Comments on: Sad about Import Cycles …

By: chris k

chris k — Tue, 06 Jan 2009 16:39:58 +0000

The haskell-src-exts package can now parse and produce the {-# SOURCE #-} pragmas and I am using it to generate Haskell code for a different project.

By: alson

alson — Mon, 05 Jan 2009 19:13:48 +0000

maltem,

"hack-ish" is just a grumpy way to say "work-around-ish" and .hs-boot is definitely "work-around-ish". I'd love GHC to support Haskell98 in this area.

By: maltem

maltem — Mon, 05 Jan 2009 18:53:14 +0000

hs-boot files are just a way of doing some work that should actually be done by the compiler. It’s awfully tedious, but it isn’t hackish.

By: alson

alson — Mon, 05 Jan 2009 06:57:30 +0000

dmwit,

I think that the .hs-boot thing has been supported for a while (also, I referenced that link at the top of my post). I've never been able to get the .hs-boot thing to work too well and I'm really afraid to make it part of building Turbinado... But maybe I should relax and see if I can get the code generator to generate the .hs-boot files. It still feels awfully hack-ish, though.

By: dmwit

dmwit — Mon, 05 Jan 2009 05:09:03 +0000

But, but… GHC supports recursive modules! Take a look at the documentation on .hs-boot files: http://www.haskell.org/ghc/docs/latest/html/users_guide/separate-compilation.html#mutual-recursion

By: alson

alson — Mon, 05 Jan 2009 02:20:25 +0000

Saizan,

Funny. I'd had it in my mind that importing Types twice would be a problem, but it makes sense that it wouldn't be...

... in which case, I'm left with the problem that CommentModel requires functions from PageModel and PageModel requires functions from CommentModel ...

... also, there's the problem with overlapping "_id" fields in Types.hs...

By: alson

alson — Mon, 05 Jan 2009 02:15:55 +0000

Derek,

Yeah, Haskell98 does require that. I know it's a low priority, but it'd be great if GHC supported that requirement, too...

Neil,

I just bumped into a nasty problem with using a single Types file. My data types are generated from the database tables. A lot of the tables have overlapping field names (e.g. _id). So Types.hs has something like:

data Page = Page {_id :: Int, ...} data Comment = Comment {_id :: Int, ...}

Since the field labels are just functions, GHC complains about _id being defined twice... Doh.

Christopher,

That would separate like/related bits of code from each other. I'm kinda sad that a compiler issue is forcing me to organize my code into non-intuitive blocks...

Victor,

Doh! As mentioned above, Types.hs won't work (because of multiply defined "_id")...

Argh... This is getting ugly.

Alson

By: Saizan

Saizan — Mon, 05 Jan 2009 02:07:32 +0000

I don’t get why Fix #1 shouldn’t work as it is. If Page is defined in Types it doesn’t matter if both PageModel and CommentModel re-export it, the compiler knows that it’s the same Page type.

By: Victor

Victor — Sun, 04 Jan 2009 22:42:23 +0000

Why should you export Types.hs from other models? What about not doing it and requireing user to manually export Types.hs in order to refer any model?

By: Christopher Lane Hinson

Christopher Lane Hinson — Sun, 04 Jan 2009 16:13:47 +0000

If you still have import cycles after pushing types into Types.hs, then start moving functions into new modules.

Either pull them up into something higher-level that knows about both Page and Comment models, or push them down into something primitive that doesn't know about either.

Mutually recursive modules are stinky in any language!