day03-mull-it-over.hs

#!/usr/bin/env stack
-- stack runghc

module Main where
import Data.Functor (($>))
import Data.Void (Void)
import Text.Megaparsec ((<|>), Parsec, anySingle, atEnd, empty, eof, failure, label, lookAhead, many, runParser, takeP, try)
import Text.Megaparsec.Char (char, newline, string)
import Text.Megaparsec.Char.Lexer (decimal)

-- Test:
--  % echo -e 'xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))' | ./day03-mull-it-over.hs # = 161

data Instruction = MulInstr Int Int deriving (Show)

type Parser = Parsec Void String

instrs :: Parser [Instruction]
instrs = many instr <* eof

mulinstr :: Parser Instruction
mulinstr = string "mul(" *> (MulInstr <$> (decimal <* char ',') <*> (decimal <* char ')'))

instr :: Parser Instruction
instr = newline $> MulInstr 0 0
        <|> try mulinstr
        <|> anySingle *> instr

-- How can this work? I don't want to match on newlines and insert dummy multiplications.
-- But this yields a `TrivialError 72 (Just EndOfInput)`; how do I break out of the 'many' parser?
instr' :: Parser Instruction
instr' = try mulinstr
         <|> anySingle *> instr

solve :: Either a [Instruction] -> Int
solve (Right is) = sum [op1 * op2 | MulInstr op1 op2 <- is]

main :: IO ()
main = interact $ show . solve . runParser instrs "<stdin>"