[Haskell-cafe] Need help to get started with GHC.Generics

Thu Sep 10 16:40:56 UTC 2020

Hi Li-yao,

Thanks for all your useful comments.

Maybe we have different ideas about what genericity is all about.

I think that my type class foe `to1 is pretty generic.

And yes, it is not about ADTs.

Thanks again

Luc

On Thu, Sep 10, 2020 at 6:38 PM Li-yao Xia <lysxia at gmail.com> wrote:

> This doesn't sound like a use case for generics then. Just to spare you
> the trouble of following a red herring.
>
> On 9/10/2020 12:26 PM, YueCompl wrote:
> > Li-yao, thanks for the pointer. And my case is not really about ADTs,
> > but to introspect the arguments an arbitrary Haskell function takes,
> > including how many and what type each argument is, so as to extract
> > proper values from a given ArgsPack, then call that Haskell function
> > with those values as args it expects.
> >
> > I'm not sure at a glance, that generics-eot has demonstrated how to
> > obtain argument list with type info for a function, and will look into
> > the details as I can.
> >
> > Thanks with regards,
> > Compl
> >
> >
> >> On 2020-09-10, at 23:08, Li-yao Xia <lysxia at gmail.com
> >> <mailto:lysxia at gmail.com>> wrote:
> >>
> >> Hi Compl,
> >>
> >> I couldn't tell what's generic (in the sense of GHC.Generics) about
> >> this example. A clearer example would be to give two applications with
> >> different algebraic data types, and to show how they consist of the
> >> same boilerplate, where the differences are only due to the differing
> >> numbers of fields and constructors.
> >>
> >> As for tutorials on generics, a good starting point might be
> >> generics-eot. Its documentation comes with a series of tutorials:
> >>
> >> https://generics-eot.readthedocs.io/en/stable/
> >>
> >> Li-yao
> >>
> >> On 9/10/2020 9:44 AM, YueCompl via Haskell-Cafe wrote:
> >>> Dear Cafe,
> >>> I'm tinkering with the idea for arbitrary Haskell functions to be
> >>> easily called from scripting code, I see auto derive
> >>> with GHC.Generics might be the most promising tool, but I'm lost
> >>> after read https://wiki.haskell.org/GHC.Generics and hackage docs. I
> >>> have no clue so far with how to start with it.
> >>> Specifically I want the section highlighted in blue get auto
> >>> generated, within the following `runghc` ready example:
> >>> ```
> >>> {-# LANGUAGEBangPatterns#-}
> >>> moduleMain where
> >>> importPrelude
> >>> importGHC.Generics
> >>> importData.Dynamic
> >>> -- * minimum data structures as interface with scripting code
> >>> typeAttrKey=String
> >>> dataAttrVal=NilValue
> >>> |IntValue!Integer
> >>> |StrValue!String
> >>> deriving(Eq,Ord,Typeable)
> >>> instanceShowAttrValwhere
> >>> show NilValue="nil"
> >>> show (IntValue!x)=show x
> >>> show (StrValue!x)=show x
> >>> dataArgsPack=ArgsPack{
> >>> positional'args::[AttrVal]
> >>> ,keyword'args::[(AttrKey,AttrVal)]
> >>> }
> >>> instanceSemigroupArgsPackwhere
> >>> (ArgsPackp1 kw1)<>(ArgsPackp2 kw2)=ArgsPack(p1 ++p2)(kw1 ++kw2)
> >>> instanceMonoidArgsPackwhere
> >>> mempty =ArgsPack[][]
> >>> classCallableawhere
> >>> call::a->ArgsPack->(AttrVal->IO())->IO()
> >>> -- * functions to be callable from scripting code
> >>> newtypeAssert=Assert(
> >>> Expect->MaybeTarget->Message->IOMessage
> >>> )
> >>> typeExpect=AttrVal
> >>> typeTarget=AttrVal
> >>> typeMessage=String
> >>> instanceCallableAssertwhere
> >>> -- can this get auto-generated ? with
> >>> https://wiki.haskell.org/GHC.Generics
> >>> call (Assert!assert)(ArgsPack!args !kwargs)!exit =do
> >>> (expect,target,message)<-parseApk
> >>> result <-assert expect target message
> >>> exit $StrValueresult
> >>> where
> >>> parseApk::IO(Expect,MaybeTarget,Message)
> >>> parseApk =goParse
> >>> (Left"missing arg: expect",Nothing,Left"missing arg: message")
> >>> args
> >>> kwargs
> >>> goParse (got'expect,got'target,got'message)[][]=casegot'expect of
> >>> Leftmsg ->error msg
> >>> Rightexpect ->casegot'message of
> >>> Leftmsg ->error msg
> >>> Rightmessage ->return (expect,got'target,message)
> >>> goParse (got'expect,got'target,got'message)args' ((name,val):kwargs')
> >>> =casename of
> >>> "expect"->casegot'expect of
> >>> Right{}->error "duplicate arg: expect"
> >>> Left{}->goParse (Rightval,got'target,got'message)args' kwargs'
> >>> "target"->casegot'target of
> >>> Just{}->error "duplicate arg: target"
> >>> Nothing->goParse (got'expect,Justval,got'message)args' kwargs'
> >>> "message"->casegot'message of
> >>> Right{}->error "duplicate arg: message"
> >>> Left{}->caseval of
> >>> StrValuemessage ->
> >>> goParse (got'expect,got'target,Rightmessage)args' kwargs'
> >>> _ ->error "bad arg type for: message"
> >>> _ ->error "unexpected keyword args"
> >>> goParse (got'expect,got'target,got'message)(val :args')[]=
> >>> casegot'expect of
> >>> Left{}->goParse (Rightval,got'target,got'message)args' []
> >>> Right{}->casegot'target of
> >>> Nothing->goParse (got'expect,Justval,got'message)args' []
> >>> Just{}->casegot'message of
> >>> Left{}->caseval of
> >>> StrValuemessage ->
> >>> goParse (got'expect,got'target,Rightmessage)args' []
> >>> _ ->error "bad arg type for: message"
> >>> Right{}->error "extranous positional args"
> >>> -- mockup & test out
> >>> main::IO()
> >>> main =
> >>> call
> >>> (Assertassert)
> >>> (ArgsPack[IntValue333,StrValue"as good will"]
> >>> [("target",IntValue333)]
> >>> )
> >>> $\result ->putStrLn $"Got result: "<>show result
> >>> -- | plain Haskell function meant to be easily called by scripting code
> >>> assert::Expect->MaybeTarget->Message->IOMessage
> >>> assert !expect !maybeTarget !message =casemaybeTarget of
> >>> Nothing->return $"* assertion not applicable: "<>message
> >>> Justtarget ->ifexpect ==target
> >>> thenreturn $"* assertion passed: "<>message
> >>> elseerror $"* assertion failed: "<>message
> >>> ```
> >>> I tried to understand how
> >>>  * The compiler can provide a default generic implementation for
> >>>    |parseJSON
> >>>    <
> https://hackage.haskell.org/package/aeson-1.5.4.0/docs/Data-Aeson.html#v:parseJSON
> >|.
> >>> is implemented in [aeson](https://hackage.haskell.org/package/aeson)
> >>> and it is overwhelming to me at the moment ...
> >>> Is there easier scaffold template for me to start with GHC.Generics?
> >>> Or there're even better techniques to achieve my final goal?
> >>> Help please!
> >>> Best regards,
> >>> Compl
> >>> _______________________________________________
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