I meant that 'pop' and 'push' should have been written with 'modify', 'get', and 'set' instead of the raw constructor, not as a drop-in replacement. I can show you examples later if this isn't clear, unless I'm not understanding your code above. Antoine On Nov 24, 2010 10:46 AM, "Adam Miezianko" wrote:

On Wed, 24 Nov 2010 06:17:47 -0800, Antoine Latter wrote:

...

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:

pop :: State Stack Int

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

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

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

Thanks!

My documentation didn't have any version identification, so discovering that there was a breaking interface change is no surprise now. Next time hoogle doesn't return me what I want I'll first suspect it's been removed before questioning the accuracy of the search results.

On a somewhat related note, I also found the link you provide, but I found

hackage.haskell.org to be intermittently timing out for me this last week.

...

However you might be better served using the 'modify' function.

Looking at the type signature of modify, I'm unsure how to use it. It does not look like I can just drop it in as a replacement for State (StateT, state). Could someone provide an example? The tutorials seem to mostly use the State constructor that's now gone.

-- Adam Miezianko