[Haskell-cafe] [web-devel] Odd "Ambiguous type variable `a0' in the constraint:" error (with Yesod)

[Mathew de Detrich]

......

You tell me this now ;)

I was actually wanting to look at scaffolding, but the section for it in the
Yesod book is not completed yet (http://www.yesodweb.com/book/scaffold)

Well that was like 4 hours wasted

Do you have a quick example of how scaffolding is done with mkYesodData and
mkYesodDispatch (I only need something trivial)?

On Mon, May 2, 2011 at 11:18 PM, Michael Snoyman <michael at snoyman.com>wrote:

> Actually, there's a much simpler solution already implemented in the
> scaffolded site: instead of using mkYesod, use mkYesodData and
> mkYesodDispatch. mkYesod is really just a combination of the two. The
> former defines your route datatype, and the latter creates the
> YesodDispatch instance. This allows you to create your route in one
> module, put your handlers in their own modules, and then import those
> handlers in the final module that calls mkYesodDispatch.
>
> HTH,
> Michael
>
> On Mon, May 2, 2011 at 4:14 PM, Mathew de Detrich <deteego at gmail.com>
> wrote:
> > Ok I have found the source issue, in my case it was an issue that ended
> up
> > turning into how the modules for my Webserver is organized, and that
> > compiler error (about an ambiguous type) occurred because my main
> webserver
> > datatype was not instantiated yet in that module (using where aproot).
> > In essence there were 2 issues
> > The original problem (with the ambigous type error) was fixed by just
> simply
> > providing a type (in this case RepHtml) to the function definition
> > Once this was done, the second problem occured due to using splicing with
> > Template Haskell (from mkYesod). What I was attempting to do is to
> seperate
> > the handlers (of the form get/post****R) from the routes created with
> > mkYesod. This wasn't originally an issue until I tried to create widgets,
> > and it was due to the use of defaultLayout.
> > Handlers using defaultLayout needs to be placed *after* the instantiation
> of
> > yesod (where you do instance yesod *** where aproot *****) however, the
> > mkYesod requires the handlers (of the form get/post****R) to be placed
> > before the routes. Handlers without a defaultLayout do not require the
> Yesod
> > instantiation to compile (which is why the error never came up before, I
> > never used defaultLayout prior to attempting to use widgets). This
> created
> > some horrific cyclic module dependably, where I was forced to use hs-boot
> > files along with creating a dummy module which just contains the instance
> > Yesod ** where ****** by itself. Splitting off that instantiation into
> > a separate module was required since hs-boot files don't work with
> functions
> > that do splicing due to template haskell
> > Of course if GHC supported cyclic module dependencies out of the box (and
> > support for function splices with template haskell in those hs-boot files
> > are added) then this would have been much less painful, is there any plan
> to
> > support automatic creating of hs-boot files to GHC anytime soon?
> > On Sun, May 1, 2011 at 11:00 PM, Michael Snoyman <michael at snoyman.com>
> > wrote:
> >>
> >> Without seeing the actual code that's causing the breakage, there's
> >> not much I can tell you. (If you'd like me to take a look off-list,
> >> feel free to send me a private email.) My best recommendation is to
> >> try putting a type signature on getRootR.
> >>
> >> As a general issue: polymorphic Hamlet is a very convenient feature,
> >> but I think it leads to too much complexity in the type system. I've
> >> added some code for non-polymorphic Hamlet to the Github repo and will
> >> be releasing it as its own module (Text.Hamlet.NonPoly) in the next
> >> release. Assuming all goes well, it will be replacing the current
> >> Hamlet. That essentially means that you'll need to replace your code
> >> with something like:
> >>
> >> getRootR = defaultLayout $ do
> >>    setTitle "Polymorphic Hamlet"
> >>    addHtml [$html|<p>I was added with addHtml|]
> >>    addHamlet [$hamlet|<p>I was added with addHamlet|]
> >>    addWidget [$whamlet|<p>I was added with addWidget|]
> >>
> >> And just to make everyone curious: I've also added i18n support to
> >> non-poly Hamlet. I've got a long train ride on Tuesday, and I'm
> >> planning on documenting it then.
> >>
> >> Michael
> >>
> >> On Sun, May 1, 2011 at 6:50 AM, Mathew de Detrich <deteego at gmail.com>
> >> wrote:
> >> > Ok so I have a problem that was described here
> >> > (http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431) in
> regards
> >> > to
> >> > returning a  "Ambiguous type variable `a0' in the constraint error"
> when
> >> > compiling. Originally I thought it was due to the way I was coding
> that
> >> > part
> >> > of the code (or to be more accurate the code specifically for creating
> >> > the
> >> > so called widgets with addHTML/addWidget,addHamlet). So I copied the
> >> > example
> >> > code given here exactly (http://www.yesodweb.com/book/example-widgets
> ).
> >> > Compiling this worked fine, so at the next point I changed the
> >> > definition
> >> > of getRootR to
> >> > getRootR = defaultLayout $ wrapper $ do
> >> >     setTitle "Polymorphic Hamlet"
> >> >     addHtml [$hamlet|<p>I was added with addHtml|]
> >> >     addHamlet [$hamlet|<p>I was added with addHamlet|]
> >> >     addWidget [$hamlet|<p>I was added with addWidget|]
> >> > and then to
> >> > getRootR = defaultLayout $ do
> >> >     setTitle "Polymorphic Hamlet"
> >> >     addHtml [$hamlet|<p>I was added with addHtml|]
> >> >     addHamlet [$hamlet|<p>I was added with addHamlet|]
> >> >     addWidget [$hamlet|<p>I was added with addWidget|]
> >> > Both times compiled fine, so the issue wasn't what I originally
> thought
> >> > that
> >> > it was (as described
> >> > in http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431). The
> >> > problem
> >> > is, that when I use the above example code in my WebServer, I get this
> >> > Ambigious type error when compiling (even though I have set up the
> route
> >> > the
> >> > exact same way). Unfortunatley I can't provide the code for my
> >> > webserver,
> >> > since its commercially owned (and it would be pointless because its
> just
> >> > renamed variables, but otherwise its the same), so does anyone have
> any
> >> > ideas what could possibly cause such an error (such as a missing/extra
> >> > import or some package or something), or possibly some missing
> >> > instances?
> >> > Also sorry for creating another mailing list, but its a different
> issue
> >> > then
> >> > what I thought it was originally (and I also wanted to put it on
> >> > haskell-cafe since its a more general issue)
> >> > _______________________________________________
> >> > web-devel mailing list
> >> > web-devel at haskell.org
> >> > http://www.haskell.org/mailman/listinfo/web-devel
> >> >
> >> >
> >
> >
>
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