[Haskell-cafe] TemplateHaskell forces manual declaration ordering?

[edwardsbenj@gmail.com]

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 at 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.
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