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

10 Sep 2020

      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 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:

```
{-# LANGUAGE BangPatterns #-}

module Main where

import           Prelude
import           GHC.Generics
import           Data.Dynamic

-- * minimum data structures as interface with scripting code

type AttrKey = String
data AttrVal = NilValue
  | IntValue !Integer
  | StrValue !String
  deriving (Eq, Ord, Typeable)
instance Show AttrVal where
  show NilValue      = "nil"
  show (IntValue !x) = show x
  show (StrValue !x) = show x

data ArgsPack = ArgsPack {
    positional'args :: [AttrVal]
  , keyword'args    :: [(AttrKey, AttrVal)]
  }
instance Semigroup ArgsPack where
  (ArgsPack p1 kw1) <> (ArgsPack p2 kw2) = ArgsPack (p1 ++ p2) (kw1 ++ kw2)
instance Monoid ArgsPack where
  mempty = ArgsPack [] []

class Callable a where
  call :: a -> ArgsPack -> (AttrVal -> IO ()) -> IO ()

-- * functions to be callable from scripting code

newtype Assert = Assert (
    Expect -> Maybe Target -> Message -> IO Message
  )
type Expect = AttrVal
type Target = AttrVal
type Message = String

instance Callable Assert where

  -- 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 $ StrValue result
   where

    parseApk :: IO (Expect, Maybe Target, Message)
    parseApk = goParse
      (Left "missing arg: expect", Nothing, Left "missing arg: message")
      args
      kwargs

    goParse (got'expect, got'target, got'message) [] [] = case got'expect of
      Left  msg    -> error msg
      Right expect -> case got'message of
        Left  msg     -> error msg
        Right message -> return (expect, got'target, message)
    goParse (got'expect, got'target, got'message) args' ((name, val) : kwargs')
      = case name of
        "expect" -> case got'expect of
          Right{} -> error "duplicate arg: expect"
          Left{}  -> goParse (Right val, got'target, got'message) args' kwargs'
        "target" -> case got'target of
          Just{}  -> error "duplicate arg: target"
          Nothing -> goParse (got'expect, Just val, got'message) args' kwargs'
        "message" -> case got'message of
          Right{} -> error "duplicate arg: message"
          Left{}  -> case val of
            StrValue message ->
              goParse (got'expect, got'target, Right message) args' kwargs'
            _ -> error "bad arg type for: message"
        _ -> error "unexpected keyword args"
    goParse (got'expect, got'target, got'message) (val : args') [] =
      case got'expect of
        Left{}  -> goParse (Right val, got'target, got'message) args' []
        Right{} -> case got'target of
          Nothing -> goParse (got'expect, Just val, got'message) args' []
          Just{}  -> case got'message of
            Left{} -> case val of
              StrValue message ->
                goParse (got'expect, got'target, Right message) args' []
              _ -> error "bad arg type for: message"
            Right{} -> error "extranous positional args"

-- mockup & test out
main :: IO ()
main =
  call
      (Assert assert)
      (ArgsPack [IntValue 333, StrValue "as good will"]
                [("target", IntValue 333)]
      )
    $ \result -> putStrLn $ "Got result: " <> show result

-- | plain Haskell function meant to be easily called by scripting code
assert :: Expect -> Maybe Target -> Message -> IO Message
assert !expect !maybeTarget !message = case maybeTarget of
  Nothing     -> return $ "* assertion not applicable: " <> message
  Just target -> if expect == target
    then return $ "* assertion passed: " <> message
    else error $ "* 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 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] Need help to get started with GHC.Generics

YueCompl