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

[Li-yao Xia]

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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