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

[Michael Snoyman]

Type:

    yesod init

It will ask you some questions and then generate a bootstrap site.

Michael

On Mon, May 2, 2011 at 4:40 PM, Mathew de Detrich <deteego at gmail.com> wrote:
> 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
>> >
>> >
>
>