Hi Michael, This is a known limitation. You can't refer to items in splices that haven't already been mentioned in the source. I don't know why it isn't (prominently) documented, it's definitely part of the folklore. Ben On Tue May 20 2014 at 11:43:25, Michael Alan Dorman < mdorman@ironicdesign.com> wrote:

Hey, all,

While refactoring some code, I moved a newtype declaration, and suddenly the compiler (up-to-date ghc-7.6.3 on Debian) started complaining that it wasn't in scope when I attempted to use it in an ADT.

I'm still new to Haskell, so I assumed it was my misunderstanding, and looked for documentation of declaration ordering constraints and couldn't find any. So then I went to make a toy case to demonstrate the problem, and it worked:

data Baz = Baz { foo :: Foo, bar :: !(Maybe Bar) } newtype Foo = Foo String newtype Bar = Bar String

So I started looking at the differences between that and my real code, and the only thing that seemed at all signficant was that I was using TH to generate some lenses. And sure enough, adding that caused it to fail:

{-# LANGUAGE TemplateHaskell #-} import Control.Lens data Baz = Baz { foo :: Foo, bar :: !(Maybe Bar) } makeLenses ''Baz newtype Foo = Foo String newtype Bar = Bar String

If you feed this to ghci, you now get:

test.hs:3:25: Not in scope: type constructor or class `Foo'

test.hs:3:45: Not in scope: type constructor or class `Bar' Perhaps you meant `Baz' (line 3)

Wondering if lens was somehow doing something so exotic it was breaking things, I tried aeson's TH support instead:

{-# LANGUAGE TemplateHaskell #-} import Data.Aeson data Baz = Baz { foo :: Foo, bar :: !(Maybe Bar) } deriveJSON defaultOptions ''Baz newtype Foo = Foo String newtype Bar = Bar String

Exact same error.

That said, if I move the splices to the end of the file, everything works:

{-# LANGUAGE TemplateHaskell #-} import Data.Aeson data Baz = Baz { foo :: Foo, bar :: !(Maybe Bar) } newtype Foo = Foo String newtype Bar = Bar String deriveJSON defaultOptions ''Baz

I looked at the GHC docs on TH, and read the wiki page, and didn't see anything suggesting that this was a known limitation, nor did I see a bug in the known bug list that seemed pertinent.

So, I'm wondering if I've found an actual bug, or a known limitation whose documentation should be made more prominent (since it seems to me that it has pretty dramatic implications for program structure), or what? And is there any way around it other than moving all the splices to the end of the source file?

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