Haskell implementation of FastOrderLine parser

jferris · January 20, 2014, 3:04pm

This is a Haskell reimplementation of the problem posed here: http://forum.thoughtbot.com/t/balancing-negative-numbers-from-text/1700/10

The Ruby version that I’m implementing is here: Refactoring from the Weekly Iteration http://forum.thoughtbot.com/t/balancing-negative-numbers-from-text/1700/8 · GitHub

Here’s the Haskell version:

gist.github.com

https://gist.github.com/jferris/2c63e6b2b2385fa992d1

Cli.hs

import Control.Applicative ((<$>))

import Combinable (combineAll)
import LineParser (parseInput)
import Order (Order, orderWithDefaults, removeZeroQty, validate)

main :: IO ()
main = display . parse =<< getContents

parse :: String -> Either String [Order]

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Combinable.hs

module Combinable (Combinable, combine, combineAll, similar) where

import Data.Sequence ((<|), empty, findIndexL, fromList, Seq, adjust)
import qualified Data.Foldable as Foldable

class Combinable a where
    similar :: Combinable a => a -> a -> Bool
    combine :: Combinable a => a -> a -> a

combineAll :: Combinable a => [a] -> [a]

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LineParser.hs

module LineParser (parseInput) where

import Text.Regex.PCRE ((=~), getAllTextSubmatches)

parseInput :: String -> ([String] -> Maybe a) -> String -> Either String [a]
parseInput pattern parser = mapEither (parser . matchLine pattern) . lines

matchLine :: String -> String -> [String]
matchLine pattern line = tail $ getAllTextSubmatches $ line =~ pattern

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The size of the code is roughly the same as the size in Ruby, but I think a few of the constructs are pretty clean in Haskell.

I ended up defining a type class (Combinable) for this which I’m sure is already implemented somewhere in the Haskell world in some form, but I couldn’t find it. I’d be particularly interested in knowing if there’s a Category for things that can be reduced by combining some of the elements in some way; I’ve had to solve that problem a few times now.

mike-burns · January 20, 2014, 4:18pm

I wonder if MonadPlus would solve your Combinable issue. Or Monoid?

Since this is a parser, would Parsec have helped here?

jferris · January 20, 2014, 4:41pm

It definitely seems close to Monoid/MonadPlus, but there isn’t really a zero case - how would I define mempty or mzero?

I originally implemented this using Parsec. It ended up being more verbose, because I couldn’t find a good way to have it parse patterns like F12/05/20’ and return them as a single string. Because of that, I had to part out the individual parts, let bindings, and then recombine them again. Maybe there’s a better way to use Parsec? If so, that would be nice, because Parsec handles the “Error on 1: whatever” logic for you (which I implemented in LineParser).

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Haskell implementation of FastOrderLine parser

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