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{-# LANGUAGE DataKinds, PolyKinds, TypeOperators #-}
{-# LANGUAGE TypeFamilies, FlexibleInstances, ScopedTypeVariables #-}
{-# LANGUAGE InstanceSigs #-}

module CmdLiner where

import GHC.TypeLits

-- Grammar
-- I care about the following:
-- * required/optional command line options
-- * required/optional envvars
-- * command line flags (not values necessary)
-- * positional arguments
-- * format of stdin capture (if relevant)
-- * format of stdout/stderr
-- * signals
data RequiredOpt name t     = RequiredOpt Symbol t
data RequiredEnv name t     = RequiredEnv Symbol t
data OptionalOpt name t def = OpionalOpt Symbol t def
data OptionalEnv name t def = OptionalEnv Symbol t def
data Flag name def          = Flag Symbol Bool
data Positional n v t       = Positional Nat Symbol t
data CaptureInput format t
data RenderOutput outstream format t
-- much like ^T in ping etc.
data Signal name = Signal Symbol

-- Combinators

data a :|| b    = a :|| b
data (a :: k) :>> (b :: *)

infixr 9 :>>
infixr 8 :||

-- Data types, type classes, families, etc.

data Proxy a = Proxy
class HasCli segments
type family Command segments :: *

-- TODO: define type instances for each part of possible type expression

-- Public interface

usage :: HasCli segments
      => Proxy segments
      -> Command segments
      -> [String]
      -> IO String
usage = undefined

-- TODO: validator, documenter, completer, ...

-- Example Declaration

-- My usage of API

data Format = JSON | YAML

type MyCli0 = RequiredOpt "output" Format
type MyCli1 = MyCli0 :>> Flag "no-ssl-verify" False
type MyCli2 = MyCli1 :>> Positional 1 "ARG1" Int