overlays.hs

{-# language OverloadedLists #-}

{-# OPTIONS_GHC -Wall #-}

module Overlays where

import Prelude hiding ((.))

import GHC.Stack
import Debug.Trace
import Data.Map (Map)
import qualified Data.Map.Lazy as Map

type PkgSet =
    Map String Derivation

type Derivation =
    Int

nixpkgs :: PkgSet
nixpkgs =
    [ "gcc" =: 1
    , "ghc" =: 2
    ]

(=:) :: a -> b -> (a, b)
a =: b = (a, b)

infixr 0 =:

type Overlay =
    PkgSet -> PkgSet -> PkgSet

mkPkgSet :: PkgSet -> [Overlay] -> PkgSet
mkPkgSet prev [] =
    prev
mkPkgSet prev (fn : fns) =
    let
        this =
            fn final prev
        rest =
            mkPkgSet this fns
        final =
            rest <> prev
    in
        final

-- $> myPackage
myPackage :: HasCallStack => PkgSet
myPackage =
    mkPkgSet nixpkgs
        [ \self super ->
            [ "hello" =: self."gcc" + 1
            ]
        , \self super ->
            [ "hello" =: (super."hello" + 4)
            , "helloOld" =: super."hello"
            , "helloSelf" =: self."hello"
            , "dang" =: self."hello" + 1
            , "second" =: self."third" - 1
            ]
        , \self super ->
            [ "hello" =: 1234
            , "finalHello" =: self."hello"
            , "helloFrom3Super" =: super."hello"
            , "third" =: 3
            ]
        ]

infixr 9 .

(.) :: PkgSet -> String -> Derivation
pkgSet . attr =
    case Map.lookup attr pkgSet of
        Nothing ->
            error $ mconcat (
                [ "The attribute "
                , attr
                , " was not found in the package set."
                ] :: [String])
        Just a ->
            a