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

[Mathew de Detrich]

Im not sure what you mean exactly by "run the scaffolder", (just to be
clear, I am not exactly sure what technically scaffolding is apart from it
being referenced once or twice in your documentation)

I assume you are talking about setting up the handlers for a specific route,
and then creating that single route on its own (instead of all at once with
mkYesod)?

On Mon, May 2, 2011 at 11:30 PM, Michael Snoyman <michael at snoyman.com>wrote:

> My best advice is to just run the scaffolder. Any other examples I can
> point you to (like the Haskellers source code) will contain a lot of
> extra information that won't necessarily apply to your case.
>
> Michael
>
> On Mon, May 2, 2011 at 4:28 PM, Mathew de Detrich <deteego at gmail.com>
> wrote:
> > ......
> > 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
> >> >> >
> >> >> >
> >> >
> >> >
> >
> >
> > _______________________________________________
> > web-devel mailing list
> > web-devel at haskell.org
> > http://www.haskell.org/mailman/listinfo/web-devel
> >
> >
>
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