On Tue, Nov 23, 2010 at 6:58 PM, Adam Miezianko wrote:

I'm working through Learn You a Haskell for Great Good [1] and getting a compiler error while playing around with some of the code.  I have this:

-- BEGIN state.hs import Control.Monad.State

type Stack = [Int]

pop :: State Stack Int pop = State $ \(x:xs) -> (x,xs)

push :: Int -> State Stack () push x = State $ \xs -> ((), x:xs)

stackManip :: State Stack Int stackManip = do    push 3    pop    pop -- END state.hs

But when I try to load it into ghci I get the following errors:

 Prelude> :load "/home/admi/.pe/state.hs"  [1 of 1] Compiling Main             ( /home/admi/.pe/state.hs, interpreted )

 /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State'

 /home/admi/.pe/state.hs:9:9: Not in scope: data constructor `State'  Failed, modules loaded: none.  Prelude>

Now, I'm not exactly sure how to read the documentation for Control.Monad.State [2] but it seems that newtype State s a = State {...} defines a constructor, or am I wrong on that point too?  So, what am I missing here?  In case it matters, I am using mtl-2.0.1.0 and ghci 6.12.3.

[1] http://learnyouahaskell.com/for-a-few-monads-more [2] http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html

It looks like your documentation doesn't match the library you're using. The documentation for mtl-2.x is here: http://hackage.haskell.org/package/mtl Starting in version 2.0, the mtl package no longer exports a constructor called 'State' - but there is a function 'state' which servers the same purpose. However you might be better served using the 'modify' function. I hope that helps, Antoine

-- Adam Miezianko _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe