I decided to write it myself for fun. I decided that “From Scratch” means:

• No parser libraries (parsec/happy/etc)
• No using read from Prelude
• No hacky meta-parsing

Here is what I came up with (using my favourite parsing method: parser combinators):

import Control.Monad ((>=>))
import Control.Applicative (Alternative (..), asum, optional)
import Data.Maybe (fromMaybe)
import Data.Functor (($>))
import Data.List (singleton)
import Data.Map (Map, fromList)
import Data.Bifunctor (second)
import Data.Char (toLower, chr)

newtype Parser i o = Parser { parse :: i -> Maybe (i, o) } deriving (Functor)

instance Applicative (Parser i) where
  pure a = Parser $ \i -> Just (i, a)
  a <*> b = Parser $ parse a >=> \(i, f) -> second f <$> parse b i
instance Alternative (Parser i) where
  empty = Parser $ const Nothing
  a <|> b = Parser $ \i -> parse a i <|> parse b i
instance Monad (Parser i) where
  a >>= f = Parser $ parse a >=> \(i, b) -> parse (f b) i
instance Semigroup o => Semigroup (Parser i o) where
  a <> b = (<>) <$> a <*> b
instance Monoid o => Monoid (Parser i o) where
  mempty = pure mempty

type SParser = Parser String

charIf :: (a -> Bool) -> Parser [a] a
charIf cond = Parser $ \i -> case i of
  (x:xs) | cond x -> Just (xs, x)
  _ -> Nothing

char :: Eq a => a -> Parser [a] a
char c = charIf (== c)

one :: Parser i a -> Parser i [a]
one = fmap singleton

str :: Eq a => [a] -> Parser [a] [a]
str (c:cs) = one (char c) <> str cs
str _ = pure []

sepBy :: Parser i a -> Parser i b -> Parser i [a]
sepBy a b = (one a <> many (b *> a)) <|> mempty

data Decimal = Decimal { mantissa :: Integer, exponent :: Int } deriving Show

data JSON = Object (Map String JSON) | Array [JSON] | Bool Bool | Number Decimal | String String | Null deriving Show

whitespace :: SParser String
whitespace = many $ asum $ map char [' ', '\t', '\r', '\n']

digit :: Int -> SParser Int
digit base = asum $ take base [asum [char c, char (toLower c)] $> n | (c, n) <- zip (['0'..'9'] <> ['A'..'Z']) [0..]]

unsignedInteger :: Int -> SParser Integer
unsignedInteger base = foldl (\acc x -> acc * fromIntegral base + fromIntegral x) 0 <$> some (digit base)

integer :: SParser Integer
integer = do
  sign <- fromIntegral <$> asum [char '-' $> (-1), char '+' $> 1, str "" $> 1]
  (sign *) <$> unsignedInteger 10

-- This is the ceil of the log10 and also very inefficient
log10 :: Integer -> Int
log10 n
  | n < 1 = 0
  | otherwise = 1 + log10 (n `div` 10)

jsonNumber :: SParser Decimal
jsonNumber = do
  whole <- integer
  fraction <- fromMaybe 0 <$> optional (str "." *> unsignedInteger 10)
  e <- fromIntegral <$> fromMaybe 0 <$> optional ((str "E" <|> str "e") *> integer)
  pure $ Decimal (whole * 10^(log10 fraction) + signum whole * fraction) (e - log10 fraction)

escapeChar :: SParser Char
escapeChar = char '\\'
  *> asum [
    str "'" $> '\'',
    str "\"" $> '"',
    str "\\" $> '\\',
    str "n" $> '\n',
    str "r" $> '\r',
    str "t" $> '\t',
    str "b" $> '\b',
    str "f" $> '\f',
    str "u" *> (chr . fromIntegral <$> unsignedInteger 16)
  ]

jsonString :: SParser String
jsonString =
  char '"'
  *> many (asum [charIf (\c -> c /= '"' && c /= '\\'), escapeChar])
  <* char '"'

jsonObjectPair :: SParser (String, JSON)
jsonObjectPair = (,) <$> (whitespace *> jsonString <* whitespace <* char ':') <*> json

json :: SParser JSON
json =
  whitespace *>
    asum [
      Object <$> fromList <$> (char '{' *> jsonObjectPair `sepBy` char ',' <* char '}'),
      Array <$> (char '[' *> json `sepBy` char ',' <* char ']'),
      Bool <$> asum [str "true" $> True, str "false" $> False],
      Number <$> jsonNumber,
      String <$> jsonString,
      Null <$ str "null"
    ]
    <* whitespace

main :: IO ()
main = interact $ show . parse json

This parses numbers as my own weird Decimal type, in order to preserve all information (converting to Double is lossy). I didn’t bother implementing any methods on the Decimal, because there are other libraries that do that and we’re just writing a parser.

It’s also slow as hell but hey, that’s naive implementations for you!

It ended up being 113 lines. I think I could reduce it a bit more if I was willing to sacrifice readability and/or just inline things instead of implementing stdlib typeclasses.