Re: [Haskell-cafe] Confused about missing data constructor

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:

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

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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.

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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