[Haskell-cafe] Is there already a lib to parse a line of string with a pattern and generate key-value map or a record?

Neil Mayhew neil_mayhew at users.sourceforge.net
Wed Apr 17 16:14:00 UTC 2019

I'm sorry, the formatting got mangled in my last message, because I was 
using MarkdownHere. I'll try again without converting it to HTML.

I'm a fan of `regex-applicative`. It's a combinator library modelled on 
the parsec family but because it parses only regular languages rather 
than context-free ones, it's a bit simpler to use and is a better match 
for some tasks. It uses `OverloadedStrings` to make it easier to include 
literal string matches and provides a non-greedy repeating combinator 
called `few` that avoids having to specify exclusive matches. I think it 
solves this problem quite nicely:

{-# LANGUAGE OverloadedStrings #-}

import Text.Regex.Applicative

-- This is just <*> with right associativity
(<&>) :: Applicative f => f (a -> b) -> f a -> f b
(<&>) = (<*>)
infixr 3 <&>

type Item = (String, String)

item :: String -> RE Char ([Item] -> [Item])
item key = (:) . (,) key <$> few anySym

-- ${year}/${month}/${day} ${hour}:${minute} User ${username} runs 
command ${command}.
pattern :: RE Char [Item]
pattern =
     item "year" <* "/" <&> item "month" <* "/" <&> item "day" <* " " <&>
     item "hour" <* ":" <&> item "minute" <* " User " <&>
     item "username" <* " runs command " <&> item "command" <* "." <&>
     pure []

input :: String
input = "2019/04/17 17:27 User magicloud runs command ls."

output :: Maybe [Item]
output = match pattern input
-- Just 

(Also available as a 

Obviously the combinator version is less compact and therefore could be 
considered less readable, but the implementation details could probably 
be tweaked a bit. It would also be relatively easy to write a 
quasi-quoter that turns the original input syntax (with `${variable}`) 
into the equivalent I've shown here.
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