has a labeled constructor:
http://www.haskell.org/onlinereport/exps.html#sect3.15.2
yet the Examples section:
http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona...
...
shows, AFAICT, no call to the State CTOR:
tick :: State Int Int
 tick = do n <- get
          put (n+1)
          return n
          I assume the n must be an Int (since it's in n+1), but
what are
...
the
get and put.  There is a get and put described here:
http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona...
...
however, I don't see how they are relevant since they are defined for
an already existing MonadState, and AFAICT, tick hasn't been defined
yet since it's on the lhs of the =.
Please, what am I missing?
Hi Larry.   Event thought 'get' and 'put' are defined in MonadState,
the concrete implementation of them for "State s a" uses the 'State'
constructor.  They probably look something like so:
get = State (\s -> (s,s))
put newS = State (\oldS ->( (), newS  ))
That still leaves me confused about:

  put (n+1)

AFAICT, with the preceding |n <- get|, this means:

  put (get+1)

and since the get has type, |State Int Int|, and there's
not + operator defined on that type (because the following:

  let get1 = get::(State Int Int)
  print (evalState get1 99)
  let get1p1 = get1+1
  putStrLn "get1p1="

compiles with error:

{---cut here---

GenMonads2Arrows.hs:40:15:
    No instance for (Num (State Int Int))
      arising from a use of `+' at GenMonads2Arrows.hs:40:15-20
    Possible fix: add an instance declaration for (Num (State Int Int))
    In the expression: get1 + 1
    In the definition of `get1p1': get1p1 = get1 + 1
    In the expression:
        do putStr "plusOneCtor="
           print (plusOneCtor int1)
           putStr "plusOne="
           print (plusOne int1)

}---cut here---

So, looking (again ;) at:

http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona...

shows:

  get ::m s

where, based on:

  Instances:
    MonadState s (State s)

from the Control-Monad_state.hmtl#4,   

  m is |State Int| and s is |Int|

however, that's just what was used above that wasn't compatible with
+1.

So, how can get be used in |get+1|?  Is there some way
to trace what's happening in the tick rhs?  Could
template haskell be any help here?  I can't use ghci
because I have to explicity type the get result, and
that leads to the above compile error.  I'm stuck!
Help!

[snip]
...
As long as you aren't writing anything that needs to dig to much into
the workings of 'State', you shouldn't have to worry too much about
it.
I'm trying to understand how to use MonadState. I would have
expected tick would have been defined:

  tick = State (\n -> (n+1,n+1))

instead of the more obscure:

  tick = do n <- get
            put (n+1)
            return n

I figured that understanding how the more obscure definition worked
might show me why it was used instead of the simpler definition.
However, as noted above, I can't figure out how the more obscure
defintion works and I don't even know if the two definitions comput
the same results.
...
I hope I haven't made things more confused ^_^
You've helped.  Thanks.

 -Larry