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

10 Sep 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  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 mailto:lysxia@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:par...
|.
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
_______________________________________________
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.
_______________________________________________
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.
-- 
   __~O
  -\ <,
(*)/ (*)

reality goes far beyond imagination

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

Luc Duponcheel